Serial RS232 Information & FAQ

DataPro RS232 Serial & Parallel Products

TROUBLE-SHOOTING SERIAL INTERFACES

Due to the diversity of implementations in the RS-232-C standard, there are literally hundreds of existing pinouts and wiring configurations for RS232 serial cables. Fortunately, most cables can be classified as either "straight/synchronous" or "crossover/null-modem", and if the devices are competent in handling signals, the correct type of cable should be suitable in most situations.

However, even with a known-good cable you may encounter problems such as garbage data response or no response at all. Such problems are created by other elements of the installation which need to be checked, and in such situations the below troubleshooting guide may be of use.

Problem: GARBAGE DATA RESPONSE

If this occurs, check the switch settings to ensure a proper match for baud rate, parity, start and stop bits, and data word length. Also be sure both devices use the same protocol e.g., X/ON, X/OFF or READY/BUSY.

NOTE that garbage data response is usually not a cable problem, although it can be. Usually it is caused by a mis-match, i.e., settings on the two devices do not match. You must match up:

Baud rate (speed)
Parity
Start and stop bits
Word size
Protocol (also called "flow control" or "handshake")
1. XON/XOFF (software)
2. RTS/CTS (hