Login | Catalog | Contact | Support | Tech Info

DataPro International Inc.


DataPro Tech Info > SCSI FAQ

The Official SCSI FAQ

Frequently Asked Questions List for comp.periphs.scsi

Copyright Gary Field, 1994-2001, all rights reserved, permission granted for non-commercial distribution in un-modified form.

The official resource for finding out those things you always wanted to know about SCSI, but were afraid to ask.

Current Editor: Gary Field (

(Where you see reference to [Editor(GF)] that means me.)

Last updated: September 18, 2001

Note: Please allow the whole file to load before clicking on any links. If you click on a link before the portion of the page it points to has loaded, you just get sent to the top of the document.

What is the SCSI FAQ:
The SCSI FAQ is a living document that attempts to serve as a reference for people who are trying to learn about SCSI
and/or troubleshooting a SCSI system. I don't look at the FAQ as a standards document. There are many topics for which I could simply regurgitate the SCSI standards documents, but instead, I generally choose to describe a useful, "practical" answer instead of an obtuse, detailed, answer which would only further confuse a newcomer to SCSI. I feel more people are served by this method. I will NEVER knowingly include wrong information in the interest of simplicity however. Believe me, when I put something in  that's incorrect (it has happened on rare occasions), I get plenty of email pointing out the error to me. Sometimes my opinion even plays into an answer (Oh well, you get what you pay for).

Occasionally, something I have written strikes controversy, usually either due to a gray area in one of the standards or due to a misintrepretation of what I wrote or included. When I get notified of the disagreement, I research the topic, listen to reasonable arguments, and make the corrections or not as I see fit.

Articles get into the FAQ in several ways:
 1) I see a question appear a number of times and write an  answer for that topic and put it in.
 2) Someone else writes up an answer for a question they feel needs answering and sends it to me. I edit it for accuracy
           if necessary, correct the grammar/spelling somewhat, edit the text into a consistent format, and put it in.
 3) I see what I feel is a well written response to a question posted in comp.periphs.scsi and ask the author if I can include it.

Submitting articles for the FAQ:
If you feel the urge to write up an article for the SCSI FAQ that you feel qualified to answer, please format it the way the existing articles are and send it to me, (preferrably in HTML format, but .txt or .doc is OK too).

FAQ history: Created by Johnathan Vail ( from articles submitted to him by comp.periph.scsi readers.

Maintained by Johnathan Vail until November 1993.
Maintained by Gary Field from November 1993 to the present.

About the editor:
My credentials in SCSI technology are pretty substantial. I've been working with this stuff on a daily basis, continuously since 1985 on both PCs and various UNIX platforms. I write and enhance SCSI device drivers for a living. All of my own computers use all SCSI I/O.
I also wrote/re-wrote "The Book of SCSI: I/O for the New Millennium", and wrote the UNIX chapter in Brian Sawert's book "The Programmer's Guide to SCSI".

There are areas of SCSI which I am not expert on, and when a question of fact comes up in one of those areas, I research the issue using the SCSI standards documents, or, ask my colleagues who are expert in that area, about it.

In general I get nothing but compliments about the FAQ. The most common complaint is that it's not always up to date on certain topics. I try my best to keep it updated, but SCSI marches on...

"Who pays for the "SCSI Info Central" Web site where the FAQ is distributed from"? you ask.

SCSI Info Central

is my own personal web site that I've registered as, connected to the Internet via AT&T(formerly MediaOne) cable modem. I spend a considerable amount of my own time and money maintaining the site, and I hope people benefit from it.

Thanks to David Sanderson and Roland Bauer for helping me improve the quality and compatibility of the HTML in this document. I would also like to thank the denizens of comp.periphs.scsi for their stimulating banter which triggered the writing of many of the articles in the FAQ.

The Eternal Question
Should I buy SCSI or just go with EIDE/ATA?

This has to be the most commonly asked question regarding SCSI!
I hope this will summarize my thoughts on that issue:

For someone to who doesn't need a real multi-tasking workstation or server, the only reason for paying the extra money for SCSI is flexibility. EIDE/ATA is strictly for "inside the case" peripherals. SCSI allows you to attach a large collection of add-ons like scanners, CD recorders, tape drives (or even devices not conceived of yet), either inside or outside the CPU case in whatever manner suits your needs or wishes.

If you like non-technical analogies:

SCSI is like a palace, with an architecture that was well thought out from the beginning and built upon over a period of time to make it even greater than originally envisioned.

IDE/ATA is like a log cabin, with a dirt floor, built from whatever was found lying around in late Fall just before the snow came. It can't be expanded because it has no foundation and would collapse under its own weight.

Both provide shelter. SCSI costs more (but not as much as a palace :-)).

Take your pick.

If automobile analogies are more to your liking:

A Ford Escort will get you to work just as fast as a Volvo station wagon. Which would you rather go on vacation in? Which would you rather be in if an accident occurs?

If your computer is nothing more than a machine that's only purpose is to perform a certain set of tasks, and you don't expect to want any more out of it, IDE is probably for you.

On the other hand, if you enjoy computing and are always looking for more things your computer can do for you, SCSI will help enhance the experience for you. You won't regret the investment.

Just as with a palace however, you need to learn your way around. That's where this FAQ comes in!

Where to get the latest copy of this FAQ:

SCSIFAQ.ORG - Now at a browser near you!

  • Pointers to the comp.periphs.scsi FAQ are posted to Usenet during the first week of each month (and usually other times too).
  • via World Wide Web (WWW): (I'm looking for  mirrors in So. America, Africa, Asia, and Australia)
If you just can't get enough SCSI, you might also want to look at:
SCSI Info Central where you will also find The SCSI Game Rules

Attention SCSI vendors: There are a few articles in this FAQ where vendor contact information, and in a few cases, part numbers, are listed. This is not an attempt to steer business to any particular vendor but only to provide possible sources of certain "hard to find" SCSI accessories (particularly special cables, adapters and terminators). If you want to be listed in one or more articles please send your contact info and which items you can provide to the FAQ editor.

I will not include pointers for devices like hard disks, tapes, CDROMs etc., which I consider readily available.

Table of Contents:


Generic SCSI Questions: Table of Contents

SCSI Documentation and Books:

Table of Contents

SCSI Performance Determination and Enhancement:

Table of Contents

SCSI Manufacturer Contact Information:

How can I contact:

Table of Contents

Manufacturer Specific Questions:

Table of Contents

Platform Specific Questions:

Table of Contents

Host Adapter Model Specific Questions:

Table of Contents

The Future of SCSI and Storage in General:

Table of Contents


Answers to the Questions:


ANSWER From: LSD, L.J.Sak@Kub. Edited by Gary Field (

SCSI stands for Small Computer System Interface. It's a standard for connecting peripherals to your computer via a standard hardware interface, which uses standard SCSI commands. The SCSI standard can be divided into SCSI (SCSI1) and SCSI2 (SCSI wide and SCSI wide and fast) and now SCSI-3 which is made up of at least 14 separate standards documents.

SCSI2 is the most popular version of the SCSI command specification and allows for scanners, hard disk drives, CD-ROM players, tapes [and many other devices]. SCSI-3 resolves many long time "gray areas" and adds much new functionality and performance improvements. It also adds new types of SCSI busses like fibre channel which uses a 4 pin copper connection or a pair of glass fibre optic cables instead of the familiar ribbon cable connection.

Table of Contents

QUESTION: SCSI sounds interesting. What do I need to get started?

ANSWER From: Gary Field (

In order to put together a PC with SCSI I/O you'll need:
  • A SCSI host adapter (also called a SCSI controller by sales types)
  • A SCSI cable (either WIDE, 68 pin, or narrow, 50 pin, external or internal)
  • A SCSI device of some sort (disk, tape, CD-ROM, CD-RW, DVD-ROM, scanner)
The SCSI device will most likely have a built in terminator that can be enabled or disabled. If it doesn't you'll also need a terminator (active terminator perferably). You'll find that there is quite a variety of SCSI cables out there. This is due to the fact that SCSI is so flexible. You are not limited to one SCSI device of course, that's just a minimum.

Table of Contents

QUESTION: What information should I provide when asking a question in the comp.periphs.scsi newsgroup?

ANSWER From: Gary Field (

In order for most SCSI problems to be resolved, one needs to provide at least the following:
  • Type of system (PC, SPARC or Alpha Workstation, etc.)
  • If PC, what type of motherboard?
  • Operating System (DOS, Windows 3.x, Win 95/98, Win NT 4/5, Linux, other UNIX)
  • Specific SCSI host adapter (Symbios xxxx, Adaptec xxxx, etc)
  • List of attached devices (and for disks, whether they're WIDE or NARROW)
  • Which connectors on the host adapter the cables are connected to
  • Length of SCSI bus
  • Where the terminators are located (90% of all SCSI problems result from mis-placed terminators).
  • Whether the configuration is new, or was working before.
It may seem like a lot of information to provide, but unless you have some SCSI experience, you may not realize how many factors can affect whether the system works properly or not.

If you don't know what some of these things mean, read the rest of this document until you do. You'll get much more help if you appear to have made an effort to find the answer on your own before asking for help.

Asking a question like "My scanner doesn't work, how come?" may not even get you a response.

PLEASE DO NOT ASK "Which is better IDE or SCSI"?
Please spare us all the aggravation of the week long tirade that will result from asking this seemingly innocent question!

Table of Contents

QUESTION: What do all these SCSI buzzwords mean?

ANSWER From: (Hennes Passmann)[Editor(GF)]

Host adapter
Also called a Host Bus Adapter or HBA. The card that connects your computer to the SCSI-bus. Usually called SCSI-controller by marketing droids. An example would be a PCI SCSI host adapter like the Adaptec 2940UW.
Terminators (passive)
A group of resistors on the physical ends of a single ended SCSI-bus (and only on these ends) that dampens reflected signals from the ends of the bus. Each terminated signal is connected by:
  • 220 Ohm to +5 volt (TERMPWR)
  • 330 Ohm to ground.
For NARROW SCSI the 18 signals that are terminated are:

I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ... DB(0).

For WIDE SCSI there are 9 more signals; DB(p1),  DB(8) ... DB(15)

Terminators (active)
Rather than passive terminators that use TERMPWR which may not be exactly +5v, active terminators use a voltage regulator. Basically it is a set of 110 Ohm resistors from each signal to a 2.8 Volt regulated Voltage source.
Single ended
"Normal" electrical signals. Uses open collector drivers to drive the SCSI bus.
[usually] survives wrong cable insertion.
DIFFSENSE signal is used to detect connection of differential devices and prevent damage.
The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm allowed to connect a device to the main cable. Most devices are single ended.
Differential (Now called High Voltage Differential to distinguish it from LVD)
Uses two wires to drive one signal.
Max. cable length of 25 meters.
Electrically incompatible with single ended devices!
Much more expensive than single ended.
Used from SCSI-1 upwards.
Apple kludge
The single ended 50 pins cable has been reduced to 25 pins by tying most grounds together. DB25 connector (like a parallel port). Often used as the external SCSI connector. Unfortunately, this abomination is being perpetuated by being used on devices like the Zip drive!

Asynchronous SCSI:
A way of sending data over the SCSI-bus.
The initiator sends a command or data over the bus and then waits until it receives a reply (e.g. an ACKnowledge). All commands are sent asynchronously over the 8 bit part of the SCSI-bus.
Synchronous SCSI
Rather than waiting for an ACK, devices that both support synchronous SCSI can send multiple bytes over the bus in the following way:
send data1 : send data2 : ... : send data3 (max outstanding bytes)
: wait : wait : response1 : reponse2: ...
This improves throughput, especially if you use long cables. (The time that a signal travels from one end of the cable to the other end of the cable IS relevant.)
Fast SCSI allows faster timing on the bus. ( 10MHz instead of 5MHz )
On a 8 bit SCSI-bus this increases the *theoretical* maximum speed from 5MB/s to 10MB/s.
Ultra SCSI
Synchronous data transfer option which allows up to 20MHz data clocking on the bus. Also called FAST20.
Ultra2 SCSI
Synchronous data transfer option which allows up to 40MHz data clocking on the bus. Also called FAST40.
Use of this option also requires the use of LVD bus drivers.

Ultra3 SCSI
Synchronous data transfer option which allows up to 80MHz data clocking on the bus. Also called Ultra160. Use of this option also requires the use of LVD bus drivers.
Uses an extra cable (or more commonly a 68 pin P cable) to send the data 16 or 32 bits wide. This allows for double or quadruple speed over the SCSI-bus.

RAID [Added by Editor(GF) Corrected by Fredrik Bjork (]
A Redundant Array of Independent Disks is a set of disk drives connected  in such a way as to allow certain types of access optimization, or data security. This can be accomplished in hardware using a special dual ported SCSI adapter, or completely in software in a special device driver.
A RAID 0 array stripes the data across multiple drives to decrease data latency. A RAID 1 array mirrors the data on multiple drives for increased data integrity. A RAID 5 array uses extra drives in a distributed manner to store parity information that can be used to apply data correction and recover any data in the event of any individual disk failure. This provides high reliability.
The following was submitted by
The minimum number of drives required for each RAID level is:
  RAID 0 - TWO (2) drives
  RAID 1 - TWO (2) drives
  RAID 0+1 - FOUR (4) drives
  RAID 3 - THREE (3) drives
  RAID 4 - THREE (3) drives
  RAID 5 - THREE (3) drives
Addition by:
Disconnect/reconnect (also called reselect)
This feature of the SCSI protocol allows a device to temporarily give up control of the SCSI bus. This is typically done when the device is performing an operation which will take some time. For example, it is very important for tape drives which would otherwise lock out other devices during long operations such as rewind.
Addition by: Editor GF (
Bus Segment
A portion of a SCSI bus isolated by a signal conditioner chip. A bus segment is logically part of a single SCSI bus (e.g. SCSI IDs must be unique) but electrically separated such that reflections on the segment do not affect other segments. Using bus segments allows longer busses because the signals get cleaned up (edges re-clocked etc) by going through the signal conditioner chips. Each segment must have its own terminations; One at the signal conditioner chip, and one at the far end of the segment.
Logical Unit Number (LUN)
A LUN is a sub-unit of a target. Most of the time, the LUN is just 0 since most types of target devices don't have sub-units. One example of where you might use LUNs is with multi-disc CDROM changers. Many of these units refer to each disc in the changer as a LUN. e.g. with the CDROM drive set as target ID 4, the first CD disc would be ID 4, LUN 0, the next would be ID 4, LUN 1 and so forth.
Another example is a optical disk jukebox where the optical drive might be LUN 0 and the changer might be LUN 1.
Some host adapters ignore LUNs unless the "Enable LUNs" option is set in the host adapter BIOS or operating system driver config. They default to not using LUNs because it speeds up the bus scan process and most targets don't support LUNs anyway.
LUN numbers are generally defined by the manufacturer and can't be changed by the user.
The Adaptec 2940 series BIOS has changed the place in the BIOS that LUN support is controlled several times.
A sketchy history:
  • AHA-2940U BIOS ver 1.11 = No LUN support
  • AHA-2940UW BIOS ver 1.25 = Under Advanced Config, "Multiple LUN support (enable/disable)"
  • ASUS P2B-LS* Adaptec AIC-7890 BIOS = Under SCSI device config, "Support Multiple LUNs (on/off)" on a per device basis.
* Note: The built-in SCSI adapter on this motherboard is quite similar to the Adaptec 2940U2W.
Table of Contents
QUESTION: What is the history of SCSI (What is SASI)?

ANSWER From: (Hennes Passmann)

#include <stddisclaimer.h>
The disk drive manufacturer Shugart begin working on a new drive interface with logical rather than physical adressing. It used 6 byte commands.
Shugart Associates Systems Interface (20 pages long) made public.
A few SASI drives are developed
Attempt to make SASI an ANSI standard failed.
Shugart and NCR request an ANSI committee be formed for SASI.
ANSI committee X3T9.2 is formed.
SCSI adds the ATN signal to the bus and creates the message protocol.
Development of SCSI drives and ST-506 to SCSI bridges begins.
CCS (Common Command Set) used in most disk drives.
Only disk and tape commands were adequately specified.
Work begins on SCSI-2.
SCSI-1 becomes official as ANSI X3.131-1986 (yes, after the work had begun on SCSI-2)
6 and 10 byte commands.
SCSI-2 specifies CDROM commands.
Production of SCSI-2 devices begins.
Work begins on SCSI-3.
SCSI-2 becomes official as X3.131-1994.
SCSI-2 is backward compatible with SCSI-1 and adds the following:
  • Fast SCSI-2. Optional bus speed of 10MHz instead of 5MHz.
  • Wide Optional 16 or 32 bit cable instead of 8 bits.
  • more commands defined, many optional (I'm not going to type the entire list here)
  • broader support for non-disk devices (tape.CDROM,Scanners....)
SCSI-2 devices can talk to the host adapter on their own inititive. (e.g. to set in which mode they should operate, FAST or not, wide, extra wide or normal ...) This can confuse some older SCSI-1 HA.
Production of drives that have some SCSI-3 enhancements.
Ultra SCSI: Bus speed of 20MHz?
SCSI-3 proposals include:
  • Support for graphical commands.
  • Fibre channel protocol (fibre channel) (FCP)
  • Serial packet protocol (IEEE 1394) (SBP)
  • SCSI-3 general packet protocol (almost all serial interfaces) and of course the old SCSI-2 commands and more.
  • Low Voltage Differential Parallel interface (SPI-2)
  • CD-R command set and algorithms (MMC)


Ultra2: Bus Speed of 40 MHz. LVD only.
Ultra3: Bus Speed of 80 MHz. LVD only.
Future (after 1998):
SCSI-3 becomes official
SCSI becomes a more network-like environment where devices can be physically distributed and shared more easily.
Table of Contents
QUESTION: Can I access SASI drive with SCSI controller?

ANSWER From: Gary Field (

Well, the answer is a definite maybe, but very unlikely. Old low performance SCSI adapters and drivers that use only a minimal subset of the SCSI commands may work with SASI devices that happen to support the INQUIRY command. Newer adapters and drivers expect to be able to use messages and will get very upset with a SASI device that doesn't understand them.

In reality, there is no practical reason to do this. Any SASI device is so obsolete that is has no real value in a system being used in 1990 or later.

Table of Contents

QUESTION:How should I lay out my SCSI bus? What should I avoid?
QUESTION: Where do I put the terminators?
QUESTION: Where should the adapter card be placed?

Answers From: Nick Kralevich <>

edited by Gary Field (

One confusing thing about SCSI is what the SCSI bus is supposed to look like, and how devices should be placed on the bus.

The SCSI bus MUST run continuously from one device to another, like this:

DEVICE A --------- DEVICE B --------- DEVICE C -------- DEVICE D

Where device A, B, C, and D can either be internal or external devices.

The devices on the SCSI bus should have at least 4 to 6 inches of cable between devices. This is to satisfy the SCSI-2 requirement that "stubs" be placed at least .1 meters apart. Some devices that have a lot of internal wiring between the connector and the SCSI chip can look like a "stub" or bus discontinuity. The reason for all these requirements is that a SCSI bus is really 18 "transmission lines" in the wave theory sense. A pulse propagating along it will "reflect" from any part of the transmission line that is different from the rest of it. These reflections add and subtract in odd combinations and cause the original pulse to be distorted and corrupted. The terminators "absorb" the energy from the pulses and prevent reflections from the ends of the bus. They do this because they (hopefully) have the same impedance as the rest of the transmission line.

The SCSI bus must not have any "Y" shape cabling. For example, setting up a cable that looks like this is NOT allowed:


            >------------- DEVICE C ----------- DEVICE D
Where do I put the terminators?
Termination must be present at two and ONLY two positions on the SCSI bus, at the beginning of the SCSI bus, and at the end of the SCSI bus. There MUST be no more than two, and no less than two, terminators on the bus.

Termination must occur within 4 inches (.1 meter) of the ends of the SCSI bus.

The following ARE acceptable:
   |            |          |           |           |         |
DEVICE A  Unconnected Unconnected  DEVICE B    DEVICE C  Adapter
Terminated                                               Terminated
   |            |          |           |           |         |
DEVICE A  Unconnected  DEVICE B  Unconnected   Adapter  DEVICE C 
Terminated                                              Terminated

   |            |          |           |           |         |
Adapter    DEVICE A   DEVICE B Unconnected Unconnected  DEVICE C
Terminated                                             Terminated
   |            |          |           |           |         |
Adapter    DEVICE A   DEVICE B Unconnected Unconnected   Termination

The following ARE NOT allowed:
   |            |          |           |                   |
DEVICE A    DEVICE B    Adapter    Unconnected        Unconnected    Dangling cable end
Terminated             Terminated
   |            |          |           |           |
Termination DEVICE A   DEVICE B  DEVICE C       Adapter             Termination in middle of bus
 [Editor GF]
Helpful hint:
I have found that it is much better in the long run to always disable the internal terminators in all of your devices and place a terminator block at the end of the cable itself. I'll grant you that this costs a little more because you need to buy a separate terminator. But, you never need to be concerned in the future when you re-arrange devices in your system, which device had its terminator enabled (none of them do). With the arrival of LVD and SCA, devices are starting to be shipped which don't even have internal terminators anyway, so getting used to the idea of terminating the cable end and not the device is a good practice. This is just my two cents worth... (backed by 15 years of tinkering with SCSI...).
Old wive's tale:
I still hear people say "If you put a terminator part way down your SCSI bus, the devices beyond it won't be seen". This is a total misconception of what terminators do. Putting a termination part way down the bus is incorrect and does cause problems, but it is quite unpredictable what the effect will be. It's not simply a matter of making the devices beyond the terminator invisible to the host adapter. Many people believe this myth and it will probably never go away, but I hope to convince at least a few people that this is not a valid way to envision how termination works.
Where Should I place the SCSI host adapter on the SCSI bus?

The placement of the SCSI adapter card can be on the end, at the beginning, or somewhere in the middle of the SCSI bus.

Quite frankly, placement of the controller card isn't special.

The adapter card is just another device on the SCSI bus.

As long as the rules above and in other sections of this FAQ are followed, there should be no problem placing the adapter card anywhere on the SCSI bus.

However, if you place the adapter card somewhere in the middle of the SCSI bus, you must be sure to disable termination on the adapter card. As noted previously, a SCSI device is only allowed to have termination if it's at the end of the bus. Only two terminators are allowed to terminate the SCSI bus, one at each end.

One last note: It doesn't make any difference where each SCSI ID is placed along the bus. It only matters that no two devices have the same ID. Don't forget that the adapter has an ID too. (Usually ID 7).

Table of Contents

QUESTION: What is a SCSI terminator? Why do I need them?

ANSWER From: Gary Field (
Updated:  May, 1999

A SCSI bus is a transmission line. To prevent reflections from the ends of the bus, you need a device which makes the transmission line appear to be of infinite length. This is done by attaching resistors, which have the same resistance as the characteristic impedance of the transmission line, to the ends of the bus. Also, since SCSI line drivers are open-collector (which can only pull a signal low), a pull-up resistor is needed to pull the signal high when it's not asserted.

If the ends of the bus are not terminated, the signal pulses will reflect off these open ends and travel back along the bus in the other direction. The resultant adding and cancelling of signal amplitudes distorts and corrupts the SCSI signals.

There are two basic types of terminators, active and passive:

  • Passive terminators consist of pairs of resistors. A 220 Ohm pulling each signal up to TERMPWR and a 330 Ohm pulling each signal down to GROUND. Passive terminators were considered adequate in SCSI-1 when the bus only ran at 5 MHz. In SCSI-2, passive terminators were given the name "Alternative 1".
  • Active terminators consist of 110 Ohm resistors connected from each signal line to a common 2.85 Volt regulated power supply. Active terminators both terminate the bus better (less reflection), and supply cleaner pull-up current (due to their Voltage regulation).  They were first defined in SCSI-2 and were given the name "Alternative 2" to distinguish them from passive terminators.

Recommendations and requirements:
In SCSI-2 when the fastest defined speed was 10 MHz, passive terminators were allowed, but active terminators were recommended.
In SCSI-3, the "alternative X" terminology has been discarded, and the SPI-2 standard only allows active termination for single-ended buses regardless of speed.
My personal recommendation is not to buy any new passive terminators. If you want to use up the old ones you have lying around, on older systems, with short buses and no more than 4 devices, that don't have any devices faster than 10 MHz, I can't argue with that, but ONLY BUY ACTIVE (or preferrably LVD) terminators for any new systems. If you run into problems, switching to an active terminator might well solve them.
Other people will tell you that only active terminators are ever acceptable at any speed. I leave the choice up to the individual at Fast10 and below, above that, active is absolutely the only acceptable choice.

I often hear the whine "It seems to be working, why should I bother with the terminators?"
The following appropriate analogy was given to me by Kevin Kilzer:
"It only seems to work fine because you have not seen an error.  It's like having  mice in your house.  If you never see one, you don't realize they are there."
Suddenly a problem will arise and you won't even realize it's associated with the fact that you added a device to your SCSI bus two weeks ago. Termination problems can manifest themselves in many ways. The best solution is to avoid them by following the rules to the letter.

A final nit to pick:
As I was reminded in looking back at the standards, technically SCSI-2 did not sanction Fast10 on single ended buses. It was only spec'd for differential. However, as was the case with WIDE SCSI using the 68 pin P cable, the industry latched onto it and it later became standardized in SCSI-3 SPI.

Table of Contents

QUESTION: What is terminator power (TERMPWR)? Why do I need it? Where does it come from?

ANSWER From:  Roger J. Hamlett (
Updated:  November, 1999

TERMPWR is the power source for the SCSI terminators. Terminators (both active and passive) require power because in addition to providing the correct impedance to prevent reflections on the SCSI bus, they source pull-up current to the SCSI signals.
The SCSI spec. allows for multiple devices to supply power, but also limits the maximum current that should be available. The "rule" is that "initiators shall supply TERMPWR". Hence a SCSI controller (host adapter) should supply TERMPWR, and on longer buses it is worth having a device near the end to also supply it. However, no more than about four devices should supply it, because in the event of a failure (shorted cable etc), there could be dangerous currents available.  Not all devices are designed to be able to supply TERMPWR, but many can. Usually this is done by setting one or two jumpers to select where TERMPWR will go. For example:
  • TERMPWR to on drive terminator only
  • TERMPWR to SCSI bus
  • On drive terminator gets its TERMPWR from SCSI bus
Addition by: [EditorGF] Even though the spec. says that host adapters should supply TERMPWR, PCMCIA type host adapters do NOT do it. This is because PCMCIA cards are generally plugged into laptop computers that run on batteries and can't afford the extra current drain. Another reason is because the contacts in a PCMCIA connector are so tiny that the 1 Amp TERMPWR current load is beyond their ratings. This being the case, at least one of the devices that you wish to attach to a PCMCIA host adapter needs to be able to supply TERMPWR, or you must provide a special terminator that has a power connection for this purpose.

Table of Contents

QUESTION: Is the spacing of connectors on a SCSI cable important?

ANSWER From: Gary Field (

The ANSI SCSI spec's say that "stubs" on a SCSI bus must not be any more than .1 meters (4 in.) long. In SCSI-2 there are also guidelines that say you shouldn't place "stubs" any closer than .3 meters (12 in.) apart. Since each device attached acts as a "stub", you really shouldn't place connectors any closer than this. This gets to be more important as your bus performance goes up. i.e. with Fast20 it is very important, but with SCSI-1 it doesn't really matter much. Since Fast20 also limits your overall bus length to 1.5 meters (for single ended) this also means you shouldn't really connect more than 5 devices for best reliability.

Another minor enhancement involves altering the spacing of adjacent connectors to prevent reflection resonance.

e.g. place connectors at one end, then .3m, then .56m then .86m then 1.12m

Overall, the cable impedance and configuration (straight vs. twisted pair) are probably more significant factors than connector spacing. However, if there is room for the extra cable, I recommend spacing the connectors as described above for best reliability.

Excess cable length is also a bad thing, so basically all these factors must traded off against each other to build the best SCSI cable for a given situation.

Table of Contents

QUESTION: How long can my SCSI bus be?

ANSWER From: Gary Field (

The SCSI bus length limits are based on the speed of the fastest device attached to the bus.

Here's a table which shows the limits:

Speed of FASTEST device Max. Single-Ended bus length Max. HV Diff. bus len. Max. LVD bus length
5 MHz (SCSI1 synch.) 6 meters 25 meters 12 meters
10 MHz (SCSI2 FAST) 3 meters (not recommended in SCSI-2) 25 meters 12 meters
20 MHz (Ultra or Fast20) I recommend no more than 1.5 meters. The SCSI-3 SPI spec. gives a much more complicated reommendation. 25 meters  12 meters
40 MHz (Ultra2 or Fast40) Not recommended 12 meters 12 meters
80 MHz (Ultra160) Not recommended Not recommended 12 meters

These limits assume the use of good quality cable,  and the use of active terminators or LVD/SE terminators at each end of the bus.

Notice that I used the term MHz to specify speed since MB/sec. changes with the bus width.

Note: Bus width doesn't change the maximum allowable length. The bus width is independent of bus length or speed.

The above table assumes that you know the max. speed of your devices (usually by looking in the manuals). Some software (like Adaptec EZ-SCSI) provides a driver status monitor which will tell you what mode the devices are actually in. This is important, since any synchronous speed must be negotiated by either the device, or the adapter. The speed actually used will be the least common denominator between the two.

For example, if a Fast20 disk is attached to a "SCSI2" host adapter that only goes up to Fast10, the device will only run at 10 MHz.

In systems with high performance disks and external peripherals which require long cables (i.e. external scanners, tapes or CDROM changers), you may want to put the external devices on their own bus to avoid having to slow down the fast disks. There are dual channel host adapters to make this simpler (avoids using multiple IRQs etc).

The SCSI Trade Association also has a handy table.

Table of Contents

QUESTION:How should I set the IDs of my devices?

ANSWER From: Gary Field ( (updated February 2001)

The main rule of SCSI IDs is that they all need to be unique on a per bus basis. Each device on a particular bus must be set to a different ID so that they can address each other without confusion. Some devices have a sticker on the drive which shows the ID pins, but if your does not, you'll need the data sheet for the device. The pins to jumper are obvious if you know how to count in binary, if not there is usually a table of jumper combinations on the data sheet for each ID setting.

There is a secondary consideration in setting IDs though; Higher ID numbers have a higher priority on the bus.
The overall ID priority order on a WIDE bus is as follows (highest to lowest): 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.
There are at least two philosophies on how to use the device priorities to best advantage:
Method 1:
Set the host adapter's ID to 7. The next lower IDs (6, 5, 4 ...) would then be used for any time critical devices you may have such as streaming tape drives or CD-RW drives. Your hard disks would be set to lower priority IDs because they are generally the fastest devices on the bus and if given too high a priority will hog all the bus bandwidth and "starve out" the slower but time critical devices.

Method 2:
This philosophy maintains that devices that create the load should be given low priorities and devices that relieve the load should be given higher priorities. In this view, the host adapter creates the load (I/O to be done), therefore, set the host adapter's ID to 0(or even 15 if no narrow devices will be attached). The time critical devices (streaming tape and CD-RW) would then be assigned highest priorities. Everything else (including disks) would be assigned IDs in between. The placement of the load creator at low priority pretty much prevents the "starvation" scenario.

To my knowledge no benchmarks have been published that show one method to be superior to the other. I would appreciate it if anyone would run some good tests to prove what the best method is or point to existing published results supporting one of these methods (or even another method).

Method 1 is apparently supported by Adaptec since they set all their host adapters to ID 7 by default.
I personally doubt that it makes very much difference which method you choose except on very heavily loaded systems where the drivers take full advantage of tagged command queueing etc.

Special consideration for older host adapters:
Many older host adapters make the assumption that the boot disk will be at ID 0. Most newer ones however, allow the user to specify which ID to boot from.

See also 1

Table of Contents

QUESTION:How do I connect up the device ID switch in an external case?

ANSWER From: Gary Field (
February 2001

The ID switches in external SCSI cases are designed to be as flexible as possible because there is no standard for how ID pins on SCSI devices are to be layed out. This flexibility however means that the user has to be creative!
Usually there are 4 wires coming from the switch (five if it's WIDE). They are:
  • ID bit 0 (value of 1)
  • ID bit 1 (value of 2)
  • ID bit 2 (value of 4)
  • ID bit 3 (if WIDE) (value of 8)
  • Common ground
The ID pins are usually specified pretty clearly on the documentation that came with the drive, but they rarely show which pins are grounds. To find the common ground pins (without guessing) you need an Ohmeter. Find which side of the pin pairs is connected to the drive chassis and to each other. Connect the common ground lead from the switch to any one of these pins.

Table of Contents

QUESTION: What are the pros and cons regarding SCSI vs IDE/ATA ?

ANSWER From: Gary Field (
Updated: April, 2001

Pros of IDE/ATA:

  • Inexpensive due to high volume of production and simplified firmwar development and testing requirements.
  • Supported directly by system BIOS in most cases. (unless you want DMA support)
  • Less overhead per command
Cons of IDE/ATA:
  • Very limited device attachment (two drives (including CDROMs) per channel, and two channels per system max.) (Recent versions of Linux (and I hear Win NT) support four or more ATA adapters)
  • Single threaded (commands do not overlap even with a second drive)
  • CPU is tied up transferring all data (actually newer EIDE controllers can do DMA as well if special drivers are loaded)
  • IDE/ATA and ATAPI evolved from the ancient ST-506 interface as one kludge on top of another
  • Cannot handle scatter/gather operations well (important in good Virtual Memory operating systems)
  • Maximum disk drive capacity of 128 GB
Pros of SCSI:
  • Flexible device attachment (up to 7 or 15 devices per SCSI bus) (inside or outside of case)
  • Longer cable lengths allowed (up to 12 meters using LVD)
  • Support for almost any peripheral type (disks, tape, CDROM, optical, scanner etc)
  • All commands can overlap with commands on other devices. Usually uses DMA to transfer data (which frees CPU for other tasks)
  • Interface and protocol is carefully specified by ANSI.
  • Largest, highest performance devices are available in SCSI before IDE
  • Most adapters can do scatter/gather DMA which is a necessity in virtual memory systems (Like Unix, NT, 2000) (Win 95/98 ?)
  • Max drive capacity of 2048 GB
Cons of SCSI:
  • Generally more expensive than IDE/ATA, due to more complex firmware and extra testing required. (not to mention greater performance commanding a higher price).
  • Slightly more complicated to install than IDE/ATA, due to termination requirements.
  • Seems scary to novice users because of amount of terminology and connector/protocol options.

Some people point to the need to set IDs in SCSI as making it more complicated, but it's really no more complicated than choosing master/slave jumpers in IDE.


Now that I've said that, here's an article to show that there's more than one opinion on this subject:

From: Ed Schernau <>

Subject: FYI: EIDE and DMA/Scatter-Gather

The Western Digital Caviar EIDE drive that came in what is now the file server in our office came with a Win3.x 32 BDA driver which allowed the user to select DMA type (B or F) and to implement scatter-gather.

Also, the Intel Triton chipset implements 2 EIDE controllers, and I know that at least the 1 on the PCI bus supports bus-mastering, as well as DMA. However, PIO transfers can be faster, the infamous Mode 4 can in theory, do 16.6 MB/sec and I've heard of a Mode 5 which can do 22 MB/sec. Which [PIO] is only a benefit in single-tasking systems like DOS or Win3.x. Sounds like Intel is trying to make EIDE into SCSI, eh?

Table of Contents

QUESTION:Should I spend the extra money on SCSI or just get IDE?

ANSWER From: Andrew Korn (

For home users this is a difficult question to answer in general. It totally depends on how you use your system, what operating systems are installed, and whether you will add more I/O devices in the future.

For server systems in a corporate environment the only sensible answer is to go with SCSI peripherals.

IDE has been improved a lot in the past few years, so in most cases it will be an acceptable choice for home users. You should consider the following

(we are mostly talking PC hardware from now on):

1. Your motherboard probably has an integrated EIDE controller capable of supporting up to four devices. (Older motherboards may not have a dual-channel IDE controller, in which case only two drives can be connected; even older motherboards may not be equipped with an IDE controller at all.) If not, an IDE controller for your system should cost less than $30, which is about half of what a decent SCSI host adapter (Symbios 53C810 based) would cost you. On the other hand, some high-end motherboards come with integrated SCSI host adapters.

2. EIDE is a single threaded architecture. This means that of the two drives connected to an IDE channel, one will always be idle while the other is executing a command. If you only want a hard disk and a CD-ROM drive, you can install the CD-ROM on the secondary IDE channel (the hard disk will probably be the primary 'master' drive); in this case, the aforementioned limitation does not affect you. Also, if you only plan on using single-tasking operating systems such as DOS, you needn't be concerned about this single-threadedness.

SCSI devices share the bus bandwidth efficiently by allowing one device to transfer data while another is seeking or rewinding its media. This will, however, only gain you performance if you use a proper multi-tasking operating system (such as Linux/UNIX [Editor(GF): or Windows NT/2000]).

3. By default, IDE devices use PIO (Programmed I/O) to communicate with the rest of the system. This has the drawback of consuming a lot of CPU time. However, most newer EIDE controllers support bus-mastering and most drives support DMA or even UDMA transfer modes. Using bus-master DMA decreases CPU consumption to almost zero. (It may not be easy to activate the DMA transfer mode under DOS, however.)

Early SCSI host adapters had much the same problem, but all newer ones support DMA transfers.

4. If you plan to use only two drives (one per IDE channel), IDE will probably be slightly faster and definitely less expensive than SCSI.

If you think you need more than two drives, plan to use a multi-tasking environment (such as Unix, OS/2, Netware or Windows 95/98/NT/2000), and think that performance is more important than cost, SCSI is the way to go.

Anything bigger than a small low-cost Linux-based server should probably use SCSI.

5. IDE tapes are not as cool as SCSI tapes. They tend to be slower, less reliable and less compatible with each other than SCSI tape drives. SCSI tapes are more expensive, however.

6. IDE is probably slightly easier to install. Termination is not an issue, and it will probably require no effort on your part to make the system aware of any new devices you add. In some increasingly rare cases this may not be true for SCSI. (You know what SCSI stands for? "System Can't See It." :))

Especially with older systems it may not be trivial (or, in rare cases, even possible) to make the computer boot from a SCSI drive.

7. It is problematic to add more than four IDE drives to a system. If you think you will need more than that, SCSI is probably the choice for you.

If your motherboard came with an integrated EIDE controller, however, there is no need to ignore that feature; you can have a mixed system with both IDE and SCSI devices. (Remember to buy SCSI where performance and parallelism is an issue; but there is no need to buy an expensive SCSI CD-ROM drive if an IDE drive would suit your needs.)

8. If you need high reliability, you want to buy a RAID capable SCSI host adapter. Be aware, however, that it is possible to emulate RAID from software; Linux can do RAID 0, 1, 4 and 5 with any mixture of SCSI and IDE disks. This software-based solution is probably less reliable and slower than a true RAID controller, but certainly also much less expensive. [Editor(GF): You can't boot from a software RAID set either].

[Editor(GF): ATA and IDE are basically the same thing, and the terms are used interchangably in this document.]

Here's a discussion that shows some of the advantages of SCSI in more detail:

from: Greg Smith (

Under DOS (and DOS/win3.1), there is very little useful work the host can do while waiting for a disk operation to complete. So handing off some work from a 66 MHz 486 to, say, an 8 MHz Z80 (on the controller) does result in a performance loss. Under EVERY other OS worth discussing (Unix, Netware, NT, OS/2, Win95/98 etc) the processor can go off and do something else while the access is in progress, so the work done by the other CPU can result in a performance increase. In such systems, due to virtual memory, a 64K byte 'contiguous' read requested by a process may be spread to 16 separate physical pages. A good SCSI controller, given a single request, can perform this 'scatter/gather' operation autonomously. ATA requires significant interrupt service overhead from the host to handle this.

Another big issue: ATA does not allow more than one I/O request to be outstanding on a single cable, even to different drives. SCSI allows multiple I/O requests to be outstanding, and they may be completed out of order. For instance, process 'A' needs to read a block. The request is sent to the drive, the disk head starts to move, and process 'A' blocks waiting for it. Then, process 'B' is allowed to run; it also reads a block from the disk. Process B's block may be sitting in a RAM cache on the SCSI controller, or on the drive itself. Or the block may be closer to the head than process A's block, or on a different drive on the same cable. SCSI allows process B's request to be completed ahead of process A's, which means that process B can be running sooner, so that the most expensive chip - the system CPU - tends to spend less time twiddling its thumbs. Under ATA, the process B request cannot even be sent to the drive until the process A request is complete. These SCSI capabilities are very valuable in a true multi-tasking environment, especialy important in a busy file server, and useless under DOS, which cannot take advantage of them.

I tend to hear from people, 'Well, I never use multitasking' because they never actively run two programs at once - all but one are 'just sitting there'. Consider what happens though, when you minimize a window which uncovers parts of four other application windows. Each of those applications is sent a message telling it to update part of its window; under win95, they will all run concurrently to perform the update. If they need to access disk (usually because of virtual memory) the smoothness of the update can depend a lot on the disk system's ability to respond to multiple independent read requests and finish them all as quickly as possible; SCSI is better at this.

So, yes, ATA can be faster under DOS; but SCSI provides advantages which are inaccessible to DOS. They will benefit Win95 however. The cost of intelligent, fast SCSI controllers and drives should decrease as people discover these advantages and start buying them.

I should add that many of SCSI's advantages are NOT available with some of the simpler SCSI controllers which were targeted only to the DOS market or part of cheap CDROM add-on kits.

Furthermore, SCSI allows far greater flexibility of interconnect. I concede that for the mass market, which likes to buy pre-configured machines, this is but a small advantage.

Table of Contents

QUESTION:Why do SCSI disks cost so much more than IDE/ATA disks?

ANSWER From: Gary Field (

 The increased cost of SCSI disks is primarily due to four factors:

  • SCSI drives are developed first for the high performance market (servers and high end workstations).  After the drive technology matures, they slap an IDE controller card onto it and start shipping high volume. That's why IDE/ATA drives are mostly 5400 and 7200 RPM when SCSI drives are mostly 10k and 15k RPM.
  • SCSI drive firmware is far more complicated than IDE firmware. This is because SCSI supports multiple initiators, disconnect/reconnect and tagged command queueing. SCSI protocol also has many more commands.  So, it costs more to develop, debug and test the firmware.
  • The drive manufacturers know that for high end systems SCSI is required and  IDE isn't really even a choice, so they tack on a premium to pay for the above two factors (charge what the traffic will bear).
  • Because SCSI drives are produced in lower volume they cost more to produce. Conversely, since SCSI drives cost more, people buy IDE instead and SCSI drives never get the advantage of high volume production.



    Interestingly, with other types of SCSI devices, the price difference isn't as great. SCSI CD-RW drives for example are usually only a little more expensive than IDE/ATAPI ones of similar ratings.

Table of Contents

QUESTION: Can I have both IDE/ATA drives and SCSI in the same system?

ANSWER From: Gary Field (

The short answer is YES. There are a few issues to consider however.

The main issue is which device will be used for booting the system. Under MSDOS, The system BIOS determined this completely. A couple third party BIOSes (like MRBIOS) allowed the user to choose the boot source, but most conventional BIOSes just booted from the IDE if it was present. If no IDE was present then the standard option card BIOS scan would find the SCSI card's BIOS and use it to boot.

Under Windows 95 and Windows NT, there are more options. Since the motherboard BIOS is used to load the boot sector that will still happen according to the same rules as under MSDOS described above. After the boot sector is loaded, the O/S's device drivers take over and those can be unloaded or drive letters re-ordered via the O/S configuration tools.

Update: As of 1999, this issue has largeley been solved. Most BIOSes allow the user sufficient control to boot from whatever device they want to (either SCSI or IDE).

Table of Contents

QUESTION: Is it possible for two computers to access/share the same SCSI devices?

ANSWER From: Gary Field ( with input from Michael Burke and Cees de Groot and Sheldon E Smith.
Updated: Jan, 2001

Yes, two (or more) hosts can be on the same SCSI bus as other SCSI devices. As long as all the SCSI hardware requirements are met, multiple hosts can share the SCSI bus. With that said, let's look at what you're getting yourself into.

This discussion refers primarily to PCs. There are high end systems that do allow full  of sharing SCSI devices. Usually, this is to allow fault tolerance. Two systems are connected to the same set of SCSI storage devices and when one of them fails, the other takes control. AIX with HACMP, Compaq Tru64 UNIX with TruClusters, and Compaq VMS clusters are examples of systems that allow this.  The combined group of systems with shared storage is often referred to as a cluster. Open VMS and Tru64 V5.x and newer also have a feature called the Ditributed Lock Manager which allows multiple hosts to actually share the filesystems on shared disks locking individual records as needed.
I am told Windows 2000 server now supports host fail-over for fault tolerant support of shared disks/RAIDs.

Some basic things you need to watch out for:

  • Each host's device drivers must use SCSI RESERVE/RELEASE commands to lock access. This locks the drive for access by only one host adapter. The conflicting host gets a RESERVATION CONFLICT status until the controlling host sends a RELEASE cmd or a bus reset is issued. The bus reset is necessary when the controlling host has failed and left the shared device reserved.
  • The host adapters on each host as well as each device on the shared bus must have unique IDs. Some host adapters may not let you set their ID.
  • Good and common electrical grounding of both systems and the devices.
  • SCSI length limits are not violated. For practical bus lengths you may need differential (either HVD or LVD but not mixed).
  • Make sure both hosts select the same data transfer mode (synch or asynch).
  • Only shareable devices can be on the shared bus.
  • Neither host adapter resets the SCSI bus except when needing to break stuck reservations.
Depending on the type of SCSI devices you want to share, there are different different issues. We'll discuss them class by class:

For Disks:
It would not make sense for two hosts to go about treating shared disks as if they each owned the device. Data would be destroyed pretty quickly. Even if the user tries to only access the shared disk from one host at a time, each host retains a cache of filesystem meta-data(directories etc) and neither host can know what the other host is changing on the disk. Therefore, unless one of the hosts restricts itself to read-only access, the filesystem will get corrupted.
Another way of letting multiple hosts share a disk is to break up the disk into partitions that are reserved for each host. Each host "owning" its own file system. Some provision needs to be made to prevent either host from using the wrong partition however. I'm not aware of a good way to do this.

For CD-ROMs and scanners:
CD-ROM drives and scanners can be shared (for data access) pretty easily because they are by definition READ-ONLY, but you can't be reading data from one host and playing music on the same drive from another. With scanners, discipline will be required to avoid attempting to access the scanner from both hosts simultaneously.

For Tapes:
Sharing tape devices is straightforward unless you need to handle failing-over from one host to another. If you want to do this the tape backup application needs to be written to know how to determine the position on the tape where writing was last successful and take up from there. If the RESERVE/RELEASE mechanism is used to reserve the tape drive, a bus reset will need to be used to break the reservation upon host fail-over. The reset will start the tape drive rewinding, so the application will need to find where it left off before it starts writing again, or start the backup over again.

For host to host communication:
Some people get the bright idea that it would be cool to transfer data directly from one host to another via the SCSI bus. While this might be cool indeed, you'll have your work cut out for you to get it to work! In order to do this one of the host adapters needs to flip into "target mode". In this mode the "target mode" host is made to appear like a disk or tape to the other host and data can be written/read to/from it in the usual manner. The snag is that software that does this is very rare and only works on specific host adapters. See here for more info on target mode and vendors that supply software.

Before considering implementing a shared SCSI bus configuration, you should examine your motives for wanting to do this. If you simply want to transfer data between the systems, a network (10/100 base T) is a MUCH simpler solution. A pair of ethernet cards costs about $50 and all the software you need is built into both Win95/98/NT/ME/2000 or Linux.
If you need fault tolerance, maybe you really do need a shared bus, but be prepared for lots of expense or years of  painful software development.

Table of Contents

QUESTION:What is the problem with the Adaptec 1542C and external cables?

ANSWER From: Scot Stelter, Adaptec (Product Manager for the AHA-1540)

Several articles lately have cited the importance of SCSI-2-compliant cables when cabling SCSI bus subsystems. Perhaps the most accurate and technically detailed one was published in Computer Technology Review in March '93 (Volume XIII, No. 3. PP. 6). In short, it explains the double-clocking mechanism that can occur due to cables whose impedance falls below the 90-Ohm SCSI-2 spec. Steep edge speeds on the REQ and ACK lines of the SCSI bus exacerbate the problem, but non-compliant cables are the root cause. Both LAN TIMES in the US (5/24/93, page 115) and CT Magazine in Germany (7/93, page 18) cite this cable problem.

In an extensive survey of cables available in the US and Europe, we found that more than half of the cables available have single-ended impedances in the 65 to 80 Ohm range -- below the 90 to 132 Ohms specified in the SCSI-2 spec. It seems that some (not all) cable vendors do not understand the specification, describing their cables as SCSI-2 compliant when they are not. A common misconception is that SCSI-2 means a high-density connector. In fact, there are several connector options. I have published a technical bulletin that summarizes the critical requirements (TB 001, April 1993). An artifact of its faster design left the AHA-1540C with faster edge-speeds than its predecessor, the AHA-1540B. As I have said, this can exacerbate the effect of bad cables. This explains why some users could get their AHA-1540B to work when an early AHA-1540C might not.

Essentially, the 1540B was more forgiving than the early 1540Cs. Good cables fixed the problem, but unfortunately for the user, good cables are hard to find.

After surveying the cable market and many of our customers, we decided that bad cables were going to be here for a while, and we had to make the 1540C as forgiving as the 1540B was. At the end of April '93 we made a change to the AHA-1540C that involved using a passive filter to reduce the slew rate of the ACK line, the signal that the host adapter drives during normal data transfers. Extensive testing with many intentionally illegal configurations confirms that we succeeded. Prior to release, we tested the AHA-1540C with over 200 peripherals, systems and demanding software programs with no failures. Then, a second team retested the AHA-1540C across a wild combination of temperatures, humidities and other stresses. This testing gives me confidence that the AHA-1540 line continues to serve as the gold standard for SCSI compatibility.

Table of Contents

QUESTION: What is the difference between the Adaptec 1542A and 1542B?

ANSWER From: (Harvey Fishman)

The AHA-1542A is obsolete and no longer supported by Adaptec. They stopped providing firmware upgrades at some level prior to the equivalence to the 3.10 level of the AHA-1542B firmware. I am not sure just where though. The present latest AHA-1542B firmware is version 3.20, and supports drives up to 8GB under MS-DOS.

Table of Contents

QUESTION:What are the differences between the Adaptec 1542B and the 1542C?

ANSWER from: Terry Kennedy (

The 1542C is an an updated model which replaces the 1542B. The 1542C features jumperless setup, having only 8 DIP switches. All other configuration options are set using the 1542C's built-in BIOS configuration utility. Configurable features not found on the 1542B are:

  • Ability to enable/disable sync negotiation on a per-ID basis (the 1542B could only do it for all ID's on the SCSI bus)
  • Ability to send "start unit" commands on a per-ID basis
  • BIOS works with alternate I/O port settings on the adapter.
  • Ability to boot from ID's other than 0
  • Software-selectable termination
  • Software-selectable geometry translation
  • Additional DMA speeds of 3.3 and 10 MB/sec
Additionally, the 1542C uses a Z80 CPU and 8Kb buffer instead of an 8085 and 2Kb buffer as on the 1542B.

Table of Contents

QUESTION: What are the differences between the 1542C and the 1542CF?

ANSWER from: Terry Kennedy (

The 1542CF includes all of the 1542C features, and adds "Fast" SCSI operation, providing SCSI data rates of up to 10MB/sec (compared with an upper limit of 5MB/sec on the 1542C). This is unrelated to the host DMA rate. It also has a software configurable address for the floppy controller and a "self-healing" fuse for termination power.

Table of Contents

QUESTION: Where can I get obsolete Adaptec files and utilities?

ANSWER From: Gary Field (

Try here or here.
You can get ASPI spec's from Adaptec's web site in the developer section.]

Table of Contents

QUESTION:What kinds of optical drives are available?
ANSWER From: Psycho Bob <>[Editor(GF)]

UPDATE: November 1999

 Optical storage has good points going for it; Immunity to stray magnetic fields; Potential for higher storage capacity per unit area; and relatively low media cost.

Current optical storage solution offers two different types of storage --rewritable and non-rewritable. The non-rewritable represents storage method in which the data becomes permanent after being written onto the disc. Rewritables, on the other hand, allows you to alter the data after it has been written -- just like the magnetic storage devices. And for
rewritables, two different technologies are available -- magneto-optical ("MO") and phase-change ("PD").


As the name implies, MO uses both magnetic and optical technology to store data on the disc. The disc itself is rare earth metal substrate. When data is to be written, the particular spot is first heated by the laser to the Curie point, and the magnetic field is generated while the spot cools. By varying the magnetic field angle, the substrate is polarized in a certain way that it will reflect back the laser beam differently depending on the magnetic field angle present when the particular spot was cooling down.

MO comes in many sizes and capacities. Consumers were first exposed to MO in Steve Jobs' NeXT computer in the mid-1980s. Although 5.25" had a slow start due to initial high cost, it has been evolving quite nicely.

The more popular ISO capacities for 5.25" MO are 4.8GB/5.2GB, 2.4GB/2.6GB, and 1.2GB/1.3GB. In 3.5" form, MO is available in 540MB/640MB, 230MB,and the 128MB. There are also some 12" MO, 14" MO, and other odd sizes in odd capacities -- particularly the hybrid 3.5" 1.3GB MO/PD drive. But they are limited to niche markets due to high cost and rarity.

Sony MiniDisc-Data is derived from the Mini-Disc (MD) audio format cartridge introduced earlier. MD-Data is to MD as CD-ROM is to digital audio compact disc (CD-DA). MD-Data (and digital audio MD) is based on the same magneto-optical technology, which partially explains the initial high-cost of the consumer MD audio recordable units. Fow now, MD-Data is the smallest of the MO family. With 2.5" form factor, it can store either 140MB or 650MB of uncompressed data.

Sony pushed the original MD-Data in the mid-'90s, but it did not catch on due to high cost (for the capacity offered) and Sony's decision to separate MD audio function from MD-Data. And for few years, MD format has lagged behind the capacity and speed of fellow MO breathens. In November 1999, Sony announced the MD-Data-2 format and has gotten the format up-to-date. It now has 650MB storage capacity with equal increase in transfer speed. The MD-Data-2 debuts with Sony's MPEG-2 camcorder in the Japanese market in December of 1999. The most important technical advancement MD-Data brought for MO in general is the one-pass recording. Prior to 5.25" 2.4GB/2.6GB MO and 3.5" 540MB/640MB MO, practically all MO used two passes to write data onto the disc -- one pass to erase the whole track, and a follow-up pass to write the updated data. MD's one pass recording, called light intensity modulation, direct over-write (LIM-DOW, ISO 14517) has been incorporated into several later-generation MO formats to speed up the writing speed.

Anyway, what's the limit of erase/write cycle can MO endure? Well, it doesn't look like anyone is really sure about it. Few years past, it is guessed to be around 1 million times with 30 years of archival stability. Today, Maxoptix says MO can sustain "greater than 1 trillion" cycles with greater than 50 years of archival storage life.

Today, the popular MO formats are 3.5" and 5.25" that follow the ISO standard. (Yes, there are others. But they're far and few in between...) The drives and medias are available from Fujitsu, IBM, Maxoptix, Pinnacle Micro, Pioneer Sony, Toshiba, and others.

Panasonic phase-change double-function (PD)

In around mid-'95, Panasonic released a proprietary optical storage format called phase-change double-function (PD) drive. The PD uses substrate that will reflect the light differently when heated to different temperatures (and then cooled). Write-once-read-multiple (WORM) medias were actually the first phase-change formats, but PD is the first *reversible* (that is, "re-writable") phase-change format. Panasonic PD stores 650MB per PD cartridge. Panasonic's own PD drive has also gone away with Sony's MD-Data, but the technology lives on in forms of CD-RW and DVD-RAM. The PD media is said to take approximately 1,000 erase/write cycles. After about 1,000 cycles, the substrate will be fatigued to the point where the two different states of the crystalline structure will become difficult to differentiate reliably.


Write-once-read-multiple (WORM) format is a *write once* format -- once you have written the data to the disc, the written data cannot be changed. Put it another way, the data recorded on the disc media is *permanent*.

WORM was the first popular format for optical storage, before being eclipsed by MO. WORM is still used by big companies and the government for archival purposes since it has the characteristic of not being able to be altered without damaging the media (good audit trail).

The new WORM formats being introduced are tending to be proprietary. There is rarely any interchangability between different vendor's drives and media. During the WORM to MO transition period, a curious format called continuous composite write-once (CCW) appeared. CCW cartridges function as WORM cartridges, writable using the installed base of WORM drives. But put it into MO drive, CCW cartridges becomes rewritable. Simply put, CCW is MO in WORM's clothing. Many of today's 5.25" MO drives still have the capability to read CCW cartridges. And practically all WORM cartridges sold today are CCW variety.

Sony-Philips Compact Disc (CD-R/CD-RW)...

WORM (in "MO" form) was once limited to niche market, but made one heck of a come-back with form of CD-R. CD-R is based on the Sony-Philips' proprietary CD-DA, commonly referred as "CD" (you know, those shiny disc things that America-On-Line sends you). CD-R offers standard capacity of 650MB of data per disc, and can be used to store data or record music (and be played in common CD players). But here, only the data-storage facet of the CD-R/CD-RW is discussed.

As far as data storage is concerned, the specifications are written in the "Orange Book." The Orange Book established three physical format for recordable CDs -- CD-MO, CD-R (previously known as "CD-WO"), and CD-RW (previously known as "CD-E").

CD-MO is a MO medium in CD format. As far as I know, this format only exists only on paper. The popular formats to come out of it are CD-R and CD-RW.

CD-R/CD-RW Incompatibility
Sony and Philips finally agreed on a standard for compact disc re-writable (CD-RW), together with HP, Matsushita, etc. Long story short, the CD-RW uses phase-change media -- same as Panasonic proprietary PD format. Not only that, it also stores 650MB like PD. And also like the PD, the CD-RW media cannot be read in existing CD-ROM drives! CD-ROM drives manufactured in 1997 and after will read CD-RW discs though.
CD-R and CD-RW are known for their incompatibilities. There are combinations of CD-R media, CD-R recorder, and CD-ROM player that simply wouldn't work. CD-RW is worse -- virtually no audio CD players will play CD-RW disc. The problem stems from the fact that reflectivity of the CD-R is less than a factory-pressed CD-ROM. And CD-RW is worse in that respect than the CD-R. As such, only the very recent CD-ROM player labled "Multi-Read" can read the CD-RW discs.


Possibly the most soap-operatic of all data-storage formats. With convergence of computers and audio/video equipment, DVD was the most talked about format for years as several companies fighting for what "DVD" format should be.

Writable DVD formats:
For now, DVD-RAM is not made to be playable in DVD-ROM players that are so popular for its good pictures. If you've looked at it, you'll notice DVD-RAM is encased in a carriage case (a la MO-style) but "video" DVDs aren't. Although I think it may be possible to take the DVD-RAM media out of the case and stick it into computer DVD-ROM to read the recorded data.

What's the difference between DVD-RAM, DVD-RW, and DVD+RW? (March 2000)
The names differ depending on whose specification the DVD storage is based on. If it's Matsushita (Panasonic), then it's DVD-RAM; if it's Sony/Philips, then it's DVD+RW; or if it's Pioneer, then the name becomes DVD-RW. The majority of current DVD storage devices follow Matsushita's DVD-RAM standard. Pioneer currently has its DVD-RW in the form of a DVD video recorder in Japan. Each has slightly different storage capacities. It's still unknown whether they'll be truly compatible with each other. But all three specs have been submitted and all are regarded as DVD re-writable "standards."

The Future?

Future optical storage will likely get bigger and bigger capacities, and faster and faster transfer rates.

Anyway, MO is here to stay, so are CD and the DVD family of formats. As for DVD... The competitions should prove to be entertaining (not). DVD is not much about technology but more about politics. But since so many electronic and entertianment giants are backing the DVD, you probably won't go wrong if you buy one. (Just hope the one you buy will not be orphaned at the turn of the hat by DVD consortium.) Some form(s) of DVD recordable will eventually standardized, but don't expect it to have more storage/speed than what MO/PD/WORM formats offer.

Summary of optical disk formats
Format *  
per disk
per sector
# of sides
per side
MO 1p
2048/ 2336
Sony MD-Data
MO 1p 2.5" 650MB 2048/2336 single 650MB/720MB Sony MD-Data-2
MO 2p
ISO/IEC 10090, ECMA 154
MO 2p
ISO/IEC 13963, ECMA 201
MO 1p




DIS(ISO/IEC) 15041
MO/PD 1p 3.5" 1.3GB 2048 single 1.3GB ISO/IEC 14760
MO 2p




ISO/IEC 10089

ANSI X3.2121-1992

MO 1p
5.25" 654MB ?? single 654MB Pioneer
MO 2p




ISO 13481
MO 2p




ISO/IEC 13549
ECMA 184
MO 1p




DIS(ISO/IEC) 14517
MO 2p

MO 1p
5.25" 1.7GB ?? Dual 654MB Pioneer
MO 1p
Pinnacle Micro "Apex"
MO 2p 5.25" 4.1GB
Dual 2.1GB ISO/IEC 15286
Kodak System 2000
WORM 5.25" 1.3GB ?? Dual 650MB ISO/IEC 11560
DIS(ISO/IEC) 15486
ISO/IEC 9171 Format A

5.25" 654MB ?? single 654MB Pioneer
ISO/IEC 10091
WORM 5.25" 1.4GB ?? Dual 700MB Panasonic

5.25" 1.7GB ?? Dual 654MB Pioneer
WORM 5.25" 2.6GB ?? Dual 1.3GB ISO/IEC 15486

5.25" 5.2GB ?? Dual 2.6GB DIS (ISO/IEC) 18093
WORM 12" 19GB ?? Dual ?? Pioneer LD-R

14" 14.8GB
DIS (ISO/IEC) 15898
PD 1p
CD-R 3.5" 130MB 2048 Single 130MB Orange Book
Orange Book
CD-RW 1p
2048 Single 650MB
Orange Book








DVD-R 3.5" 2.4GB 2048 dual 1.2GB 1.2GB, ECMA-279

5.25" 3.95GB
2048 Single 3.95GB

5.25" 8GB 2048 dual 4GB ECMA-279

5.25" 2.6GB
2048 Single
2.6GB ECMA-272, ECMA-273
DVD-RAM 1p 5.25" 4.7GB
2048 Single
UDF ISO-13346
DVD-RW 1p 5.25" 3GB

*technology: 1p -- one-pass write

             2p -- two-pass write
Standards for storage are set by many organizations. International Standards Organization (ISO), European Computer Manufacturers Association (ECMA), Deutsche Institut fur Normung (DIN), Japanese Industrial Standards Committee (JISC), and American National Standards Institute (ANSI) set the main optical disc storage standards. The ISO standards take precedence over all other standards.

In the above table, the heading defines one standard -- e.g. 5.25" MO 1.2GB/1.3GB has both ISO 13549 and ECMA 184 listed for it. IT IS NOT THAT 1.2GB FOLLOWS ISO 13549 AND 1.3GB FOLLOWS ECMA 184.

Of CD standards...

Funny as it seems, CD is actually considered as a proprietary format made by Sony and Phillips. The physical format for derivatives like CD-ROM and CD-R are "written in mutual agreement" in form of Red Book, Yellow Book, Orange Book, etc.

Of bytes/sector and usability...

As many of you might notice (especially on 5.25" MOs), there are different sized sectors. Many O/Ses assume one sector to contain 512 bytes. If you buy any of the media that use different than 512 byte/sector, you will need a software driver of some sort to use the media.

In optical media, the sectors are "hard sectored" at factory -- in other words, you cannot change the number of sectors by reformatting (low-level formatting) them. Take the 5.25" 1.2GB/1.3GB MO for example again. The 1.3GB media is sectored at 1024 bytes per sector. So the 1.3GB media has total of 637,041 sectors (per side) on it. If you do not use a software driver and your operating system does not properly recognize it, the 1.3GB media will become a 650MB cartridge (~325MB per side)!!

The safest bet is to use the 512 bytes/sector media. That should make the drive and media usable on most operating systems.

Table of Contents

QUESTION: Where can I get various SCSI documentation?
QUESTION: How can I find out about the emerging SCSI standards?
Updated: August, 2000

Thanks to John Lohmeyer of LSI Logic (formerly Symbios Logic, AT&T GIS, NCR Microelectronics), a number of SCSI related files are freely available.

This is the place to find more information about I/O Interfaces, especially SCSI, SCSI-2, and SCSI-3 including SPI, Fast-20 (Ultra
SCSI), Fast-40 (Ultra2 SCSI), Low Voltage Differential (LVD), SPI-3 (Ultra3 SCSI or Ultra160), SPI-4 (Ultra320), CAM, and much more. There are also pointers here to other web sites on Fibre Channel, ATA (IDE), and ATAPI.

The information is accessible from:


SASI Spec. - (.PDF format):

SCSI-1 draft standard - (Plain text, no figures, Dec. 1985): OR here

SCSI-2 draft standard (converted to HTML) -

SCA Specification

From: Gary Watson (

Small Form Factor (SFF) Committee documents (like the SCA spec's) are available by FaxAccess at:

(408) 741-1600 You will be asked to order documents by number.

For example: to get information on the Single Connector Attach spec.

The SCA-1 spec. is document #8015

The SCA-2 spec. is document #8046 (8451?)

document #8000 is an index to the other documents.

SCA-2 pinout data can also be found in the SCSI3 SPI-4 document referred to as "Alternative 4".

This FaxAccess service is available to all, but please keep in mind that unless you have engineering-level understanding of peripheral interfaces, you _will_not_ be able to understand any of it and you are wasting your own time and the bandwidth of these resources. If you are trying to learn more about SCSI, you are better off reading the magazine articles and books listed elsewhere in this FAQ.

The SCSI, SFF, SSA, and Fibre Channel reflectors:

A list of these is available on the T10 site.

"The SCSI, SFF, SSA, and Fibre Channel reflectors are for review and commentary on the respective specifications, not for asking questions about the interfaces (unless related to a specific ambiguity in a specification) nor for recruiting nor for technical support nor any purpose other than what is stated. The reflectors _are_ available for public review and commentary as required by ANSI and ISO."

Any spec on the reflectors or on the BBS or on the ftp sites are **proposed** or **preliminary** and are often subject to major substantive changes during the committee process. Actual, released, final specs are *only* available from Global Engineering Documents.

For Fibre Channel Info:

For Firewire (IEEE-1394) Info:

Table of Contents

QUESTION:Where can I get official ANSI SCSI documents?

ANSWER #1 From: (Kevin Jones)

and jmatrow@donald.WichitaKS.NCR.COM (John Matrow)

The SCSI specification: Available from:


11 West 42nd St. - 13th floor

New York, NY 10036

Sales Dept. (212) 642-4900


Global Engineering Documents

15 Inverness Way East

Englewood Co 80112-5704

(800) 854-7179 or (303) 792-2181

Int'l Sales Fax: (303) 397-2740

SCSI-1: X3.131-1986

SCSI-2: X3.131-199x

SCSI-3 X3T9.2/91-010R4 Working Draft

[Editor(GF):] The official ANSI standards are NOT available free of charge from any source. Only draft versions are freely distributable.

Table of Contents

QUESTION: What SCSI books and tutorials are available?

ANSWER From: Gary Field (

Updated: June, 2000

The Book of SCSI : I/O for the New Millennium, by Gary Field, Peter Ridge et al

Published by No Starch Press, San Francisco, CA

ISBN # 1-886411-10-7 , List Price $49.95.

A very complete reference and tutorial on almost all aspects of SCSI, including all the latest advances like Ultra2WIDE/LVD, and all the previous standard SCSI features. It addresses everything you need to know to install and debug SCSI I/O on a PC running  Windows 95/98/NT and information on Linux as well. Also includes a CD-ROM with useful SCSI utilities and information.
The technical editor was none other than John Lohmeyer (chairman of the ANSI SCSI committee since the beginning of SCSI), so you know the facts are straight!

IN-DEPTH EXPLORATION OF SCSI can be obtained from Solution Technology, Attn: SCSI Publications, POB 104, Boulder Creek, CA 95006, (408)338-4285, FAX (408)338-4374

THE SCSI ENCYLOPEDIA and the SCSI BENCH REFERENCE can be obtained from ENDL Publishing, 14426 Black Walnut Ct., Saratoga, CA 95090,

(408)867-6642, FAX (408)867-2115

SCSI: UNDERSTANDING THE SMALL COMPUTER SYSTEM INTERFACE was published by Prentice-Hall, ISBN 0-13-796855-8 (Seems to be out of print)

A neat little book called "Basics of SCSI" second edition, was sent to me free of charge by Ancot Corporation, Menlo Park, CA (415) 322-5322. It gives a simplified description of how most aspects of the SCSI bus work and includes some discussion of SCSI-2 issues.

"Programmer's Guide to SCSI" with CDROM - by Brian Sawert.

Published by Addison Wesley, Reading, MA. SRP $39.95

ISBN # 0-201-18538-5

Includes a chapter on UNIX SCSI subsystems written by Gary Field.

Brian's own web site

Addition by: (

'The SCSI Bus and IDE Interface' 2nd edition by Friedhelm Scmidt,

Addison-Wesley Publishing, $34.95 (I think). It includes a diskette with examples of source code to handle SCSI and IDE devices from a low-level programmer's perspective, and it has very detailed technical descriptions of both subsystems.

Not a book for beginners, but I heartily recommend it for anyone who's serious about learning the technical ropes.

ANSWER #2 From: Runar Jorgensen (

There was a two part article in Byte Magazine. The first part was in Feb 1990 issue, p. 267-274 and the second was in Mar 1990 issue, p. 291-298.

Another two part article appeared in Byte in May 1986 and June 1986.

Table of Contents

QUESTION: Where can I find SCSI info on the Web?

ANSWER FROM: Gary Field (

Try some of these: Table of Contents

QUESTION: Where can I get information on various disk drives and controllers?

ANSWER: (Eric Krieger) (Updated Sep. 30, 1994)

Drive and Controller Guide, Version 4.3

THEREF(tm) is a comprehensive Directory of Hard Drives, Floppy Drives, Optical Drives, and Drive Controllers & Host Adapters. It is designed to help the novice and pro alike with integration problems and system setups.

Information is provided in two handy formats; Portrait mode, for those who prefer a normal book-binding type print format, and(or) do not have a printer with Landscape capability, and Landscape mode, for those who pre-fer a computer-printout type format.

For printing, a Laserjet is preferred, but not necessary, and setup info is provided. For viewing, LIST(tm) by Vernon Buerg, will provide an excellent result, and allow text searches for finding specific models.

By F. Robert Falbo

Due many reports about the unavailablity of this file/archive I made sure that the file does exist at the following site:

You should find the archive at:



(In that directory-path there is also a sub-directory Seagate, where you also can find info/files about Seagate-drives).

Before you actually get this file, be sure to get/read the file /README.FILETYPES since it explains the used file-extension and which (de-)archiver should be used (and where to find/get them!).

Note: In the archive there are files containing Extended ASCII or ANSI characters (mostly used with IBM- and compatible PC's), so it may be a bit unreadable when reading it on non-PC systems, or without using a proper Characterset/Font!

Table of Contents

QUESTION: Where can I get technical information and jumper settings for HP drives?

ANSWER From: Rodney Brown (

Update From: Martin C Mueller ( )

Table of Contents

QUESTION: How can I contact Adaptec?

Also: Future Domain, Corel CD Creator, Trantor, Incat systems.

ANSWER From: (Jon D Caples)

408 945-8600 Main number
800 959 7274 tech support
800 442 7274 orders, doc, new bios, etc.
408 945-7727 BBS

Adaptec's general inquiry number, 800-959-7274, affords access to a FAX-based information retrieval system. In order to preserve the accuracy of this information, I won't go into details about how to use it (since Adaptec may change things without telling me :) ).

For those outside the CAN-US area, or local to Adaptec the direct FAX info number is (408) 957-7150.

There are three general topics as of this writing:

  • General Information
  • Sales Information
  • Technical Information
Give it a call and request the directory! As of this writing there are over 130 documents available. You need a touchtone phone and the fax number. You'll also be asked for an extension number to stamp on the FAX which will be used to identify the recipient.

[Editor(GF): As of July 1993 Adaptec bought Trantor.

Try (800) 872-6867 (TRA-NTOR)]

World Wide Web (WWW) URL:

[(from: Andrew Lockhart ( ]

You can address Adaptec support by email. The address is An auto-responder will bounce a message back acknowledging receipt of your email. This message will also detail other current forms of Adaptec Technical support. They promise a, no more than, 5 day turn-around. We have found the response brief, but satisfactory to our needs. We should add, we mention we are dealers in our email (which may improve Adaptec's response).

Table of Contents

QUESTION:How can I contact Archive Corporation?


Archive was bought by Conner Peripherals in 1993

Table of Contents

QUESTION: How can I contact BusLogic/Bustek/Mylex?

ANSWER From: Gary Field (

Table of Contents

QUESTION: How can I contact Corel?

ANSWER From: Gary Field (

Main Number: (800) 772-6735
Tech. Support: (613) 728-1010

ANSWER From: Gerrit Visser (

ftp: (SCSI is under Multimedia)

For Corel CD Creator Software contact Adaptec

Table of Contents

QUESTION: How can I contact Fujitsu?

ANSWER From: Ken Porter (

Fujitsu FactsLine FAX Back service (408) 428-0456

A six page catalog of available documents can be ordered.

ANSWER From: Mike Henry (anonymous)

A while back, Fujitsu created a product called Fujitsu Knowledge System (FKS) (long available on Compuserve (GO FUJITSU)). It is a Windows Help File (.HLP) listing of many Fujitsu disk, tape, and optical products. It includes drive switch/jumper settings and meanings. It is available via anonymous ftp from in the /pub/fks directory, filename: fks.exe

It is self-extracting and mostly self-documenting.

Table of Contents

QUESTION: How can I contact Quantum?

ANSWER From: (Kevin Martinez)

Quantum Corporation
500 McCarthy Blvd.
Milpitas, CA
Technical Support Telephone Numbers:
800 826-8022 Main Technical Support Number
408 894-3282 Technical Support Fax
408 894-3214 Technical Support BBS V.32 8N1
408 434-9262 Technical Support for Plus Development Products
408 894-4000 Main Quantum Phone number
800 4DISKFAX FAX on demand (From Thanh Ma

Note: In October, 2000 Maxtor merged with Quantum.

Table of Contents

QUESTION: How can I contact Seagate?

ANSWER From: John McDonald (

Technical Support Services

Online Services

Using a modem, you can obtain troubleshooting tips, free utility programs, drive specifications, and jumper settings for Seagate's entire product line. You can also download software for installing and analyzing your drive.

  • SeaNET

  • You can obtain technical information about Seagate products over the Internet from Seagate's World Wide Web home page (, Seagate's FTP server ( or e-mail. Send your e-mail questions to or
  • SeaBOARD

  • SeaBOARD is a computer bulletin board system that contains information about Seagate disc and tape drive products and is available 24 hours daily. Set your communications software to eight data bits, no parity and one stop bit (8-N-1).

    Location Phone number

    • Australia 61-2-9756-2359
    • France 33 1-48 25 35 95
    • Germany 49-89-1409331
    • Taiwan 886-2-2719-6075
    • Thailand 662-531-8111
    • UK 44-1628-478011
    • USA Disc: 405-936-1600
    • Tape: 405-936-1630
FAX Services
  • SeaFAX

  • You can use a touch-tone telephone to access Seagate's automated FAX system to receive technical support information by return FAX. This service is available 24 hours daily.

    Location Phone number

    • Australia 61-2-9756-5170
    • Germany 49-89-14305102
    • UK 44-1628-894084
    • USA 1-800-SEAGATE or
    • Disc: 405-936-1620
    • Tape: 405-936-1640
  • Seagate Technical Support FAX

  • You can FAX questions or comments to technical support specialists 24 hours daily. Responses are sent during business hours.

    Location Phone number

    • Australia 61-2-9725-4052
    • France 33 1-46 04 42 50
    • Germany 49-89-14305100
    • Hong Kong 852-2368 7173
    • Japan 81-3-5462-2979
    • Korea 82-2-556-7395
    • 82-2-556-4251
    • Singapore 65-488-7528
    • Taiwan 886-2-2715-2923
    • UK 44-1628-890660
    • USA Disc: 405-936-1685
    • Tape: 405-936-1683
Telephone Services
  • SeaFONE

  • 1-800-SEAGATE

    Seagate's 800 number (1-800-732-4283) allows toll-free access to automated self-help services that provide answers to commonly asked questions, troubleshooting tips, and specifications for disc drives and tape drives. This service is available 24 hours daily and requires a touch-tone phone. International callers can reach this automated self-help service by calling 405-936-1234.

  • Seagate Telephone Technical Support

  • For one-on-one help, you can talk to a technical support specialist during local business hours. Before calling, note your system configuration and drive model number (STnnnn).

    Location Phone number

    • Australia 61
Table of Contents
QUESTION: How can I contact Conner Peripherals? 

Conner Peripherals was bought by Seagate

Table of Contents

QUESTION: How can I contact Maxtor?

ANSWER From: David G North (

In October 2000, Maxtor merged with Quantum.
Their web site is still available however.


Table of Contents

QUESTION: How can I contact NCR?

NCR Microelectronics division was bought by AT&T and then by Symbios Logic.

See "How can I contact Symbios Logic"

Table of Contents

QUESTION:How can I contact Philips?

ANSWER From: S. C. Mentzer (

Philips Consumer Electronics Co.

One Philips Drive

Knoxville, TN 37914-1810

(615) 521-4316

(615) 521-4891 (FAX)



Table of Contents

QUESTION: How can I contact Symbios Logic?

ANSWER From: Symbios Logic

Update From: Wade Adams (

[Editor(GF)]: In Feb., 1998, Adaptec attempted to purchase Symbios Logic. The Federal Trade Commission told them they couldn't and subsequently, Symbios was sold to LSI Logic. In 2014 LSI was acquired by Broadcom Limited.

For literature on any Symbios Logic product please contact:

Phone: (800) 636-8022
            (800) 856-3093
            (719) 536-3300
 Fax:     (719) 536-3301


Technical Support:

Phone: (719) 533-7230


Table of Contents

QUESTION: How can I contact UltraStor? (Out of business)

ANSWER From: Ultrastor

UltraStor Corporation

13766 Alton Parkway suite 144

Irvine, CA 92718

General (714) 581-4100

Tech. Support (714) 581-4016

FAX (714) 581-4102

BBS (714) 581-4125




Table of Contents

QUESTION: How can I contact Tecmar Technologies (formerly Wangtek, WangDAT,Sytron, and Rexon)?

ANSWER FROM: from: Jay Long - ( and

Peter Dyballa (

Tecmar Technologies, Inc.

1900 Pike Rd., Bldg. E

Longmont, CO USA

phone: (303) 682-3700

(303) 776-7706

FAX: (303)776-1085

faxback: (800) 4BACKUP


European Office

Unit 15 Suttons Business Park

Suttons Park Avenue

Earley, Reading, UK RG6 1AZ

(44) 1189-660063

(44) 1189-660065 FAX

Singapore Office

Blk. 35 Marsiling Industrial Estate Road 3 #05-01/ 06

Singapore 739257

(65) 269-2228

(65) 360-0888 fax

Table of Contents

QUESTION:How can I contact Western Digital?



Western Digital Corporation

8105 Irvine Center Drive

Irvine, CA USA 92718

Online Services:

Tech Support BBS 714-753-1234 (up to 28.8 KBS)



AOL (keyword) WDC or Western Digital

MSN (go word) WDC

Table of Contents

QUESTION: How can I contact DPT (Distributed Processing Technology)?

ANSWER: From: Gary Field (

voice: (407) 830-5522

FAX: (407) 260-6690



Table of Contents

QUESTION: How can I contactMicropolis?

ANSWER: From: Richard Ravich (

[Editor(GF): As of late 1997 Micropolis is now out of business -- RIP.

Table of Contents

QUESTION: How can I contact Legacy Storage Systems?

ANSWER: From: Gregory Smith (

General: (905) 475-1077

Sales/Tech support/Service: (905) 475-0550

U.S. Tech Support: (800) 361-5685

Fax: (905) 475-1088


Legacy Storage Systems

43 Riviera Drive

Markham, ON Canada L3R 5J6

Table of Contents

QUESTION: How can I find out about IBM hard disks?

ANSWER: From: Gary Field (

Table of Contents

QUESTION: How can I contact Initio?

ANSWER: From: Gary Field (

Table of Contents


ANSWER From: (Kevin Jones)

There are 2 handshaking modes on the SCSI bus, used for transferring data:


ASYNCHRONOUS is a classic Req/Ack handshake.

SYNCHRONOUS is "sort of" Req/Ack, only it allows you to issue multiple Req's before receiving Ack's. What this means in practice is that SYNCHRONOUS transfers are approx 3 times faster than ASYNCHRONOUS.

SCSI1 allowed asynchronous transfers at up to 1.5 Mbytes/Sec and synchronous transfers at up to 5.0 Mbytes/Sec.

SCSI2 had some of the timing margins "shaved" in order that faster handshaking could occur. The result is that asynchronous transfers can run at up to 3.0 Mbytes/Sec and synchronous transfers at up to 10.0 Mbytes/Sec.

The term "FAST" is generally applied to a SCSI device which can do syncrhonous transfers at speeds in excess of 5.0 Mbytes/Sec. This term can only be applied to SCSI2 devices since SCSI1 didn't have the timing margins that allow for FAST transfers.

Table of Contents

QUESTION: How can I check a passive SCSI terminator?

ANSWER From: (Steve Ligett)

With an Ohmmeter:

The terminator contains 18 220-ohm resistors from signals to TERMPWR, and 18 330-ohm resistors from those signals to GROUND. I've drawn that below:

     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
    R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1
     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
sig  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o
     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
    R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2
     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

R1 = 220 Ohms, R2 = 330 Ohms
When you measure from any one signal to termpower, you aren't measuring that resistor in isolation, you are measuring that resistor IN PARALLEL with the combination of the corresponding 330 ohm resistor plus 17 220+330 ohm resistor pairs in series.
 I've redrawn the schematic to
make this easier to see:

|     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|    R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1 R1
|     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|     o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o  o
|     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|    R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2 R2
|     |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
|   --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
R1 |
|  |
| R2
| /
 o   <--------- 17 other pairs in parallel ---------->
We're trying to measure that one resistor from a signal to TERMPWR, but there's a ton of other stuff in parallel. The resistance of that "stuff" is 330 + 550/17 ohms (the 330 ohm resistor, in series with a parallel combination of 17 550 ohm resistors). The general formula for the equivalent of two resistances in parallel is R1*R2/(R1+R2).

Whipping out my trusty spreadsheet, I find that the "stuff" has a resistance of about 362 ohms, and that, in parallel with 220 ohms is about 137 ohms.

Table of Contents

QUESTION:Can someone explain to me the difference between 'normal' SCSI and differential SCSI?

ANSWER From: ralf@wpi.WPI.EDU (Ralph Valentino)

"Normal" SCSI is also called "Single-ended" SCSI. For each signal that needs to be sent across the bus, there exists a wire to carry it. With differential SCSI, for each signal that needs to be sent across the bus, there exists a pair of wires to carry it. The first in this pair carries the same type of signal the single-ended SCSI carries. The second in this pair, however, carries its logical inversion. The receiver takes the difference of the pair (thus the name differential), which makes it less susceptible to noise and allows for greater cable length.

Table of Contents
QUESTION: What are the pinouts for differential SCSI?

ANSWER From: ralf@wpi.WPI.EDU (Ralph Valentino)

Differential SCSI Connector Pinouts
---------------------------------------- ----------------------------------------
| SCSI   |         | MINI   |          | | SCSI   |         | MINI   |           |
| SIGNAL | DD-50P  | MICRO  | DD-50SA  | | SIGNAL | DD-50P  | MICRO  | DD-50SA   |
---------------------------------------- ----------------------------------------
| -GND   |    2    |   26   |   34     | | (open) |    1    |    1   |     1     |
| -DB(0) |    4    |   27   |    2     | | +DB(0) |    3    |    2   |    18     |
| -DB(1) |    6    |   28   |   19     | | +DB(1) |    5    |    3   |    35     |
| -DB(2) |    8    |   29   |   36     | | +DB(2) |    7    |    4   |     3     |
| -DB(3) |   10    |   30   |    4     | | +DB(3) |    9    |    5   |    20     |
| -DB(4) |   12    |   31   |   21     | | +DB(4) |   11    |    6   |    37     |
| -DB(5) |   14    |   32   |   38     | | +DB(5) |   13    |    7   |     5     |
| -DB(6) |   16    |   33   |    6     | | +DB(6) |   15    |    8   |    22     |
| -DB(7) |   18    |   34   |   23     | | +DB(7) |   17    |    9   |    39     |
| -DB(P) |   20    |   35   |   40     | | +DB(P) |   19    |   10   |     7     |
|  GND   |   22    |   36   |    8     | |DIFSENS |   21    |   11   |    24     |
|  GND   |   24    |   37   |   25     | | GND    |   23    |   12   |    41     |
|TERMPWR |   26    |   38   |   42     | |TERMPWR |   25    |   13   |     9     |
|  GND   |   28    |   39   |   10     | | GND    |   27    |   14   |    26     |
| -ATN   |   30    |   40   |   27     | | +ATN   |   29    |   15   |    43     |
|  GND   |   32    |   41   |   44     | | GND    |   31    |   16   |    11     |
| -BSY   |   34    |   42   |   12     | | +BSY   |   33    |   17   |    28     |
| -ACK   |   36    |   43   |   29     | | +ACK   |   35    |   18   |    45     |
| -RST   |   38    |   44   |   46     | | +RST   |   37    |   19   |    13     |
| -MSG   |   40    |   45   |   14     | | +MSG   |   39    |   20   |    30     |
| -SEL   |   42    |   46   |   31     | | +SEL   |   41    |   21   |    47     |
| -C/D   |   44    |   47   |   48     | | +C/D   |   43    |   22   |    15     |
| -REQ   |   46    |   48   |   16     | | +REQ   |   45    |   23   |    32     |
| -I/O   |   48    |   49   |   33     | | +I/O   |   47    |   24   |    49     |
|  GND   |   50    |   50   |   50     | | GND    |   49    |   25   |    17     |
---------------------------------------- ----------------------------------------

Please note that I can only verify the DD-50P connector. The Mini

Micro and DD-50SA pinout above is a pin for pin mapping from the SCSI

pinout in this FAQ.


How can I tell if I have a single ended or a differential drive?

ANSWER From: Gary Field (

Most times the model number of the drive will end with "D".

Use an Ohm meter to check the resistance between pins 21 & 22.

On a single ended system, they should both be tied together and tied to GND.

On the differential drive, they should be open or have a significant resistance between them. Differential drives are less common than single-ended ones, because they are mainly used only where longer cable runs are necessary, and they are not generally used in PCs, but state of the art drives are available with differential interfaces. Generally only the higher performance drives have a differential option because of the added cost.

Table of Contents

QUESTION: Who manufactures SCSI extenders and Single-Ended to Differential converters ?

ANSWER From: Gary Field (

Apcon Inc.

17938 SW Boones Ferry Road

Portland, OR 97224

Phone: (503) 639-6700 Fax: (503) 639-6740



Paralan Corporation

4655 Ruffner Street
San Diego CA  92111

Phone: 619-560-7266    Fax: 619-560-8929



Rancho Technology Inc.

10783 Bell Court-Rancho


Phone: (909)987-3966; Fax: (909)989-2365;

E-Mail:; BBS: (909)980-7699


Table of Contents

QUESTION: I've got a SCSI disk with an 80 pin connector. Someone called it an SCA drive. Can I connect this to my SCSI bus?

ANSWER From: Gary Field (

Date: Updated July 1999

SCA stands for "Single Connector Attachment". It is a standard being worked on by the ANSI Small Form Factor (SFF) committee. It combines WIDE SCSI signals, Power connections and ID switch connections onto one connector.

The main reason for creating this standard was to make it easier to connect drives in a hot-swappable RAID configuration.

SCSI vendors sell adapters that bring out the three sets of signals to conventional connectors.

Some places that sell such adapters are:

(There aren't any host adapters with 80 pin connectors, so don't ask )

See: SCA Specifications for more information about SCA.

SCA adapters for LVD drives:
If your SCA drive is an Ultra2 WIDE or LVD type drive, you need to make sure to get an SCA adapter designed to work with LVD drives. Some SCA adapters don't connect the DIFFSENS signal through which prevents proper sensing of whether single ended devices are on the bus. So if your drive is LVD, be sure to ask the vendor for an adapter that's compatible with LVD.

Table of Contents

QUESTION:What are the pinouts for SCSI connectors?

ANSWER From: snively@scsi.Eng.Sun.COM (Bob Snively)

[ Edited and expanded by Gary Field ( ]

Originally dated May 23, 1990

The connector families described by the drawings have standard pin numberings which are described the same way by all vendors that I have encountered. The SCSI-2 specification identifies the standard numbering, using that convention. It happened to be documented by AMP, but all the vendors use the same convention.

The following diagrams have the outline drawings of connector sockets at the bottom. This is really for reference only, because the connector sockets and plugs are both specified as to their numbering and usually are labeled.

There are some minor problems in naming the microconnector conductor pairs, which I have corrected in the enclosed diagram. All the conductor pairs of the Mini-Micro (High Density) connector are in fact passed through on the cables. SCSI-2 defines the RSR (Reserved) lines as may be ground or may be open, but they are still passed through the cable. Most present standard SCSI devices will ground those lines.
-------------------- microSCSI to SCSI Diagram ---------------------------
SCSI Connector Pinouts (single-ended)

------------------------------------ ------------------------------------- 
| SCSI  |        | MINI  |         | | SCSI   |        | MINI  |         |
| SIGNAL| DD-50P | MICRO | DD-50SA | | SIGNAL | DD-50P | MICRO | DD-50SA |
------------------------------------ -------------------------------------
| -DB(0)|    2   |  26   |   34    | |  GND   |   1    |   1   |    1    |
| -DB(1)|    4   |  27   |    2    | |  GND   |   3    |   2   |   18    |
| -DB(2)|    6   |  28   |   19    | |  GND   |   5    |   3   |   35    |
| -DB(3)|    8   |  29   |   36    | |  GND   |   7    |   4   |    3    |
| -DB(4)|   10   |  30   |    4    | |  GND   |   9    |   5   |   20    |
| -DB(5)|   12   |  31   |   21    | |  GND   |  11    |   6   |   37    | 
| -DB(6)|   14   |  32   |   38    | |  GND   |  13    |   7   |    5    |
| -DB(7)|   16   |  33   |    6    | |  GND   |  15    |   8   |   22    |
| -DB(P)|   18   |  34   |   23    | |  GND   |  17    |   9   |   39    |
| GND   |   20   |  35   |   40    | |  GND   |  19    |  10   |    7    |
| GND   |   22   |  36   |    8    | |  GND   |  21    |  11   |   24    |
| RSR   |   24   |  37   |   25    | |  RSR   |  23    |  12   |   41    |
|TERMPWR|   26   |  38   |   42    | | OPEN   |  25    |  13   |    9    |
| RSR   |   28   |  39   |   10    | |  RSR   |  27    |  14   |   26    |
| GND   |   30   |  40   |   27    | |  GND   |  29    |  15   |   43    |
| -ATN  |   32   |  41   |   44    | |  GND   |  31    |  16   |   11    |
| GND   |   34   |  42   |   12    | |  GND   |  33    |  17   |   28    |
| -BSY  |   36   |  43   |   29    | |  GND   |  35    |  18   |   45    |
| -ACK  |   38   |  44   |   46    | |  GND   |  37    |  19   |   13    |
| -RST  |   40   |  45   |   14    | |  GND   |  39    |  20   |   30    |
| -MSG  |   42   |  46   |   31    | |  GND   |  41    |  21   |   47    |
| -SEL  |   44   |  47   |   48    | |  GND   |  43    |  22   |   15    |
| -C/D  |   46   |  48   |   16    | |  GND   |  45    |  23   |   32    |
| -REQ  |   48   |  49   |   33    | |  GND   |  47    |  24   |   49    |
| -I/O  |   50   |  50   |   50    | |  GND   |  49    |  25   |   17    |
------------------------------------ -------------------------------------
                       ________________________        MINI-MICRO
    DD-50P            |   -------------------    |    ______________________
  ______ ______       |17 \. . . . . . . . . /1  |   | _________________    |
49| . . . . . .| 1    |33  \. . . . . . . . /18  |   | 1\ - - - - - - - /25 |
50| . . . . . .|2     |50   \. . . . . . . / 34  |   | 26\- - - - - - -/50  |
  -------------       |       -------------      |   |    -------------     |
                       -------------------------      ----------------------
ribbon cable             Old style Sun SCSI               "SCSI-2"
   male                                                     male
(  1            25 )
 \ ++++++++++++++ /
  \ 26         50/
"Centronics" 50 male (use pin numbers for MINI-MICRO)
16 bit Wide SCSI-3 "P" (Primary) Connector pinout (single-ended)
--------------------   --------------------
|  SCSI  | HIGH DEN |  |  SCSI  | HIGH DEN |
| SIGNAL |  68 PIN  |  | SIGNAL |  68 PIN  |
--------------------   --------------------
| GND    |     1    |  | -DB(12)|    35    |
| GND    |     2    |  | -DB(13)|    36    | 
| GND    |     3    |  | -DB(14)|    37    | 
| GND    |     4    |  | -DB(15)|    38    | 
| GND    |     5    |  | -DB(P1)|    39    | 
| GND    |     6    |  | -DB(0) |    40    | 
| GND    |     7    |  | -DB(1) |    41    | 
| GND    |     8    |  | -DB(2) |    42    | 
| GND    |     9    |  | -DB(3) |    43    | 
| GND    |    10    |  | -DB(4) |    44    | 
| GND    |    11    |  | -DB(5) |    45    | 
| GND    |    12    |  | -DB(6) |    46    | 
| GND    |    13    |  | -DB(7) |    47    | 
| GND    |    14    |  | -DB(P) |    48    | 
| GND    |    15    |  | GND    |    49    | 
| GND    |    16    |  | GND    |    50    | 
|TERMPWR |    17    |  |TERMPWR |    51    | 
|TERMPWR |    18    |  |TERMPWR |    52    |
| RSRVD  |    19    |  | RSRVD  |    53    |
| GND    |    20    |  | GND    |    54    | 
| GND    |    21    |  | -ATN   |    55    | 
| GND    |    22    |  | GND    |    56    | 
| GND    |    23    |  | -BSY   |    57    | 
| GND    |    24    |  | -ACK   |    58    | 
| GND    |    25    |  | -RST   |    59    | 
| GND    |    26    |  | -MSG   |    60    | 
| GND    |    27    |  | -SEL   |    61    | 
| GND    |    28    |  | -C/D   |    62    | 
| GND    |    29    |  | -REQ   |    63    | 
| GND    |    30    |  | -I/O   |    64    | 
| GND    |    31    |  | -DB(8) |    65    | 
| GND    |    32    |  | -DB(9) |    66    | 
| GND    |    33    |  | -DB(10)|    67    | 
| GND    |    34    |  | -DB(11)|    68    | 
---------------------  ---------------------
| _______________________    |
| 1\ - - - - - - - - - - /34 |
| 35\- - - - - - - - - -/68  |
|    -------------------     |

IBM's "Not really SCSI" connectors: [Editor(GF)]
Note that this connector is NON-COMPLIANT WITH ANY SCSI STANDARD!
60 pin Burndy connector as used on IBM RS/6000 systems:
Pin Signal  Pin Signal
--- ------  --- -----
1    Gnd    31   Gnd
2   -DB(0)  32  -ATN
3   Gnd     33   Gnd
4   -DB(1)  34   Gnd
5   Gnd     35   Gnd
6   -DB(2)  36  -BSY
7   Gnd     37  Gnd
8   -DB(3)  38  -ACK
9   Gnd     39  Gnd
10  -DB(4)  40  -RST
11  Gnd     41  Gnd
12  -DB(5)  42  -MSG
13  Gnd     43  Gnd
14  -DB(6)  44  -SEL
15  Gnd     45  Gnd
16  -DB(7)  46  -C/D
17  Gnd     47  Gnd
18  -DB(P)  48  -REQ
19  Gnd     49  Gnd

20  Gnd     50  -I/O
21  Gnd     51  Gnd
22  Gnd     52 Reserved
23  Gnd     53 Reserved
24  Gnd     54 Reserved
25  N/C     55 Reserved
26 TERMPWR  56 Reserved
27  Gnd     57 Reserved
28  Gnd     58 Reserved
29  Gnd     59 Reserved
30  Gnd     60 Reserved
Table of Contents


ANSWER From: Gary Field (

Macintosh Plus SCSI Connector Pinouts

Note that this connector is NON COMPLIANT WITH ANY SCSI STANDARD!

The grounding is insufficient and does not allow for proper twisted-pair

transmission line implementation. It is recommended that a short adapter cable

be used to convert to the more common Centronics style 50 pin connection,

rather than extend the 25 pin connection any further than necessary.

The Macintosh Plus used a NCR 5380 SCSI chip controlled by the MC68000


| SCSI     DB-25S  |
| SIGNAL    pin(s) |
+------------------+            DB-25S (female)
| -DB(0) |   8     |      _____________________________
| -DB(1) |  21     |     13\ o o o o o o o o o o o o o /1
| -DB(2) |  22     |      25\ o o o o o o o o o o o o /14
| -DB(3) |  10     |         ------------------------ 
| -DB(4) |  23     |        View from rear of computer.
| -DB(5) |  11     |
| -DB(6) |  12     |
| -DB(7) |  13     |
| -DB(P) |  20     |
| GND    | 7,9,14  |
| GND    |16,18,24 |
| -ATN   |  17     |
| BSY    |   6     |
| -ACK   |   5     |
| -RST   |   4     |
| -MSG   |   2     |
| -SEL   |  19     |
| -C/D   |  15     |
| -REQ   |   1     |
| -I/O   |   3     |
Pin 25 is NOT CONNECTED in the Mac Plus implementation. Newer Macs connect TERMPWR to pin 25, but are otherwise the same.
Future Domain 25 pin connector pinout 
Used on TMC-830/845 and TMC-850/860/885.

Use the Macintosh pinout above for TMC-850M, TMC-1610M, TMC-1650/1670 or MCS-600

| SCSI  | DB-25S  |
| SIGNAL| pin(s)  |
+-----------------+              DB-25S (female)
| -DB(0)|   14    |        _____________________________
| -DB(1)|    2    |      13\ o o o o o o o o o o o o o /1
| -DB(2)|   15    |       25\ o o o o o o o o o o o o /14
| -DB(3)|    3    |          ------------------------ 
| -DB(4)|   16    |          View from rear of computer.
| -DB(5)|    4    |
| -DB(6)|   17    |
| -DB(7)|    5    |
| -DB(P)|   18    |
| GND   |1,6,8,13 |
| GND   |13,19,25 |
| -ATN  |   20    |
| BSY   |   23    |
| -ACK  |   22    |
| -RST  |   10    |
| -MSG  |   21    |
| -SEL  |    7    |
| -C/D  |   11    |
| -REQ  |   24    |
| -I/O  |   12    |
Table of Contents

QUESTION: Where can I find pictures of the various SCSI connectors?
ANSWER From: Charlie (

It isn't easy for a newcomer to identify the multitude of SCSI connectors by name alone.
This URL will show diagrams and photos to help with identifying them:

Table of Contents

QUESTION:What is the difference between SCSI-1 and SCSI-2?

ANSWER From Dal Allen:


In 1985, when the first SCSI standard was being finalized as an American National Standard, the X3T9.2 Task Group was approached by a group of manufacturers. The group wanted to increase the mandatory requirements of SCSI and to define further features for direct-access devices. Rather than delay the SCSI standard, X3T9.2 formed an ad hoc group to develop a working paper that was eventually called the Common Command Set (CCS). Many products were designed to this working paper.

In parallel with the development of the CCS working paper, X3T9.2 sought permission to begin working on an enhanced SCSI standard, to be called SCSI-2. SCSI-2 would include the results of the CCS working paper, caching commands, performance enhancement features, and whatever else X3T9.2 deemed worthwhile.

While SCSI-2 was to go beyond the original SCSI standard (now referred to as SCSI-1), it was to retain a high degree of compatibility with SCSI-1 devices.

How is SCSI-2 different from SCSI-1?

1. Several options were removed from SCSI-1:

a. Single initiator option was removed.

b. Non-arbitrating Systems option was removed.

c. Non-extended sense data option was removed.

d. Reservation queuing option was removed.

e. The read-only device command set was replaced by the CD-ROM command


f. The alternative 1 shielded connector was dropped.

2. There are several new low-level requirements in SCSI-2:

a. Parity must be implemented.

b. Initiators must provide TERMPWR -- Targets may provide TERMPWR.

c. The arbitration delay was extended to 2.4 us from 2.2 us.

d. Message support is now required.

3. Many options significantly enhancing SCSI were added:

a. Wide SCSI (up to 32 bits wide using a second cable)

b. Fast SCSI (synchronous data transfers of up to 10 Mega-transfers per second -- up to 40 MegaBytes per second when combined with wide SCSI)

c. Command queuing (up to 256 commands per initiator on each logical unit)

d. High-density connector alternatives were added for both shielded and non- shielded connectors.

e. Improved termination for single-ended buses (Alternative 2)

f. Asynchronous event notification

g. Extended contingent allegiance

h. Terminate I/O Process messaging for time-critical process termination.

4. New command sets were added to SCSI-2 including:

a. CD-ROM (replaces read-only devices)

b. Scanner devices

c. Optical memory devices (provides for write-once, read-only, and

erasable media)

d. Medium changer devices

e. Communications devices

5. All command sets were enhanced:

a. Device Models were added

b. Extended sense was expanded to add:

+ Additional sense codes

+ Additional sense code qualifiers

+ Field replaceable unit code

+ Sense key specific bytes

c. INQUIRY DATA was expanded to add:

+ An implemented options byte

+ Vendor identification field

+ Product identification field

+ Product revision level field

+ Vital product data (more extensive product reporting)

d. The MODE SELECT and MODE SENSE commands were paged for all device types

e. The following commands were added for all device types:






f. The COPY command definition was expanded to include information on how to handle inexact block sizes and to include an image copy option.

g. The direct-access device command set was enhanced as follows:

+ The FORMAT UNIT command provides more control over defect management

+ Cache management was added:


- PREFETCH command


- Force unit access bit

- Disable page out bit

+ Several new commands were added:





+ The sequential-access device command set was enhanced as follows:

- Partitioned media concept was added:

* LOCATE command


- Several mode pages were added

- Buffered mode 2 was added

- An immediate bit was added to the WRITE FILEMARKS command

+ The printer device command set was enhanced as follows:

- Several mode pages defined:

* Disconnect/reconnect

* Parallel printer

* Serial printer

* Printer options

+ The write-once (optical) device command set was enhanced by:

- Several new commands were added:




- Twelve-byte command descriptor blocks were defined for several

commands to accommodate larger transfer lengths.


The following article was written by Dal Allan of ENDL in April 1990. It was published nine months later in the January 1991 issue of "Computer Technology Review". While it appeared in the Tape Storage Technology Section of CTR, the article is general in nature and tape-specific. In spite of the less than timely publication, most of the information is still valid.

It is reprinted here with the permission of the author. If you copy this article, please include this notice giving "Computer Technology Review" credit for first publication.

What's New in SCSI-2

Scuzzy is the pronunciation and SCSI (Small Computer System Interface) is the acronym, for the best known and most widely used ANSI (American National Standards Institute) interface.

Despite use of the term "Small" in its name, everyone has to agree that

Scuzzy is large - in use, in market impact, in influence, and unfortunately, in documentation. The standards effort that began with a 20-page specification in 1980 has grown to a 600 page extravaganza of technical information.

Even before ANSI (American National Standards Institute) published the first run of SCSI as a standards document in 1986, ASC (Accredited Standards Committee) X3T9.2 was hard at work on SCSI-2.

No technical rationale can be offered as to why SCSI-1 ended and SCSI-2

began, or as to why SCSI-2 ended and SCSI-3 began. The justification is much more simple - you have to stop sometime and get a standard printed. Popular interfaces never stop evolving, adapting, and expanding to meet more uses than originally envisaged.

Interfaces even live far beyond their technological lifespan. SMD (Storage Module Drive) has been called technically obsolete for 5 years but every year there are more megabytes shipped on the SMD interface than the year before. This will probably continue for another year or so before the high point is reached, and it will be at least a decade before SMD is considered to be insignificant.

If SCSI enhancements are cut off at an arbitrary point, what initiates the decision? Impatience is as good an answer as any. The committee and the market get sick of promises that the revision process will "end soon," and assert pressure to "do it now."

The SCSI-3 effort is actively under way right now, and the workload of the committee seems to be no less than it was a year ago. What is pleasant, is that the political pressures have eased.

There is a major difference between the standards for SCSI in 1986 and SCSI-2 in 1990. The stated goal of compatibility between manufacturers had not been achieved in SCSI in 1986 due to a proliferation of undocumented "features."

Each implementation was different enough that new software drivers had to be written for each device. OEMs defined variations in hardware that required custom development programs and unique microcode. Out of this diversity arose a cry for commonality that turned into CCS (Common Command Set), and became so popular that it took on an identity of its own.

CCS defined the data structures of Mode Select and Mode Sense commands,

defect management on the Format command, and error recovery procedures. CCS succeeded because the goals were limited, the objectives clear and the time was right.

CCS was the beginning of SCSI-2, but it was only for disks. Tape and optical disks suffered from diversity, and so it was that the first working group efforts on SCSI-2 were focused on tapes and optical disks. However, opening up a new standards effort is like lifting the lid on Pandora's Box - it's hard to stay focused on a single task. SCSI-2 went far beyond extending and consolidating CCS for multiple device types.

SCSI-2 represents three years of creative thought by some of the best minds in the business. Many of the new features will be useful only in advanced systems; a few will find their way into the average user's system. Some may never appear in any useful form and will atrophy, as did some original SCSI features like Extended Identify.

Before beginning coverage of "what's new in SCSI-2," it might be well to list some of the things that aren't new. The silicon chips designed for SCSI are still usable. No new features were introduced which obsolete chips. The cause of silicon obsolescence has been rapid market shifts in integrating functions to provide higher performance.

Similarly, initiators which were designed properly, according to SCSI in 1986, will successfully support SCSI-2 peripherals. However, it should be pointed out that not all the initiators sold over the last few years behaved according to the standard, and they can be "blown away "by SCSI-2 targets.

The 1986 standard allows either initiators or targets to begin negotiation for synchronous transfers, and requires that both initiators and targets properly handle the sequence. A surprisingly large percentage of SCSI initiators will fail if the target begins negotiation. This has not been as much of a problem to date as it will become in the future, and you know as well as I do, that these non-compliant initiators are going to blame the SCSI-2 targets for being "incompatible."

Quirks in the 1986 standard, like 4 bytes being transferred on Request

Sense, even if the requested length was zero have been corrected in SCSI-2. Initiators which relied on this quirk instead of requesting 4 bytes will get into trouble with a SCSI-2 target.

A sincere effort has been made to ensure that a 1986-compliant initiator does not fail or have problems with a SCSI-2 target. If problems occur, look for a non-compliant initiator before you blame the SCSI-2 standard.

After that little lecture, let us turn to the features you will find in

SCSI-2 which include:

o Wide SCSI: SCSI may now transfer data at bus widths of 16 and 32 bits.

Commands, status, messages and arbitration are still 8 bits, and the B-Cable has 68 pins for data bits. Cabling was a confusing issue in the closing days of SCSI-2, because the first project of SCSI-3 was the definition of a 16-bit wide P-Cable which supported 16-bit arbitration as well as 16-bit data transfers. Although SCSI-2 does not contain a definition of the P-Cable, it is quite possible that within the year, the P-Cable will be most popular non-SCSI-2 feature on SCSI-2 products. The market responds to what it wants, not the arbitrary cutoffs of standards committees.

o Fast SCSI: A 10 MHz transfer rate for SCSI came out of a joint effort

with the IPI (Intelligent Peripheral Interface) committee in ASC X3T9.3.

Fast SCSI achieves 10 Megabytes/second on the A-Cable and with wider data paths of 16- and 32-bits can rise to 20 Megabytes/second and even 40 Megabytes/second. However, by the time the market starts demanding 40 Megabytes/second it is likely that the effort to serialize the physical interface for SCSI-3 will attract high-performance SCSI users to the Fiber Channel.

A word of caution. At this time the fast parameters cannot be met by the Single Ended electrical class, and is only suitable for Differential. One of the goals in SCSI-3 is to identify the improvements needed to achieve 10 MHz operation with Single Ended components.

o Termination: The Single Ended electrical class depends on very tight

termination tolerances, but the passive 132 ohm termination defined in 1986 is mismatched with the cable impedance (typically below 100 ohms). Although not a problem at low speeds when only a few devices are connected, reflections can cause errors when transfer rates increase and/or more devices are added. In SCSI-2, an active terminator has been defined which lowers termination to 110 ohms and is a major boost to system integrity.

o Bus Arbitration, Parity and the Identify Message were options of SCSI, but are required in SCSI-2. All but the earliest and most primitive SCSI implementations had these features anyway, so SCSI-2 only legitimizes the de facto market choices. The Identify message has been enhanced to allow the target to execute processes, so that commands can be issued to the target and not just the LUNs.

o Connectors: The tab and receptacle microconnectors chosen for SCSI-2 are available from several sources. A smaller connector was seen as essential for the shrinking form factor of disk drives and other peripherals. This selection was one of the most argued over and contentious decisions made during SCSI-2 development.

o Rotational Position Locking: A rose by any other name, this feature

defines synchronized spindles, so than an initiator can manage disk targets which have their spindles locked in a known relative position to each other.

Synchronized disks do not all have to be at Index, they can be set to an offset in time relative to the master drive. By arraying banks of

synchronized disks, faster transfer rates can be achieved.

o Contingent Allegiance: This existed in SCSI-1, even though it was not

defined, and is required to prevent the corruption of error sense data.

Targets in the Contingent Allegiance state reject all commands from other initiators until the error status is cleared by the initiator that received the Check Condition when the error occurred.

Deferred errors were a problem in the original SCSI but were not described. A deferred error occurs in buffered systems when the target advises Good Status when it accepts written data into a buffer. Some time later, if anything goes wrong when the buffer contents are being written to the media, you have a deferred error.

o Extended Contingent Allegiance (ECA): This extends the utility of the

Contingent Allegiance state for an indefinite period during which the

initiator that received the error can perform advanced recovery algorithms.

o Asynchronous Event Notification (AEN): This function compensates for a deficiency in the original SCSI which did not permit a target to advise the initiator of asynchronous events such as a cartridge being loaded into a tape drive.

o Mandatory Messages: The list of mandated messages has grown:

|      Both            |         Target           |     Initiator     |
|    Identify          |         Abort            |     Disconnect    |
|                      |                          |                   |
|   Message Reject     |      No Operation        |   Restore Pointer |
|                      |                          |                   |
| Message Parity Error |    Bus Device Reset      | Save Data Pointer |
|                      |                          |                   |
|                      | Initiator Detected Error |                   |
o Optional messages have been added to negotiate wide transfers and Tags to support command queueing. A last-minute inclusion in SCSI-2 was the ability to Terminate I/O and receive the residue information in Check Condition status (so that only the incomplete part of the command need be re-started by the initiator).

o Command Queueing: In SCSI-1, initiators were limited to one command per LUN e.g. a disk drive. Now up to 256 commands can be outstanding to one LUN.

The target is allowed to re-sequence the order of command execution to optimize seek motions. Queued commands require Tag messages which follow the Identify.

o Disk Cacheing: Two control bits are used in the CDB (Command descriptor Block) to control whether the cache is accessed on a Read or Write command, and some commands have been added to control pre-fetching and locking of data into the cache. Users do not have to change their software to take advantage of cacheing, however, as the Mode Select/Mode Sense Cache page allows parameters to be set which optimize the algorithms used in the target to maximize cache performance. Here is another area in which improvements have already been proposed in SCSI-3, and will turn up in SCSI-2 products shipping later this year.

o Sense Keys and Sense Codes have been formalized and extended. A subscript byte to the Sense Code has been added to provide specifics on the type of error being reported. Although of little value to error recovery, the additional information about error causes is useful to the engineer who has to analyze failures in the field, and can be used by host systems as input to prognostic analysis to anticipate fault conditions.

o Commands: Many old commands have been reworked and several new commands have been added.

o Pages: Some method had to be found to pass parameters between host and target, and the technique used is known as pages. The concept was introduced in CCS and has been expanded mightily in SCSI-2.

A number of new Common Commands have been added, and the opcode space for 10-byte CDBs has been doubled.

o Change Definition allows a SCSI-2 initiator to instruct a SCSI-2 target to stop executing according to the 1986 standard, and provide advanced SCSI-2 features. Most SCSI-2 targets will power on and operate according to the 1986 standard (so that there is no risk of "disturbing" the installed initiators), and will only begin operating in SCSI-2 mode, offering access to the advanced SCSI-2 capabilities, after being instructed to do so by the initiator using the Change Definition command.

o The Mode Select and Mode Sense pages which describe parameters for

operation have been greatly expanded, from practically nothing in 1986 to hundreds of items in SCSI-2. Whenever you hear of something being described as powerful and flexible tool, think complicated. Integrators are advised to be judicious in their selection of the pages they decide to support.

o The Inquiry command now provides all sorts of interesting data about the target and its LUNs. Some of this is fixed by the standard, but the main benefit may be in the Vendor Unique data segregated into the special designation of Vital Product Data, which can be used by integrators as a tool to manage the system environment.

o Select Log and Sense Log have been added so that the initiator can gather both historical (e.g. all Check Conditions) and statistical (e.g. number of soft errors requiring ECC) data from the target.

o Diagnostic capabilities have been extended on the Read/Write Buffer and Read/Write Long commands. The ways in which the target can manage bad blocks in the user data space have been defined further and regulated to reduce inconsistencies in the 1986 standard. A companion capability to Read Defect Data permits the initiator to use a standard method to be advised of drive defect lists.

o A new group of 12-byte command blocks has been defined for all optical devices to support the large volume sizes and potentially large transfer lengths. The Erase command has been added for rewritable optical disks so that areas on the media can be pre-erased for subsequent recording. Write Once disks need Media Scan, so that the user can find blank areas on the media.

o New command sets have been added for Scanners, Medium Changers, and CDROMs.

All of this technical detail can get boring, so how about some "goodies" in SCSI-2 which benefit the common man and help the struggling engineer? First, and probably the best feature in SCSI-2 is that the document has been alphabetized. No longer do you have to embark on a hunt for the Read command because you cannot remember the opcode.

In the 1986 standard, everything was in numeric sequence, and the only

engineers who could find things easily were the microprogrammers who had memorized all the message and opcode tables. Now, ordinary people can find the Read command because it is in alphabetic sequence. This reorganization may sound like a small matter but it wasn't, it required a considerable amount of effort on the part of the SCSI-2 editors. It was well worth it.

Another boon is the introduction for each device class of models which describe the device class characteristics. The tape model was the most needed, because various tape devices use the same acronym but with different meanings or different acronyms for the same meaning.

The SCSI-2 tape model defines the terms used by SCSI-2, and how they correspond to the acronyms of the different tapes. For example, on a 9-track reel, End of Tape is a warning, and there is sufficient media beyond the reflective spot to record more data and a trailer. Not so on a 1/4" tape cartridge. End of Tape means out of media and no more data can be written. This sort of difference in terms causes nightmares for standardization efforts.

So there it is; a summary of what is in SCSI-2. It's not scary, although it is daunting to imagine plowing through a 600-page document. Time for a commercial here. The "SCSI Bench Reference" available from ENDL Publications (408-867-6642), is a compaction of the standard. It takes the 10% of SCSI-2 which is constantly referenced by any implementor, and puts it in an easy-to-use reference format in a small handbook. The author is Jeff Stai, one of the earliest engineers to become involved with SCSI implementation, and a significant contributor to the development of both the 1986 standard and SCSI-2.

SCSI-2 is not yet published as a standard, but it will be available later this year. Until then, the latest revision can be purchased from Global Engineering (800-854-7179).


Consultant and analyst I. Dal Allan is the founder of ENDL and publisher of the ENDL Letter and the "SCSI Bench Reference." A pioneer and activist in the development and use of standard interfaces, he is Vice Chairman of ASC X3T9.2 (SCSI) and Chairman of the SCSI-2 Common Access Method Committee.

Table of Contents

QUESTION: What is the difference between SCSI-2 and SCSI-3?

ANSWER From: excerpts of postings by Jeff Stai and others:

(Mohit K Goyall -

Are SCSI-3 hard drives and/or controllers available yet?

Allegedly, Previous postings have said "I heard that SCSI-3 has been standardized," but I haven't seen anything firm about it. I've seen controllers advertised by JDR Microdevices and some cheap clones; the Quantum "Empire" drives are also advertised as SCSI-3 by some mail order vendors. Seagate and IBM call their fastest drives (probably comparable in speed to the Quantums, if not faster) "Wide SCSI-2."

That's a misnomer. See below.

What is the difference between SCSI-3 and Fast & Wide SCSI-2?

Wide SCSI-2 required two cables to do 16 bit wide transfers. SCSI-3 defined a single cable, single REQ/ACK 16 bit, WIDE transfer. The reason you are hearing 16-bit single cable being called SCSI-3 is that they CAN. The fact that single cable 16-bit has been around for a while just shows you how much the standardization process lags behind the real world.

SCSI-3 is really a family of standards. SCSI was broken up from a single document into different layers and command sets. This was done to allow for different physical transport layers (like fibre channel and SSA) to be defined, and to allow for smaller "bite-sized" projects that maybe get done a little faster ;-)

The family includes the following members with TLAs:

- SCSI-3 Parallel Interface (SPI): Defines the mechanical, timing, phases, and electrical parameters of the parallel cable we all know and love. Some of the electrical and cable parameters are tightened/improved over SCSI-2.

- SCSI-3 Interlock Protocol (SIP): Defines the messages and how the phases are invoked. No real change from SCSI-2, except for some new messages.

- SCSI-3 Architectural Model (SAM): In a nutshell, defines a common set of functions and services and definitions for how a physical transport properly gets commands, data, and status exchanged between two devices, complete with error handling and queueing.

- SCSI-3 Primary Commands (SPC): All of the commands executed by any and all SCSI devices, like REQUEST SENSE and INQUIRY, etc.

- SCSI-3 Block Commands (SBC): Disk commands.

- SCSI-3 Stream Commands (SBC): Tape commands.

- SCSI-3 Controller Commands (SCC): RAID box commands.

- SCSI-3 Multimedia Commands (MMC): For CDROMS etc.

- SCSI-3 Fibre Channel Protocol (FCP): SCSI commands over gigabit Fibre Channel.

- SCSI-3 Serial Bus Protocol (SBP): SCSI commands over IEEE 1394 High Speed Serial Bus (Apple's "Firewire").

- SCSI-3 Serial Storage Protocol (SSP): SCSI commands over SSA.


Table of Contents

=== QUESTION: After perusing the latest issue of Computer Shopper, I came away with the impression that companies are calling F&W SCSI-2 HD's SCSI-3. Is this an incorrect assumption, or is F&W SCSI-2 known as SCSI-3?

Is this really mostly marketing hype?

Actually, there is something to that. TECHNICALLY, what is out there is often a hybrid: SCSI-3 "SPI" silicon with some other hodgepodge of SCSI-3 proposals, all mixed in with SCSI-2 stuff.

An earlier posting said that the Quantum Empire ("SCSI-3") drives contain some commands from the SCSI-3 command set, and Adaptec suggested a specific setting on its 2940W controller to work properly with the drive.

I understand there are some drives with proposed SCSI-3 command features. These are mostly in the MODE SELECT and in error codes, as I recall. Perhaps someone who knows more about this could elaborate?

Note also that the major players (like DC Drives) don't have any "SCSI-3" stuff advertised; only JDR and some cheap clones are promoting it.

Besides, Wide SCSI-2 has yet to really catch on (mostly because only a few drives are fast enough to take advantage of it).

There is no "wide SCSI-2" because that would mean two cables. Single cable wide SCSI has always been SCSI-3, it just took too d*** long to get into a standard! :-)

Table of Contents


ANSWER From: Gary Field ( )

Yes, the asynchronous transfer option waits for each byte to be transferred before it is acknowledged. With synchronous protocol, the device sending the data is allowed to get ahead of the device receiving the data by a number of bytes (called the offset). The offset is negotiated between the initiator and the target some time prior to the transfer beginning. The synchronous protocol is considerably more efficient and therefore faster than asynchronous.

See also: 1, 2, 3

Table of Contents

QUESTION: Is the NCR 53C90 Faster than spec?

ANSWER From: kstewart@ncr-mpd.FtCollins.NCR.COM (Ken Stewart)

I've seen a few comments about our 54C90 being faster than spec. While I doubt the author was really complaining (I got twice as much as I paid for - sure makes me mad ;) I'd like to explain the situation. Along the way, I'll also show that asynchronous is faster on short cables, while synchronous is faster on long cables. The cross-over point occurs somewhere around six feet--assuming that you have our 53C90 family devices at both ends of the cable. The reason has to do with the propagation delay of the cable; the turn around time of the silicon; and the interlocked nature of the asynchronous handshake.

1) We have measured propagation delays from various cables and found an average of 1.7 nanoseconds per foot, which is roughly 5.25 ns per meter. 2) The turn-around time is the amount of time the SCSI chip takes to change an output in response to an input. If REQ is an input then ACK is an output. Or if ACK is an input then REQ is an output. Typical turn-around time for the 53C90 is 40 nanoseconds.

3)The asynchronous transfer uses an interlocked handshake where a device cannot do the next thing until it receives positive acknowledgment that the other device received the last thing.

First REQ goes true /* driven by Target */

then ACK is permitted to go true /* driven by Initiator */

then REQ is permitted to go false

then ACK is permitted to go false

Thus we have four "edges" propagating down the cable plus 4 turn-around delays. Asynchronous transfer requires 55 ns setup and no hold time (paragraph in in SCSI-1 or SCSI-2) which gives an upper speed limit around 18 MB/s. A detailed analysis (assuming 53C90 family) shows that the setup time subtracts out. This is mostly because we are running at one-third the max rate, but also because setup for the next byte can begin anytime after ACK is received true or REQ is received false, depending on who is receiving. You can either take my word for it or draw the waveforms yourself. Thus, the asynchronous transfer reduces to:

(4 * 1.7 * 1) + (4 * 40ns) = 167 ns /* 1 foot cable */

= 6 MB/s

(4 * 5.25 * 6) + (4 * 40ns) = 286 ns /* 6 meter cable */

= 3.5 MB/s

(4 * 5.25 * 25) + (4 * 40ns) = 685 ns /* 25 meter cable */

= 1.5 MB/s

note: cables longer than 6 meters require external differential transceivers which add delay and degrade the performance even more than indicated here.

Our simulations say that under very best conditions (fast silicon, low temperature, high voltage, zero length cable) we can expect more than 8 MB/s asynchronously. In the lab, I routinely measure 5 MB/s on 8 foot cables. So, if you were writing the data manual for this, how would YOU spec it?

The framers of the SCSI spec threw in synchronous mode to boost the performance on long cables. In synchronous mode, the sending device is permitted to send the next byte without receiving acknowledgment that the receiver actually received the last byte. Kind of a ship and pray method.

The acknowledgment is required to come back sometime, but we just don't have to wait for it (handwave the offset stuff and the ending boundary conditions). In this mode any external transceivers add a time shift, but not a delay. So if you negotiate for 5 MB/s, you get 5MB/s regardless how long the cable is and regardless whether you are single-ended or differential. But you can't go faster than 5.5 MB/s, except in SCSI-2.

Synchronous mode does have a hold time (unlike asynch) but again, setup and hold times subtract out. In SCSI-1 synchronous mode, the speed limit comes from the combined ASSERTION PERIOD + NEGATION PERIOD which is 90ns + 90ns = 180ns = 5.5 MB/s. Our 53C90 family doesn't quite hit the max, but we do guarentee 5.0 MB/s. In SCSI-2, anything above 5.0 MB/s is considered to be FAST. Here the maximum transfer rate is explicitly limited to 100 ns or 10MB/s; you don't have to read between the lines to deduce it.

Interesting tid-bit: given a SCSI-2 FAST period of 100 ns and a cable delay of 131 ns on a 25 meter cable, you can actually stack 1.31 bytes in the 8-bit cable. In FAST and WIDE SCSI you can stack 5.24 bytes in this copper FIFO.


Table of Contents


ANSWER From: Gary Field (
Updated: May, 2000

ASPI stands for Advanced SCSI Programming Interface. It was developed by Adaptec (and the "A" originally stood for Adaptec). It is a calling convention and set of commands that can be used to send SCSI commands via any SCSI host adapter that supports it. It is strictly for use with  machines running MSDOS, Windows( 3.1x, 95/98/NT/2000), Netware, or OS/2. There is no UNIX version of ASPI. The error reporting and recovery mechanisms are much more limited than in CAM, but ASPI gained much wider acceptance because it was available earlier.

The ASPI subsystem is divided into layers:

  • ASPI Manager Driver (the layer closest to the host adapter)
  • ASPI peripheral Driver (the code which knows the details of the SCSI peripheral, e.g. DISK, CD-ROM, scanner etc)
  • ASPI applications (utilities like SCSITool, or CD Recording packages)
ASPI peripheral drivers are fairly portable and may handle many different drives of the same type. These would be supplied by the peripheral manufacturers, or in some cases in a general purpose utility package like Adaptec EZ-SCSI.
ASPI manager drivers are available from the manufacturers of most SCSI host adapters.

Table of Contents


ANSWER From: (Clifton Jones)

CAM stands for Common Access Method.

It is an ANSI standard to make it easier to program SCSI applications by encapsulating the SCSI functions into a standardized calling convention.

[Editor(GF): It is similar to ASPI but much more elaborate and complete].

ANSWER From: (Hale Landis)


You can get the CAM spec(s) from the ANSI T10 Web site

Table of Contents

QUESTION: What is FPT (Termination)?

ANSWER From: (John Vincent)

FPT stands for Forced Perfect Termination. FPT is actually really simple, I wish I had thought of it. What it does is use diode clamps to eliminate over and undershoot. The "trick" is that instead of clamping to +5 and GND they clamp to the output of two regulated voltages. This allows the clamping diodes to turn on earlier and is therefore better at eliminating overshoot and undershoot. The block diagram for a FPTed signal is below. The resistor value is probably in the 110 Ohm range. The actual output voltages of the regulators may not be exaclty as I have shown them but ideally they are matched to the diode characteristics so that conduction occurs when the signal voltage is greater than 3.0 V or less than 0.2 V.

     +-----------*--- TERMPWR 
     |           |
 ____|___        |
|        |       |
| Vreg 1 |---------------------------------* 2.8 V
|________|       |                         |
     |           |                         |
   -----         |                         |
    ---          |                         \
     -           |                         / term resistor
                 |                         \ (110 Ohms)
                 |                         /
             ____|___                      |
            |        |                     |
            | Vreg 2 |-*--------* 2.4 V    |
            |________| |        |          |
                       |      --+--        |
                       |       / \         |
                +------+      /___\        |
                |               |          |
                |               |          |            terminated
                |               *----------*------------- signal
                |               |
                |               |
                |             --+-- 
                |              / \
                |             /___\ Both diodes are fast silicon
                |               | switching diodes (.6 V drop)
             ___|____           |
            |        |          | 

            | Vreg 3 |----------* 0.8 Volts
The diagram shows the circuit for terminating one signal. In a complete FPT there would be 36 diodes and 18 110 Ohm resistors plus the regulator chips.

Using the values shown, transients would be clamped at 0.2V and 3.0V.


Some errors in the above diagram were corrected as suggested by

Wietze van Winden (

Table of Contents

QUESTION: What is Active Termination?

ANSWER From: (Eric Smith)

and (Brent R. Largent)

An active terminator actually has one or more voltage regulators to produce the termination voltage, rather than using resistor voltage dividers.

This is a passive terminator:

TERMPWR ------/\/\/\/------+------/\/\/\/----- GND
                         SCSI signal
Notice that the termination voltage varies with the voltage on the TERMPWR line. One voltage divider (two resistors) is used for each SCSI signal.

An active terminator looks more like this (supply filter caps omitted):

            2.85 Volt Regulator
             +-----------+ +2.85V     110 Ohms
TERMPWR -----| in    out |------+------/\/\/\/-------SCSI signal
             |    gnd    |      |
             +-----------+      |
                   |            +------/\/\/\/-------SCSI signal
                   |            |
GND ---------------+            |
                                +------/\/\/\/-------SCSI signal
Assuming that the TERMPWR voltage doesn't drop below the desired termination voltage (plus the regulator's minimum drop), the SCSI signals will always be terminated to the correct voltage level.

Several vendors have started making SCSI active terminator chips, which contain the regulator and the resistors including Dallas Semiconductor, Unitrode Integrated Circuits and Motorola.

[Editor(GF): Another nice feature of active termination is that it can be disabled by a single jumper instead of needing to unplug resistor arrays.]

Table of Contents

QUESTION: Why Is Active Termination Better?

ANSWER (Brent R. Largent)

Typical passive terminators (resistors) allow signals to fluctuate directly in relation to the TERM Power Voltage. Usually terminating resistors will suffice over short distances, like 2-3 feet, but for longer distances active termination is a real advantage.

Active termination provide the following advantages:

- Helps reduce noise.

- A logic bit can be used to effectively disconnect the termination.

- Regulated termination voltage.

- SCSI-2 spec. recommends active termination on both ends of the scsi bus.

- Improved resistance tolerances (from 1% to about 3%)


- Reduces current drawn from TERMPWR line.

In FPT form:

- Provides signal overshoot/undershoot clamping on all signal lines. ]

Table of Contents

QUESTION:How can I tell whether an unmarked terminator is active or passive?

ANSWER From: Gary Field (

If you have an Ohm-meter of one kind or another, measure the resistance from the TERMPWR pin to an adjacent GROUND pin. Reverse the probes and take another reading.

If the reading is about 30.5 Ohms, with the probes both ways, you have a passive single-ended terminator.

If the reading is about 45 Ohms, with the probes both ways, you have a passive differential terminator.

Active terminators should read much higher and give very different readings with the probes interchanged.

Another method: (Suggested by Rico Tudor)

If you measure the resistance from one signal pin to another, a reading of 264 Ohms indicates a passive terminator.
A reading of 220 Ohms indicates an active terminator. This will be true for either 50 or 68 pin terminators.

If you measure the resistance from a signal pin to a GROUND pin on a 50 pin terminator, a reading of 143 Ohms indicates a passive terminator. The value would be closer to 139 Ohms on a 68 pin passive terminator. A reading of greater than 400 Ohms indicates an active terminator.

For those geeks who must know how these values were arrived at:

For passive terminators
Signal to Signal formula:     1 /  ((220 + 220) * (330 + 330) ) = 264 Ohms

Signal to Ground formula:   1 / ( (1/330) + ( 1/(220+((1/(N-1)) * 550) ) ) )= 153 Ohms (for N=18) or 145 (for N=27)

For active terminators
Signal to Signal formula:    110 + 110 = 220 Ohms
Signal to Ground formula:  It is not practical to guess what the output circuit of a 2.85 Volt regulator might look like and attempt to derive an exact formula for it. Actual measurements indicate that it will generally be high resistance.

Table of Contents

QUESTION: Where can I buy terminators ?

ANSWER From: Rodney Brown (

Info taken from Usenet postings by:

John Zatler (

Steve Schreppler (

Dave Nadler (

DataMate / Methode

Methode Electronics, Inc.

dataMate Division

7444 West Wilson Avenue

Chicago, IL 60656

(708) 867-9600

(800) 323-6858

(708) 867-3149 FAX


Brief description of terminators available.

Passive, Active, SLICK (Elaboration of FPT)) in:

Centronics 50 pin (SCSI-1) DM8[05]0-09-[0RS]

Male 3 row D-Sub (Old Sun) DM950-??-?

Male 50 position .050" Centres (SCSI-2 HD) DM20[05]0-02-[0RS]

Male 68 position .050" Centres (SCSI-3 P cable) DM2050-02-68[RS]

Male & Female for ribbon cables DM1050-02-[0RS] (M),

DM650-06-[0RS] (F)

Male/Female for pass through between device and ribbon cable DM550-06-[0RS]

Newark Electronics stocks the DataMate product line.

Newark Electronics (International orders)

4801 N. Ravenswood Ave. 500 N. Pulaski St.

Chicago IL 60640-4496 Chicago IL 60624-1019

(312)-784-5100, (FAX (312)-638-7652, TLX 6718690 NEWARK U).


Selectronix Ltd

Minerva House, Calleva Park,

Aldermaston, Reading, RG7 8NE, UK

Tel: +44 (0)118 9817387

Fax: +44 (0)118 9817608


Technical Cable Concepts Inc.


Table of Contents

QUESTION: What is Plug and Play SCSI?

ANSWER From: (Lee Fisher)

Date: August 1998

Plug and Play is the technology that supports automatic configuration of PC hardware and attached devices.

For more info see:

Table of Contents

QUESTION:Where can I get drivers (ASPI and other) for the WD7000 FASST2 host adapter?

ANSWER From: Gary Field (

(Update - March 2000)
The following information is outdated but I'm leaving it here for now just for historical purposes. My current opinion is that these cards are totally obsolete and should be replaced. The only support for these cards I know of is under Linux and FreeBSD.

Western Digital stopped producing WD7000 FASST2 cards some time in 1990. Future Domain bought the rights to produce them. Future Domain was later bought out by Adaptec and the boards are no longer produced. Columbia Data Products Inc. of Altamonte Springs, Florida still provides driver support for the card. Their SST IV driver package provides support for many types of SCSI devices including disks, tapes, and CDROM. Also included in this package is an ASPI manager driver (equivalent to the Adaptec ASPI4DOS.SYS). I have personally tested this ASPI manager and it works with GNU tar w/ASPI and the Corel CDROM driver, so most other ASPI stuff should work too. Versions of SSTASPI.SYS prior to Oct 1993 do NOT work with the above mentioned programs so be sure to check the file date. There are other useful programs in the package as well. For instance I find the TAPEUTIL program very handy for duplicating tapes. The price of this package is $99 or $85 as an upgrade of a previous version.

A pre-requisite to run this software is that the adapter card must have a BIOS ROM version of 3.36 or newer. I don't think cards manufactured before 1989 or so are compatible.

Columbia Data Products Inc.

1070 B Rainer Dr

Altamonte Springs, FL 32714 (407) 869-6700 (main number)

(407) 862-4725 (fax) (Inactive) (Columbia e-mail)

[Update to above information 1/20/97][Editor(GF)]

From: "Alan L. Welsh" <>

Subject: Western Digital 7000-Fasst SCSI Cards and CDP's SST software

Alan L. Welsh, President

Columbia Data Products, Inc.

We don't usually recommend that users purchase the upgrade for the 7000 software today. Development has ceased, Windows 95 is not supported except in DOS mode, and today I would rather recommend a popular currently manufactured Local-bus SCSI board and not an ISA 7000 board. However, there are still some companies that we do support that have standardized on 7000s and need to keep them in service for years to come. So please buy the software, sell the board, use it as-is, or buy a new board. (Inactive)



Starting in early 1987, Western Digital (WD) manufactured virtually all of the 100,000+ 7000 SCSI boards, except for a few hundred that were made by Future Domain. The first few thousand, known as 7000-ASC boards went out with no software and only a ROMBIOS that was actually written by John Sponger of WD. In the summer of 1987, Columbia Data Products (CDP) completed and shipped its first ROMBIOS for the card that enabled it to boot and operate in DOS. At that same time, CDP also completed a DOS ram-resident driver, so that DOS would recognize and operate the card without the slowness of the ROMBIOS, a DASD driver so that DOS could access additional drive letters, and to break the (then) 32 meg barrier, and partitioning software to perform the FDISK function for SCSI.

It was CDP's goal at that time to develop and provide SCSI software that would enable: any SCSI host adapter, to run any SCSI peripheral, on any operating system, in any PC-based bus. Since at that time WD had 80% of the hard drive controller market, CDP chose WD as the most logical choice to strategically market with, and so CDP supported their cards almost exclusively. During that following year, CDP continued to develop the software for the 7000 host adapters, enabling it to run faster than any other board of its time, including Adaptec's new 1540, whose hardware was actually faster.

In the fall of 1988, CDP exclusively licensed its SCSI software suite, called SST to WD. The WD 7000-asc SCSI host adapter was renamed 7000-FASST. WD was the first OEM to ship software with all SCSI boards distributed as part of the package. CDP's SST software was well received, even though SCSI was still a relatively small market. CDP was paid a royalty for each card shipped and CDP provided complete software support and limited hardware support throughout the world.

By 1991 CDP had developed support for all SCSI peripherals known, all PC operating systems such as Unix, Xenix, Windows, Dos, Netware, and even AIX, although never officially released, and a SCSI toolkit utility package.

All of the 7000-FASST's shipped had multiboot capability that allowed all of these operating systems to simultaneously coexist on a single hard drive so that one OS can be selectively booted each session.

CDP's exclusive was ending with WD, and CDP was porting the software to 25 of the most popular SCSI host adapters. Unfortunately, most of software had to be re-architected and rewritten to embrace not only all the new adapters but also the new SCSI software standards such as CAM, LADDR, ASPI, INT-4b, as well as CDP's own standard since 1987, SDLP. During the next few years WD was losing a considerable amount of money and sold many of their product lines, which included selling the SCSI board business to Future Domain. Future Domain did very little sales of the 7000 as they had competing product lines and didn't understand the value of a bus mastering SCSI board. (Bus mastering gives the card the ability to move data to and from the card and system memory directly without the CPU's involvement, making it as fast as the peripherals driving it, even on an old slow 80286!) The bus mastering 1542 product line from Adaptec is still being produced today, very popular, and is based on the same basic design as the 7000. From a pricing standpoint, the prices for this class of product has declined less than 50% in ten years. This is only amazing if you compare the price of 1MB of memory at $300 in 1987 to that of today.

CDP has continued to develop and support for the 7000-FASST continuously, even though the board hasn't been manufactured for quite a number of years.

Our last major revision of our SST-IV software was done in late 1993, although there have been some minor revisions since then. To enable CDP to continue to develop software and support the board, CDP has been selling upgrades to the large installed user base for years. Without this revenue, development and support would have ceased long ago. There are no plans to continue development at this time, as SCSI is moving from the ISA bus to Local Bus. Although Window-95 development and support was considered, the potential upgrade business wouldn't have covered the cost of development.

In 1994 CDP entered the server backup software market, shipping the first version of Snapback in March of that year. Many of our customers for years had been begging us to write our own backup software and were complaining that "restoring" their servers sometimes took days with the current backup products. For SCSI software development purposes only, CDP had been backing up and restoring hard drives containing multiple operating systems for years. CDP adapted and then rewrote this software in this first release to provide the ability to backup and restore any hard drive that contained any operating system, from DOS. CDP later wrote a device driver in Netware, that could make the backup tape look, act and perform like a hard drive from a Netware workstation. This enabled direct file retrieval and use through Netware from the backup tape, making it appear to a workstation to be just another drive letter. Since all the directories and FATs are cached, the tape is almost as fast as a hard drive. Another feature, resize, allows a Netware server's hard drive to be replaced with a larger one in an hour instead of a day's labor.

At fall COMDEX 1996, CDP released its latest version, Snapback Live! That backs up a live image of a Netware file server's hard drive, capturing all open files in the process, without impacting system performance. Watch your Computer magazine for Snapback reviews in 1997, as well as a version for NT. Innovating backup software has now become CDP's new life--from an innovative SCSI software company.

For more information, contact us at: (Inactive) OR

Table of Contents

QUESTION: What if I have a SCSI drive larger than a gigabyte (1024MB)?

ANSWER From: Gary Field (

The IBM PC/AT BIOS Int 13h disk interface was specified in about 1986 when a large disk drive was about 60 MB. IBM decided that disks wouldn't have more than 1024 cylinders and only allocated 10 bits for the CYL parameter to the INT 13h interface. By 1989, this was already a problem. When vendors began to support SCSI drives under INT 13h, they needed to come up with a translation algorithm between the CYL, HEAD, SECT parameters of INT 13h and the linear block numbers used by SCSI devices. Various vendors chose to map the two such that each INT 13h "cylinder" contained 1 MB.

In other words they emulated a drive with 32 heads and 63 sectors per track.

At the time, large drives were at about 300 MB, so this worked OK. Once drives larger than 1024 MB arrived, a problem developed. They couldn't provide cylinder values greater than 1023! Changing algorithms became necessary.

This is painful since any disk formatted with the old algorithm can't be read using the new algorithm.

By the way, different vendors chose different mappings, so drives formatted with one adapter can't necessarily be moved to a different one.

Adaptec's newer adapters (e.g. the 154xC and the 154xCF) provide a BIOS control to select the old algorithm or the new one, and they also provide BIOS PROMs for the 154xB that will use the new algorithm.

There is an absolute limit of 16 M sectors which means 8 GB assuming 512 byte sectors. Also DOS (actually the FAT 16 filesystem)  only allows 2 GB per partition.

The day when this presents another problem is not too far away (1995?)

Hopefully, we'll all be running more sophisticated O/Ses that bypass this limitation by then.

Table of Contents

QUESTION: My SCSI bus works, but is not reliable. What should I look at?

ANSWER From: Gary Field (

If you still have problems after you're sure that you have all the ID and termination and cable issues resolved, it's time to dig a little deeper (Voltmeter and Oscilloscope required).

If you get your SCSI bus to the point where it basically works, but it isn't reliable I have found that the gremlin can be the TERMPWR Voltage.

With your system fully powered up, and both terminators attached, measure the TERMPWR Voltage at the far end of your bus. It needs to be between 4.25 and 5.25 Volts. Many vendors start with the system's +5 VDC and add a regular silicon rectifier diode and fuse in series. Silicon rectifiers have an inherent voltage drop of .6 to 1.0 Volts depending on the current through them.

Schottky barrier rectifiers are much better for this application. I always use a 1N5817 myself. If the diode on the host adapter is a 1N400x type, change it to a 1N5817. If you add up the drop across the diode and the fuse and 15 feet of ribbon cable and the connector contact resistances, many times you'll find yourself below 4.0 Volts. When using passive terminators, this can shift the signal threshold and decrease the signal to noise ratio on the bus.

If you aren't able to get relief with these methods, sometimes you can solve the problem by having several devices supply TERMPWR to the bus.

Sometimes the Voltage is high enough, but there is too much noise on the TERMPWR line. This can cause really strange problems! If you can see more than about 200 mV of noise on TERMPWR, add a .1 uF and 10 uF capacitor from TERMPWR to one of the adjacent GROUND lines. You need to have the bus as active as you can get it when measuring the noise. I have actually seen over 1 Volt of noise in some severe cases.

Another way you can help to solve TERMPWR problems is to use active terminators. These don't draw as much current from the TERMPWR source and they also have a built in regulator, which can operate on lower voltage than the standard passive terminators. The regulator also tends to reduce the noise.

Table of Contents

QUESTION: Where can I find information about programming using the ASPI interface from DOS and Windows?

ANSWER From: Gary Field (

The following are the best sources of information I have found:

Table of Contents

QUESTION: What is target mode and what host adapters and drivers support it?

ANSWER From: Gary Field (

Target mode refers to the ability of some SCSI host adapters to respond on the SCSI bus as if they were a SCSI target (device) instead of an initiator (host) which is the usual behavior. Using target mode two hosts can communicate with each other over the SCSI bus at high speeds.

Although most SCSI chips can be used in target mode, not all host adapters support it. Here are some that do:

Adaptec 2940UW (
The documents that describe how target mode works for the 2940/2944 are called:
Designer Book
AIC-7880 SCSI Chip Specification
Adaptec TARGET mode firmware programming guide
(I don't think any of these is available anymore)

NexiTech provides target mode software for the 2940 adapter. (

Qlogic QLA-1040 (

Table of Contents

QUESTION: How do I replace Macintosh internal HD and terminate the SCSI chain properly?

ANSWER From: Jie Yuan PhD (Jie.Yuan@UC.Edu)

The factory installed Macintosh internal HD should be terminated. Make sure the terminator/resitor-package is installed in the drive before using it. Most vendors will install the terminator for you if you tell them it is for use in Macintosh as the system disk. Manufacturers usually have toll free numbers for SCSI termination, ID, and such. If you don't already have the terminator, they may send you one for free. BTW, Macintosh SCSI chain starts at the system disk (ID=0), and ends at the control board (ID=7). ID numbers from 1-6 should be used for any other devices on the chain.

Table of Contents

QUESTION: Will attaching a SCSI-1 device to my SCSI-2 bus hurt its performance?

ANSWER From: Gary Field (

Attaching a SCSI-1 device to a system with a SCSI-2 host adapter and several SCSI-2 devices already attached will not hurt over-all performance significantly unless it doesn't handle disconnect/reconnect well. This assumes that the host adapter keeps track of protocol options separately for each target device. Some people have the idea that attaching a SCSI-1 device to a SCSI-2 bus will cause the entire bus to run at SCSI-1 speeds. This is not true.

Table of Contents

QUESTION: Can I connect a SCSI-3 disk to my SCSI-1 host adapter?

Can I connect a SCSI-2 CDROM to a SCSI-3 host adapter?

Can I connect a Narrow SCSI2 disk to a WIDE SCSI3 host adapter?

ANSWER From: Gary Field (

Questions of this nature really cannot be answered in a useful way. There are so many aspects and options to each of the SCSI standards, you need to be much more specific about what devices and adapters you're interested in connecting. Most of the time the best thing to do is just try it! Most combinations will work, but if you're considering a purchase and looking for a guarantee from "The Net", forget it.

The issue is further complicated by the fact that vendors like to latch onto the latest acronyms before they even know what's involved. For example SCSI3 is not approved yet, but vendors are already saying their devices are SCSI3 compatible. Since there is no standards compliance testing organization, they can pretty much say what they want.

If you buy a high end host adapter (probably called SCSI3 :-) ) from a reputable vendor, and it has enough control over the various options (like synch xfer rate 5,10,20,40 mega xfers/sec and the ability to disable WIDE or FAST/Ultra negotiation), and you carefully think out what devices you connect to it (all WIDE devices nearest the host adapter end of the bus etc.), and you are careful to properly terminate not only both ends, but both halves (upper byte and lower byte) of the bus, and none of the older devices you might already have (like a Panasonic CDROM) do anything stupid (like not handle the WIDE negotiation message without hanging) then it will all work fine. :-)

Even though a host adapter may be called SCSI3 doesn't mean it can enable or disable each optional feature, yet this is vital for supporting older devices.

To make matters worse, you won't know which older devices do some of the stupid things unless you know someone who's been bitten already. Your best bet is to look for good deals on name brand devices and adapters and before you buy, ask in comp.periphs.scsi whether anyone has tried the combination you're considering. It's also important to buy from a well known vendor with reasonable return policies.

If you're looking at buying a Vendorxyz spiffydisk which claims to be SCSI-3 compatible and you have a Seagate ST-01 host adapter and you want to know if anyone else has tried this combination, then that's exactly what you should ask.

In general, most SCSI devices and adapters made less than 4 years apart will probably work together, but without specific information about exactly which devices there's no assurance of it. There's also the potential for poor performance even if it does work.

Table of Contents

QUESTION: Can I connect a WIDE device to my narrow SCSI host adapter?
QUESTION: Can I connect a narrow device to my WIDE SCSI host adapter?

ANSWER FROM: Gary Field (
Updated: June, 2000

Yes, you just need an appropriate adapter. Most WIDE devices use the 68 pin "P" connector so you need a 68 pin to 50 pin adapter. You do need to make sure that both the upper byte and lower byte of the bus will be properly terminated though. Some adapters provide Hi-9  terminators, others do not. If the wiring adapter is placed right at the SCSI host adapter, you can usually configure the host adapter's on-board terminators to only terminate the high byte. You need to be clear on what type of connectors are present where you want to do the conversion. You also need to plan your bus so that there won't be any narrow cable between any of the WIDE devices.
Certain host adapters with auto-termination make the assumption that when the low byte is terminated the high byte is also. When using WIDE/narrow adapters this assumption is not valid. Another purpose served by the hi-9 terminator is supplying pull-up current to the upper data lines which would otherwise be left floating.

Special note for LVD drives:

It is recommended that if you connect a WIDE LVD drive to a narrow bus that you use a 68 to 50 pin adapter which has high byte termination. It may seem that the termination wouldn't be needed in this case because the bus is narrow. However, the drive needs to have those signals "pulled up" (logically negated), to avoid the floating signals from confusing it.

If for some reason you attach a WIDE device to a WIDE host adapter using a narrow cable, you must be sure to disable WIDE negotiation in the host adapter BIOS or the device will hang when it is accessed.

One further caveat is that if narrow devices are attached to a WIDE adapter, the adapter's ID must be between 0 and 7 because narrow devices would not be able to see it if the ID was any higher than 7.

WIDE to NARROW adapters are available from:

 Computer Cable Makers Inc

819 Striker Ave, Suite 6
Sacramento, CA 95834

Technical Cable Concepts

1790 E. McFadden Ave., Unit 103/104
Santa Ana, CA 92705

TEL: (714) 835-1081

FAX: (714) 835-1595

Dalco Electronics

P.O. Box 550

275 South Pioneer Blvd.

Springboro, OH 45066-1180

Warning: Some 68 pin to 50 adapters have TERMPWR wired incorrectly.

It seems that the manufacturers of many of these adapters (even ones with a good reputation) have designed their adapters visually rather than by signal description/function. I say this because I have taken a couple apart and I can see where they went wrong. If you look at the layout of a circuit board which  makes the connections between a HD 68 pin and a HD 50 pin you see a nice symetrical fan-like pattern, and for the most part following this pattern gives you the correct wiring. HOWEVER; there are 3 signals that must NOT follow the obvious pattern or TERMPWR can end up shorted to GROUND. This is NOT a good thing. The pins in question are: 17,18 and 51 on the HD 68 connector.  These are TERMPWR. If you follow the obvious pattern:
 - HD68 pin 17 connects to HD50 pin 12 (which is RESERVED in SCSI-2)
 - HD68 pin 18 connects to HD50 pin 13 (which should be OPEN)
 - HD68 pin 51 connects to HD50 pin 37 (which is RESERVED in SCSI-2)

To make things worse HD50 pins 12 and 37 were originally defined as GROUND
in SCSI-1.
Also, the Pioneer DVD-U02 DVD drive neglected to leave pin 25 (which turns into HD50 pin 13) open. Which also causes the shorted condition.

Unfortunately, most of these adapters are molded in plastic so that you can't easily open it up and cut those connections. In order to fix them you need to break off the pins in question on the HD68 connector.

Table of Contents

QUESTION: How does device ID numbering work with WIDE vs NARROW devices?

ANSWER From: Gary Field (

Narrow SCSI devices can only use IDs 0 through 7. WIDE SCSI devices on a SCSI-3 system with 68 pin P cables, can use IDs 0 through 15. It is generally wise to reserve 0-7 for narrow devices though. SCSI-2 only specified the use of IDs 0-7 even for WIDE devices, but SCSI-3 allows 0-15 for WIDE devices. All devices on one bus must have unique IDs of course.

The arbitration priorities are as follows:


ID 7
ID 0

ID 15
ID 8

ID 23
ID 16

ID 31
ID 24

(I doubt you'll ever see a system using WIDE 32 which is required for use of IDs 16 thru 24)

A WIDE device that is set to ID 10 knows not to respond to selection for ID 2 because the parity bit P1 (for bits 8-15) will not be set by the initiator. During a selection of ID 10, the P parity bit (for bits 0-7) will not be set by the initiator, but the P1 bit will be.

To use both WIDE and narrow devices on the same bus, the host adapter must be set to ID 7 (or less) so that the narrow devices can talk to it.

Table of Contents

QUESTION: What is spindle-sync and why would I want it?

ANSWER From: Roger J. Hamlett (

It fundamentally affects just one aspect of performance, the 'latency'. With a single drive, if you are waiting for a sector to 'arrive' round a track, you have (on average) to wait for approximately one half the rotational time of the drive for it to arrive. So you might arrive at the track just as the sector has gone by, and have to wait one whole rotation at the worst, or the sector might arrive just as you want it, and latency would be zero. This average time, is the minimum latency achievable. There are two methods of reducing this time. The first is to increase the rotational rate of the drive. This is why for certain types of application a 7200RPM drive, will still outperform a 5400RPM drive that has the same data rate off the drive. The other method is to have multiple copies of the required data on unsynchronized drives, and take whichever copy arrives first. This can be done with mirrored drives, and gives a small improvement in the latency time. However the 'down side' of multiple drives comes when we have to wait for all the data parts to arrive. So (for instance) on a striped array, if the drives are synchronized, the latency will remain the same as for the single drives with both data 'parts' arriving together. However, if the drives are unsynchronized, the 'total' latency goes up, to 33% 'worse' than the single drive, as we now have to wait for both parts to arrive. Similar 'extensions' take place with other RAID configurations, unless the drives are synchronized. Basically, in RAID arrays, the drives should be synchronized, _unless_ the total required data can be assembled from a small fraction of the drives.

RAID 1, and RAID 1,0, are the most common configurations where synchronization is NOT advised.

Table of Contents

QUESTION: What are the general steps I need to do to install a SCSI disk to be used with Windows?

ANSWER From: Roberto Waltman (

This description assumes an Adaptec host adapter, but other types should involve about the same procedure.

Let me start from scratch and describe one by one all necessary steps:

  • Prepare a bootable MS-DOS floppy (SYS A: ) containing, in addition to the system files, the FDISK.EXE and FORMAT.COM programs. (Preferably the ones that came with your Win95 distribution). Make sure there isn't anything in the AUTOEXEC.BAT or CONFIG.SYS files that could make trouble later. Better still, delete these two files. Do not insert the floppy yet.
  • Reset your computer and enter the BIOS setup, (not the SCSI setup) and make sure that the "disk type" is set to 'none' or 'not installed' or something similar. Verify that the boot sequence is A: first.
  • Exit and reset
(If your host adapter is new, you can probably skip the next step, but if you want to avoid mysteries later, it's not a bad idea to do it.)
  • Enter the SCSI setup (CTRL-A) and go to the setup menu. Press F6 to restore all the default settings.
  • Exit and reset
  • Enter the SCSI setup (CTRL-A) and go to the utilities menu. Make sure you see your disk in the list of devices, and the name and model look OK.
  • Select the proper disk and run the "Format"
  • Choose "Verify media" to build your confidence that the drive is really working right.
  • If these two steps work ok, your disk and controller are fine and they are communicating correctly. If not, you have a hardware problem. (check cables?, terminators?, TERMPWR?, disk itself?)
(It is not necessary to wait for the verify function to finish, although it is a good idea to do it with a new disk.)
  • Exit and reset.
  • Boot from the floppy this time. While the system is coming up, a message on the screen will show up saying something like "your disk model C: 80H BIOS Installed." This means that the SCSI host adapter recognized the disk, and since there is not an IDE C: disk, it installed the necessary BIOS functions to use the SCSI disk as 'C:' It does NOT mean that the C: drive is ready for DOS/Windows. If you don't get that message check that the SCSI disk is installed as device ID 0. (With newer host adapters you can use IDs other than 0)
  • After getting the A:> prompt, run FDISK. Create a primary DOS partition. (2 GB max except for Win 95 OSR2 w/FAT32). Make that partition active.
  • Exit and reboot from the floppy.
  • At this point you already have a C: drive, but you can not use it because it has no file system. (Typing DIR C:, for example, will produce the error message 'Invalid media type', different from the 'Invalid drive specification' you got before)
  • To make a file system run FORMAT C: /S /U. The /S tells the format program to copy the system files to C: at the end of the formatting. This will make C a bootable disk. (Assuming the partition was made active above )
  • When the FORMAT program ends, you should be able to switch to C:, do a DIR, etc.
  • Remove the floppy, reset and (hopefully) reboot from the hard disk.
Notes for mixing IDE and SCSI disks on the same system?
  • The IDE disk must be defined properly in the BIOS setup ("disk type= number" or "autodetect" instead of "not installed" as above).
  • If you will only boot from the IDE disk, the SCSI disk doesn't need to be made bootable. (Some modern BIOSes let you choose to boot from SCSI even if an IDE disk is installed)
  • The BIOS in the SCSI controller will install a maximum of two disks. If you have an IDE disk installed, the SCSI BIOS will still install the (first) SCSI disk. If you have 2 IDE disks
  • You'll have to install SCSI drivers in the boot disk to access the SCSI disk or disks. If you have a system with 4 SCSI disks (no IDE) the controller's BIOS will install only the first two;
  • Again you'll have to install drivers to access the rest, etc.
Table of Contents
QUESTION: My SCSI CDROM only works when Windows 95 is installed. How can I get Windows 95 installed? Is this a catch 22?

ANSWER From: Gary Field (
Update: May, 2000

Since Windows 95 generally is installed from CDROM media, obviously your system needs to be able to read a CDROM before you can install Windows 95.

This is done by creating a boot diskette containing the necessary drivers to allow the SCSI adapter to talk to the CDROM drive.

For a system with an Adaptec 2940 host adapter this means:

  • Make an MSDOS bootable floppy.
  • Add the drivers you'll need (as mentioned below) and the MSCDEX program to the floppy.
  • Create two files on the floppy as follows:
    MSCDEX /D:MSCD000 /M:12 /L:J
If you have a different host adapter, find out from your manual what the equivalent driver names are for your card.

Once you get the system booted, select drive J (the CDROM), and run SETUP.EXE

You can also use a Windows 98 install floppy if you have one, since this contains drivers for most popular SCSI host adapters and CD-ROMs.

Table of Contents

QUESTION: Under Windows 95 OSR2 I can only see the first 8 GB of my 9 GB disk. What's going on?

ANSWER From: Gary Field (

The retail version of Windows 95 is limited to 2 GB per partition by the use of the FAT16 filesystem. Since you're getting more than 2 GB, you must be using a FAT32 filesystem.

Using FAT32 with drives larger than 8 GB requires a host adapter that supports the "INT 13 extensions". If your host adapter was built before about 1996, you may not have this feature. For example Adaptec 2940W Host adapters did not support this. Even the early 2940UW didn't have it. As of BIOS ver. 1.2x the support is present. Check with your host adapter manufacturer for an updated BIOS.

Table of Contents

QUESTION: Are there any storage related reasons to upgrade to Windows 95 OSR2?

ANSWER From: Gary Field (

Yes, Microsoft has made a few enhancements:

  • FAT-32 filesystem

  • which has the following features:
    • Default cluster size of 4k bytes for drives up to 8 GB.
    • Supports drives up to 2 Terrabytes (2048 GB).
    • Will only install on drives > 512 MB.
    • Can use the "backup" copy of the FAT if needed.
    • Is ONLY accessible from Windows 95 OSR2. (Not supported by Windows NT)
  • CDFS (ISO-9660) enhancements.
  • Drive Power Management.
  • 120 MB floptical support.
  • The mini-port driver for the Adaptec 2940xx (\windows\system\iosubsys\aic78xx.mpd) is updated. In the retail version of Windows 95 there are problems with the Microsoft supplied driver. If the above mentioned file is older than April '96, you need a new one. The updated driver is also available from

Table of Contents

QUESTION: I changed the host adapter in my system and now my disk doesn't work. Why?

ANSWER From: Gary Field (

Each manufacturer chooses their own algorithm for converting cylinder, head and sector to a SCSI logical block number. If you run into this, you need to back up your system to tape or CD-R using the old host adapter, switch host adapters, Low Level Format (LLF)the disk (using the host adapter's BIOS), re-partition (using FDISK), and re-initialize the filesystem (using FORMAT), then restore all the data from the backup media.

Not as easy as you expected huh?

Table of Contents

QUESTION: My drive letters map to different disk partitions than I think they should. What's going on?

ANSWER From: Gary Field (

Caveat: The following description assumes the use of Microsoft's FDISK. The use of tools like Linux fdisk or Partition Magic changes a lot of the rules.)

Microsoft uses the following strange algorithm to map drive letters to disk partitions:

Look at the partition tables on BOTH PHYSICAL DISKS.

First PRIMARY partition becomes Drive C:

If there is a second PRIMARY partition it becomes Drive D:

If there are no more PRIMARY partitions, look for EXTENDED partitions.

The first logical drive in the first EXTENDED partition becomes drive E:, the next logical drive in that EXTENDED partition becomes F: etc.

If there is another EXTENDED partition, the first logical drive in it becomes drive G:, the next logical drive in it becomes drive H: etc.

Then, as device drivers are loaded, any disk drives they support will be assigned consecutive drive letters. CDROMs and other "Network drives" can be assigned specific drive letters if desired, leaving holes in the lettering scheme.

Under MSDOS, Win 3.x and Windows 95 this behavior can't be changed. Under Windows NT all drive letters can be re-mapped to whatever you want.

If you don't want D: on your second disk, don't create a PRIMARY partition on it!

An example of this mapping which often confuses newbies:

INT 13h Drive 0x80:



Logical -> E:

Logical -> F:

Int 13h Drive 0x81:



Logical -> G:

Logical -> H:


Assigned to drive M:

Table of Contents

QUESTION: What benchmark tools are available for SCSI testing and where can I get them?

ANSWER From: Gary Field (

Date: August 1998

Table of Contents

QUESTION: Will USB replace SCSI?

ANSWER From: Gary Field (

Date: Updated November 2000

If we're talking about USB 1.0 or 1.1 devices, then absolutely not! USB 1.0  and 1.1 ports, are designed to connect miscellaneous low speed peripherals external to a PC. It operates at 12 Mbits/sec.. It will handle things like keyboards, mice, modems, PDAs, digital cameras, scanners, printers etc. Some low to medium performance CD recorders also have USB interfaces. It is NOT intended for general purpose mass storage devices and is NOT a serious replacement for IDE or SCSI.
USB 2.0 is just starting to appear in November 2000. USB 2.0 is supposed to be able to run at 480 Mbits/sec. If that pans out, it may be practical to put storage devices on it. Even though that's still not as fast as parallel SCSI's 160 MB/sec (1280 Mbits/sec.) or soon to come 320 MB/sec, it could cut into parallel SCSI's territory, just as IDE has.
I think USB 2.0 would be more of a threat to IEEE-1394 however.
USB and IEEE-1394 both have the advantage that termination is fixed and therefore simple. This can be done because all connections are point to point, not daisy chained like parallel SCSI. This will be attractive to many of the less technical users.
IEEE-1394 devices are getting to be much more available, so it's going to be a race to see whether USB2 or IEEE-1394 garners the biggest share of the low end I/O market.

Table of Contents

QUESTION: Will Firewire (IEEE-1394) replace SCSI?

ANSWER From: Gary Field (

Date: September 1998 (slight changes Nov 2000)

Well, not exacty.

IEEE-1394 is a 100/200/400 Mbit/sec. serial protocol. Firewire is a trademark of Apple Computer for an early version of this protocol. There is a standard (IEEE-1394) that describes the serial interface from a hardware standpoint. There are other standards that describe the software protocol that is used to transfer data over this interface. SCSI-3 has a section (SBP) that describes one protocol.

It is becoming popular for interfacing computers to video cameras for frame grabbing and editing. Even if it starts to catch on as a storage interface, it will still be SCSI. People tend to think of SCSI as meaning only the current parallel SCSI interfaces (Single-ended, Differential, and LVD and the WIDE and NARROW variations of these.) In SCSI-3 all Hell breaks loose in terms of how many options there are (Parallel(SE, HVD, LVD), Firewire, Fibre Channel(copper or fibre), SSA)!

The serial interfaces hold a lot of promise for making the interconnection of storage devices much simpler, and increasing the distance they can be from the host. When the cost of these interfaces comes down, they may well replace the parallel buses we currently use, but it will still be SCSI. I think it's possible that IEEE-1394 storage devices will replace IDE devices in mass market PCs in the future (probably not before 2003 or so though).
Probably the most welcome feature of IEEE-1394 is the fact that termination is fixed and the user doesn't need to think about it.

Table of Contents

QUESTION: What is Low Level Formatting(LLF)?

ANSWER From: Gary Field (

Date: September 1998

Each platter in a disk drive is organized as tracks and sectors. Each sector contains header and trailer information as well as error detection (CRC) data in addition to the actual user data field.

When a disk is manufactured the platters are blank (no sector layout). Before shipping, a special command (usually not documented) is issued to the drive to cause it to lay down the sector headers, blank data fields and good CRC. Also many data patterns may be written to each sector to check for media errors. Any sectors with errors are put into the "manufacturer's defect list" and the drive remembers not to use those sectors in the future. Later, after the drive is shipped, a user may decide to "Low Level Format" the drive if he is having problems, or wants to start with a "clean slate". This is done using the SCSI FORMAT command via a special utility usually supplied by the host adapter manufacturer (usually in the on-board BIOS).

Some side effects of doing a LLF:

  • The logical block size and any other saved parameters that are set using MODE SELECT will remain the same as they were last set (i.e. they will NOT return to default values). This can be over-ridden with command option bits.
  • The data fields of the sectors will be set to the manufacturer's default value (usually 00 but not always).
  • Any bad sectors that were mapped out using RE-ASSIGN BLOCK commands will be available for use again. (i.e. the "grown defect list" is cleared). This can be over-ridden with command option bits.
  • If a power failure occurs while a LLF is in progress, the drive may be left in an unusable state, requiring return to the manufacturer for repair. It is safest to do this on a system with a UPS.
Usually it is a good idea to LLF a drive when it is installed and then verify (read each sector using a special utility) it. From then on you shouldn't need to LLF it again. A good verify utility will offer the option of re-assigning any bad blocks that are found. These will then be placed in the drive's "grown defect list". The only way to recover blocks that are mistakenly added to the grown defect list is to issue FORMAT UNIT to the drive.

LLF is NOT to be confused with running the MSDOS/Windows utilities called FDISK or FORMAT. FDISK causes a "partition table" to be created which logically divides up a disk for use by multiple filesystems and/or Operating Systems. FORMAT causes a FAT16 filesystem to be initialized in an existing partition. FORMAT is equivalent to the UNIX command mkfs (Make filesystem). FORMAT also reads the entire partition and marks any bad sectors found as unusable in the File Allocation Table. This does NOT cause the drive to add them to the drive's "grown defect list", but does prevent DOS/Windows from using them.

Table of Contents

QUESTION:Why doesn't my Ultra WIDE drive perform at 40 MB/sec like the SCSI spec. says?

ANSWER From: Gary Field (

Date: January 1999

I am starting to hear "I see UDMA drives being advertised as 33 or 66 MB/sec, how come my brand new Ultra-WIDE SCSI drive can't even do 20 MB/sec.. SCSI is a bunch of hype". My response is "No, marketing people are a bunch of liars (or idiots, take your pick)!". Don't blindly accept what you see in ads. The UDMA drives won't run at 33 MB/sec. in any real sense either!

There is a common mis-conception that because Fast20 - WIDE is specified at 40 MB/sec., that your drives will benchmark at that speed. People need to understand the difference between "bus bandwidth" and "drive performance".
Bus bandwidth is the maximum speed that data can be moved across the bus.
Drive performance is made up of many parameters; Rotational latency, data clock rate, seek time,
cache efficiency and other factors.
The fastest disk drives (either SCSI or IDE) made in 1998 can only transfer  20 MB/sec. even after they
have found the portion of data they need to send.
This is because the data is only moving under the heads at about 160 Mbits/sec.
As drives start to spin their media faster and the bandwidth of read/write heads gets higher, this number increases. Current drives can spin at 10,000 RPM. The next generation will spin at 14,400 RPM.
This will probably move the maximum transfer rate for a single drive to about 28 MB/sec.

As you can see, there is a considerable difference between the drive speed and the bus speed.
You ask; "Why should I have a bus that's so much faster than the drive?"  The answer is so that you can support multiple drives without slowing any of them down.
In the example we're discussing here, a Fast 20 - WIDE bus can support two 20 MB/sec drives before it becomes the bottleneck. This is why Fast40 - WIDE (Ultra2W or U2W) was developed. Servers and other high performance systems need to handle more than two disks simultaneously. U2W is rated at 80 MB/sec., so it can support four 20 MB/sec. disks before it becomes the bottleneck. Later in 1999, Ultra3, or Fast-80, or a subset called Ultra160/m, host adapters will become available which will allow supporting up to eight 20 MB/sec. drives or five 28 MB/sec drives. At that point the PCI bus becomes the bottleneck.
By the way, you'll need a 66 MHz (or 64 bit) PCI bus to handle Ultra3/Ultra160 host adapters, since 33 MHz, 32 bit PCI  can only transfer 133 MB/sec.

Table of Contents

QUESTION:My system says my new 9.1 GB disk is only 8.5 GB. Did I get cheated?

ANSWER From: Gary Field (

Date: January 1999

Well, not exactly. It's a matter of definitions.
Marketing people like to use the prefix Mega to mean 1,000,000 and Giga to mean 1,000,000,000.
Computer engineers define Mega to mean 1,048,576, and Giga to mean 1,073,741,824.
The reason engineers use these strange looking numbers is because they are powers of 2.
(2 to the 20th and 2 to the 30th)
The ad you read when you bought your disk was written by a marketing person. The
operating system on your computer was written by an engineer. In engineering terms,
your 9,100,000,000 byte disk is about 8.5 GB.

Table of Contents

QUESTION: How much overhead does the SCSI protocol impose?

ANSWER From: Gary Field (

Date: September 1999

At first glance the SCSI protocol may seem rather verbose with 6, 10 or 12 byte commands being sent to read a block of data.  Let's look at a WORST CASE example:
  • Host waits for the bus to be free (800 ns)
  • Host arbitrates for the bus (2400 ns)
  • Host selects target(900 ns)
  • Target enters MESSAGE OUT phase
  • Host sends IDENTIFY message (1 byte @asynch. = 800 ns)
  • Target enters COMMAND phase
  • Host sends 12 byte READ command requesting one 512 byte block (12 bytes @asynch. = 10,000 ns)
  • Target enters MESSAGE IN phase
  • Target sends DISCONNECT message (1 byte @asynch. = 800 ns)
  • Target re-selects host(900 ns)
  • Target sends RESTORE POINTERS message (1 byte @asynch. = 800 ns)
  • Target enters DATA IN phase
  • Target sends 512 bytes of data using U2W synchronous xfer mode(6400 ns)
  • Target enters STATUS phase
  • Target sends STATUS byte (1 byte @asynch. = 800 ns)

Total transfer time = 24.6 us. Of that time, 18.2 us was due to the protocol. This comes out to 74% overhead!
Fortunately, in the real world, multiple blocks are read together, which brings the overhead way down.

If asynch protocol was used for the data phase, this would only be 4.2% overhead! The problem is that the synchronous speeds get faster and faster, but synchronous is not used to transfer the command bytes and many of the delays are set by physical parameters of the bus (length, capacitance etc).

I see a total of 16 extra bytes that were sent in order to read those 512 bytes of data. As a percentage 16/512 is only about 3.1%.

This is for the WORST CASE with the host only asking for 1 block and using a 12 byte command on a bus with only 1 drive active. In a more typical case, the host would ask for many blocks to be read together and use a 10 byte command. More importantly, the bus would have several drives active and the commands and messages would be transferred during the bus idle times while drives were disconnected doing their seeks.

Table of Contents

QUESTION: What is LVD (Low Voltage Differential)?

ANSWER From: Gary Field (

Date: January 1999

Low Voltage Differential is a new hardware bus driver type for SCSI-3. It first becomes important
with Fast-40 (Ultra2) devices. If single ended (S.E.) bus drivers were used with Fast-40, the bus length would
be limited to about .75 meters! Since this was clearly un-acceptable, and the older High Voltage Differential
(HVD) interface adds too much cost to a system, ANSI defined a new form of differential interface
that is less expensive to implement because the bus driver logic dissipates little enough power that
it can be included in LSI chips. They also wanted to make sure they avoided the confusion caused by
HVD (HVD and S.E. devices cannot co-exist on a bus), so they specified that if an LVD device is designed properly,
it can switch to S.E. mode and operate with S.E. devices on the same bus segment.
Another difference worth noting is that LVD devices do not contain on-board terminators. You need
to attach an LVD terminator to the end of the cable instead of using a device to terminate the bus
as was commonly done with S.E. SCSI. The big advantage to LVD is that you can have a high speed
SCSI bus up to 12 meters in length!

Table of Contents

QUESTION: My drive stopped working. Is it dead?

ANSWER From: Gary Field (
Updated: December, 1999

Before you toss that "dead" drive, here are a few things to try:
  • Substitue another power supply. Maybe it's not getting power.
  • Check the cable connection. Try another connector.
  • Re-check your terminators. Have you added a device lately?
  • If you have a VERIFY utility (such as in the Adaptec SCSI Select BIOS), run it. Perhaps only certain blocks have gone bad. The verify utility should be able to re-assign those blocks to good areas of the disk. That still leaves holes in your files where blank sectors have been substituted though. If you have a utility that can tell you what files those block numbers were in, you can just copy over those files. Even if the disk is too damaged to read the disk media successfully, the verify utility may provide some error messages or SENSE DATA that will provide a clue to what's wrong with the drive.
  • I hope it didn't have any important data on it! (You have a backup of it anyway right? Oh too bad...)
  • Try doing a low level format.
  • Is it under warranty? Check the serial number on the mfr web site. If it's under warranty, contact the mfr for an RMA number and send it back. Don't violate the warranty by tinkering any further.
  • No warranty huh? If the drive won't spin up, sometimes it's due to "sticktion". This means that the heads are "stuck" to the media by vacuum.  One method of trying to un-stick the heads is to try twisting the drive in the plane of the disk platters with a quick wrist twisting action. If that still doesn't do it,  as a last resort, giving the drive a sharp rap with a screwdriver handle will sometimes free the heads. At this point it's worth a shot (so to speak). If you do succeed in getting the drive spinning again, do NOT turn it off again until you get the data on it transferred to another drive. The sticktion problem tends to be chronic so plan on replacing the drive as soon as possible.
  • If the manufacturer has firmware for it on their website, try re-flashing the drive's firmware.
  • If you have another identical "dead" drive lying around, try swapping the controller card from it onto the drive in question. Perhaps you can make one good one out of the two bad ones. This only works if the drives really are identical.
If it still isn't working, you have a nice new paper weight! Or, for a more educational experience, you can take it apart and see first hand what those platters look like.

Additional information about what SENSE DATA can tell you (from Folkert Rienstra):
SCSI devices provide information about their condition and any errors that have occurred during the previous command execution.
Devices return this information in response to a REQUEST SENSE command.  Many device drivers simply throw this information away making it difficult to analyze problems that occur during normal operation. If you can reproduce the problem using smarter software that will issue a REQUEST SENSE command and interpret the data for you, a device can often tell you what is wrong with it.

What do you need:
1. A drive exerciser also known as a CSO (Customer Simulated Operations) program that will report the error sense information.
2. A list of SCSI errors comprised of Sense Key, Additional Sense Code (ASC) and Additional Sense Code Qualifier (ASCQ).
2. A program that translates those codes into descriptive information.

In the first category fall codeupdt from IBM and SCSI Workbench from Western Digital. They both contain a drive exerciser.
SCSIBench, the SCSI benchmark program from Adaptec is a little exerciser in it's own right: If the drive is not completely up to scratch it will complain through Sense information.
* There is also SCSITool from Bart Lagerweij which is currently under construction (Apr 2000) that has a lot of sense codes built in.
* And then there is SMARTMon from SANTOOLS (David A. Lethe) that will show you not only translated Sense Error data on Inquiry but also accumulated Sense Log data.
That is, when your drive is "supported" by SMARTMon. SMARTMon does not have a drive exerciser.
(A few words of caution: Caching parameters in Device Properties MAY crash or hang your system. Also, adding alerts to 'Alerts|Maintain rulesets ....  for statistical alerts' can result in a hung system too. David is looking into those at the moment).

In the second category falls SCSIcode from Adaptec. This is a little DOS program (that  can run in a DOS box) that accepts the Sense Key and the Additional Sense Code and translates those into the appropriate error descriptions.

Some URLs:


Table of Contents

QUESTION: My host adapter says it "auto-terminates". How does it do that?

ANSWER From: Gary Field (
Date: May, 1999

That's a very good question! I haven't done a thorough study to determine how they all do it, but the ones I've looked at sense a ground connection on certain pins. This may work in many instances, but it's certainly not foolproof. For example many people connect scanners, or ZIP drives that have (may their little metal souls be damned) 25 pin connectors, to the end of the bus. What happens to all those ground connections now? Who knows! Another problem comes in when 68 pin buses are adapted down to 50 pins. Same problem. Another problem is if the host adapter is in the middle of the bus. It might be able to sense whether there is another terminator, but how can it know if there are too many?
In my opinion, the only reliable ways to sense whether the bus is properly terminated is by looking and checking manually, or with a Time Domain Reflectometer and I don't think they'll be putting those onto SCSI host adapters any time soon (but it's not impossible).
Certain host adapters with auto-termination make the assumption that when the low byte is terminated the high byte is also. This isn't always a good assumption, especially if 68 pin to 50 pin adapters are in use.
The bottom line is that you shouldn't depend on auto-termination to do the right thing. Set the termination manually!

Table of Contents

QUESTION: Can I flash update a new BIOS on my OEM Adaptec host adapter?

ANSWER From: Folkert Rienstra
Date: March, 2000 (updated April, 2000)

 To know that, you have to know the difference between a product made by an OEM and a product intended for delivery to an OEM (vs the retail product).
The first one is the OEM (original equipment manufacturer) himself (e.g. a card maker) supplying a product that incorporates another manufacturer's product (e.g. the chip) and sells it under his own name.
The second one is a product intended for use by an OEM where the OEM (e.g. a PC maker) will use it as a part in his product. Adaptec plays both roles. It is a chip manufacturer and it also is a card manufacturer. To complicate things it manufactures cards under its own brand name, identified as AHA-series adapters to be sold to the public, the so called retail products, and cards under the AIC-xxxx chipname umbrella, these carry different deviceIDs. (I am not sure if these were sold to OEMs or as 'Adaptec on the cheap' retail products).
To complicate things further still, they produce cards under the Adaptec brandname for use by OEMs. Some of these are recognized by the special 'S' BIOS. It is possible that you can buy these as an OEM version, although Adaptec never intended for you to buy it, unless you want yourself to be called an OEM, in which case you would need to deal with Adaptec yourself, not through your retailer (and supply you own tech. support).
And to complicate things still further they also produce cards under the Adaptec brand name that identify under the AIC-xxxx chip name umbrella, although the BIOS may say something different (some 290x and 2930-x cards). I think that those are intended to come bundled with other products.
Don't take the previous information as fact, it is just my observation.

As to warranty and service, I think that Adaptec will offer support only for their retail products, which they may track through the serial number or their TSID number scheme. Of course, they can not prevent you from downloading a BIOS image for your AHA- or ASC-branded, OEM intended, card that is physically indistinguishable from the retail product.
Is there a solution for (made by) OEM cards or inboard SCSI solutions?
Yes there is, but it is not without risk and is not for the faint of heart:
When you have an OEM card or motherboard SCSI solution that has the "different from AHA branded products" PnP deviceID, you may still have a possibility to convert an Adaptec sourced retail card BIOS for use on your card or inboard solution. There are about 5 or 6 places in a ROM image where the PnP deviceID is stored. When a PCI BIOS extension starts, it uses those device IDs to find and check against the card/SCSI chipset. If that ID is not found it does not load. Change the deviceIDs in the ROM image to the ID used for your card and you are on your way.
There is one small problem however: Adaptec BIOSes are compressed BIOSes headed by a decompression routine. You will have to decompress the BIOS before you can edit it, and compress it afterwards, then paste it behind the decompression header again. Your ROM image is now ready. If you have a problem understanding what is written here (it involves the use of LHARC and  a binary editor looking for xx-lh5- and hex IDs) don't even try it.
An example:
You have a AIC-7880 based card identified as "Adaptec AIC-7880 PCI SCSI Controller" the device ID is 8078(h). Look it up in the driver inf file like Windows SCSI.INF or AIC78xx.INF that comes with the drivers. Right behind that you find "Adaptec AHA-2940U/AHA-2940UW PCI SCSI Controller" the device ID is 8178(h). You get the 2940UW bios (the file that ends in .ROM) and you replace all instances of 7881(h) in the BIOS header and the decompressed bios with 7880(h). Reassemble the ROM image and you're done. The BIOS image can now be flashed into the card or Motherboard. For motherboard solutions the decompression header may be absent because it is already part of a compressed motherboard BIOS, and the name of the section may not be obvious (e.g. the SYMBIOS BIOS in an AWARD BIOS goes under the name of PCI32.ROM). There may be other sections that need to stay at their fixed locations,  such as the AWARD decompression BIOS @ 1B000 and the AWARD bootblock BIOS @ 1E000.
* Some of this also applies to the SYMBIOS BIOS included in AWARD sourced BIOSes.

Notes from the [Editor(GF)]:
Some people have told me that if you get your Adaptec card into an unusable state when trying to re-flash the BIOS, you may be able to recover it using an older version of the FLASH.EXE utility. Try the following links to get older versions:

Table of Contents

QUESTION: Although the Adaptec BIOS identifies my drives, I cannot access my drives under DOS. What am I doing wrong?

ANSWER From: Folkert Rienstra
Date: April, 2000

 In order to access your drives under DOS they need to be enabled in the SCSISelect BIOS setup(CTRL-A during boot). The fact that you can see them at startup does not mean they are enabled. The BIOS does a device scan and shows any device it finds, but to be enabled for DOS access, the device needs to be "Included in BIOS Scan".
Devices that are enabled for BIOS control are identified with an additional "- Hard Disk n" at the end of the line.

Table of Contents

QUESTION: Why are my 80MB/s Ultra2/LVD drives only listed as 40MB/s [SE] by my OS?

ANSWER From: Dana Lacoste
Date: April, 2001

Almost all LVD drives are designed to be able to be used on a Single Ended bus if desired. In this case, the drive switches to S.E. mode. The drive determines which type of bus it's on based on the voltage present on the DIFFSENSE signal. If even one S.E. device is present on an otherwise LVD bus segment, all devices will run in S.E. mode. Make sure you have only LVD/SE terminators instead of merely Active/SE terminators  (LVD/SE terminators are also available which will indicate whether they are operating in LVD or SE mode).

Table of Contents

QUESTION: What issues are involved in connecting an Ultra160 (Ultra3) drive to an Ultra2 host adapter?

ANSWER From: Gary Field (
Date: July, 2000

 In general when new SCSI protocols are designed they are compatible with most of the previous protocols. This case is no exception. An Ultra3 drive attached to an Ultra2 host adapter will simply operate in Ultra2 LVD mode. If you connect that same Ultra3 drive to a SCSI-2 host adapter like an Adaptec 1540cf, it will operate in Fast-10 Single Ended mode. Essentially the host adapter and drive will negotiate for the best speed that they both have in common.

Table of Contents

QUESTION: What is meant by PIO, DMA, and Bus Master DMA?

ANSWER From: Gary Field (
Date: July, 2000

 PIO stands for Programmed (or Polled) Input/Output. This means that the CPU is used to read and write data directly to and from the host adapter chips. This is typically only used on low cost host adapter cards because it ties up the CPU during I/O which makes multi-tasking work poorly.
DMA, or Direct Memory Access means exactly that. Special circuitry is included either on the motherboard, or the host adapter card itself,  that can move data between system memory and the host adapter, with the CPU only performing a simple initial setup of the transfer. There are two distinct types of DMA circuitry used in PCs, third-party DMA and Bus Mastering DMA. Third-party DMA uses chips built into all PC/AT compatible motherboards to transfer the data. Bus Mastering DMA uses circuitry on the host adapter card itself to take control of the bus and transfer the data. PCI host adapters all use Bus Mastering DMA. This is the most efficient of the three data transfer methods and consequently also the most expensive. Using DMA frees the CPU from the data movement task and interleaves data transfers into bus cycles that the CPU isn't using. This makes multi-tasking work very smoothly which is what SCSI is all about!
Another term related to DMA is "scatter/gather". This refers to the ability to perform the DMA transfer to/from a collection of separate memory blocks or pages. This is vitally important in virtual memory operating systems which keep virtual memory addresses contiguous but scatter the data throughout physical memory pages. Since DMA uses physical addresses to do the transfer, the DMA chips need to be able to accept a "linked list" of physical addresses and transfer sizes rather than simply one address and size. The term "scatter/gather" refers to what happens to the data during the DMA operation. For READs from the device, the sequence of bytes from the device are "scattered" onto discontiguous physical pages of memory. During WRITEs to the device, data is "gathered" from the various physical pages and marshalled into a sequence of bytes to transfer to the device.

In my opinion, if you're going to bother using SCSI, get the full advantage and buy a PCI Bus Mastering host adapter.

Table of Contents

QUESTION: How Should I Choose a SCSI Host Adapter?

ANSWER From: Gary Field (
Date: July, 2000

 A SCSI Host Bus Adapter (sometimes called an HBA) is the heart of a SCSI subsystem. As the name implies, it connects the system's I/O bus (PCI or ISA) to the SCSI bus. You need to give some thought to what SCSI devices you will be connecting to your system before choosing one. There is a large variety to choose from. They range from really low end PIO ISA cards that are included with SCSI scanners, CD-RW drives, etc, to dual channel, Ultra160, 64 bit Bus Mastering PCI cards costing up to $400 or more.
If you only intend to connect low performance devices such as a scanner to the system, then a low end HBA is probably all you need.
But, if you want to connect SCSI hard disks, you'll probably want to get the best card you can afford. If you will have disks and assorted other slower devices, you may even want to get a dual channel card that will allow you to isolate the slower devices onto their own bus to keep the disks performing optimally.

Using an ISA card in a PCI capable system can cause really poor performance. PCI motherboards often implement the ISA slots in such a way that they are much slower than ISA slots on ISA only motherboards. There is often a temptation to use an old ISA card that came with your scanner, or CD-RW drive. Please resist this urge. These cards rarely have proper drivers available for other devices, plus they can degrade the performance of the entire system which defeats the whole purpose of using SCSI in the first place!
Another factor to consider is the type of support you require. If you really know a lot about installing and configuring PC option cards, you may want to save a few bucks and get an "OEM" card. But, if you aren't a real expert on this, you'd better stick to a "kit" packaged card that comes with manufacturer's technical support and the required cables, drivers etc.
Also, you should decide what operating systems you will be running and make sure they support the specific card you are considering. Also, check the manufacturer's web site to see if they provide driver updates on a regular basis. You will find that having updated drivers readily available makes the difference between enjoying and hating your PC.

To be thorough, you should also read this article about determining the performance that will result from a particular combination of devices. Running through these calculations may cause you to realize that even though the SCSI protocol allows each device to run at its highest speed, having slower devices mixed with your high speed devices on the same bus, can eat up the bandwidth of even a high performance host adapter.

Table of Contents

QUESTION: How can I calculate the performance I'll get with mixed SCSI devices?

ANSWER From: Gary Field (
Date: August, 2000

 When you mix SCSI devices that have different levels of performance on the same bus it can be difficult to guess what the overall performance of the bus will be. You may even find that you run out of bus bandwidth with fewer devices than you expected.

Here are some formulas I worked out, that at least in theory should come close to telling you what speed each device will perform at, and what percentage of the available bus bandwidth you'll be using with all of the specified devices running flat out.

If all the devices on the bus run at the same speed and that speed is also the max. speed of the host adapter, you can easily estimate how it will all work. For example, if you connect three Ultra2WIDE disks that are each specified to deliver 25 MB/sec sustained, to a Ultra2WIDE host adapter card, you can figure that all three drives will be able to run at their maximum performance and most of the host adapter's bandwidth will be utilized (3 * 25 = 75 MB/sec on a 80 MB/sec host adapter which amounts to 93.75% utilization).

When you connect two Ultra drives rated at 15 MB/sec sustained, and a Fast-10, 20x CD-ROM rated at 3 MB/sec sustained, to a Ultra2 host adapter, what will be the performance of that combination?

You might be tempted to say that the two 15 MB/sec disks add up to 30 MB/sec and the CD-ROM's 3 MB/sec will bring it up to 33 MB/sec on the bus, which is less than half of the 80 MB/sec the host adapter is specified for.
However, this would not be even close to reality!
The real answer is that the bus will be 180% utilized, the two disks will each benchmark at around 8.34 MB/sec and the CD-ROM at around 1.67 MB/sec!

Before we can get real answers, we need some real information:

First, you need the device specifications for all of the devices you will have connected. The two parameters you need are:

  • The device's max. burst transfer rate (usually 10, 20, 40, or 80 MB/sec). e.g. a Fast-20 WIDE disk would have a burst rate of 40 MB/sec.
  • The device's max. sustained transfer rate. This is the rate that data actually comes off the medium at. This can be from 150 KB/sec for a 1x CDROM up to about 35 MB/sec for a 15,000 RPM hard disk.
These are usually available from the manufacturer's web site.

Next, we assume that the host adapter is capable of operating at the burst rate of the fastest device that will be attached.

Lastly, we assume that a test utility is being used that causes all attached devices to be accessed continually. This means that the results will be a "worst case" scenario but may actually reflect reality for very heavily loaded servers.

Given the above information, we can estimate the utilization of the bus bandwidth and the speed each device will operate at while running benchmarks. The following formulas ignore protocol overhead however.

My Algorithm:
The way I chose to go about figuring this is to calculate how long it will take each device to transfer "one seconds worth" of data across the SCSI bus. (i.e. The sustained number of Megabytes/sec times the number of microseconds/byte at the burst rate), then, adding this figure for each device on the bus. If the total is greater than 1 second, all the data can't be transferred at full speed. Since there's not enough time to transfer all the data, we need to reduce the time allotted to each device, and see how much data it could transfer in that amount of time.

My formulae:
dev1_secs = (1/dev1_br) * dev1_sr)
dev2_secs = (1/dev2_br) * dev2_sr)
devN_secs = (1/devN_br) * devN_sr)

total_secs =  (dev1_secs + dev2_secs + ... + devN_secs )
Percent_Utilization =  total_secs * 100


  • dev1_br = The first device's burst rate in MB/sec, dev1_sr = The first device's sustained rate in MB/sec
  • dev2_br = The second device's burst rate in MB/sec, dev2_sr = The second device's sustained rate in MB/sec
  • devN_br = The Nth device's burst rate in MB/sec, devN_sr = The Nth device's sustained rate in MB/sec
If Percent_Utilization is less than 100, then each device will run pretty close to it's sustained rate and it's obvious what percentage of bus bandwidth remains for other activity.

If however, Percent_Utilization is greater than 100, we need to do more:

Overutilization = Percent_Utilization - 100
Overtime = Overutilization/100
devN_actual_secs = devN_secs - ((devN_secs / total_secs) * overtime)
Real speed of DevN = devN_actual_secs * devN_br

Note: I make the assumption that each device will pay the overutilization penalty in proportion to how much data it will transfer (i.e. how much it uses the bus). In reality it would depend on other factors too, like what SCSI ID (and therefore what bus priority) each device was at. I haven't done any testing to determine whether this is the best way to divide up the "overtime". Perhaps the faster devices should pay less of a penalty. I would welcome any data on how this works out on a real bus.

An example:
We have three devices attached to an Ultra2 (80 MB/sec) host adapter:

  • An Ultra WIDE hard disk with a 40 MB/sec burst rate, and a 15 MB/sec sustained transfer rate.
  • An Ultra narrow hard disk with a 20 MB/sec burst rate, and a 10 MB/sec sustained transfer rate.
  • An Ultra narrow 28x CD-ROM drive with a 20 MB/sec burst rate, and a 4 MB/sec sustained transfer rate.
dev1_secs = .025 * 15 = .375 secs
dev2_secs = .050 * 10 = .500 secs
dev3_secs = .050 * 4   = .200 secs
total_secs =  (.375 + .500 + .200 )
Percent_Utilization =  total_secs * 100 = 107.5%

Since the bus is greater than 100 % utilized:

Overutilization = 107.5 - 100 = 7.5 % overutilized
Overtime = 7.5/100 = .075 seconds
dev1_actual_secs = .375 - ((.375/1.075) * .075)
Real speed of Device #1 = dev1_actual_secs * dev1_br = 13.95 MB/sec
dev2_actual_secs = .500 - ((.500/1.075) * .075)
Real speed of Device #2 = dev2_actual_secs * dev2_br = 9.3 MB/sec
dev3_actual_secs = .200 - ((.200/1.075) * .075)
Real speed of Device #3 = dev3_actual_secs * dev3_br = 3.73 MB/sec

Table of Contents


ANSWER From: Gary Field (
Date: April 12, 2001

 iSCSI is a new standard for sending SCSI commands over an IP network. It is the networking industry's answer to fibre channel. Storage Area Networks are becoming very important to any company that needs large amounts of reliable storage. Fibre Channel is the defacto  standard protocol for most SANs. Cisco and other networking companies are now pushing iSCSI since they ignored fibre channel for too long.

Table of Contents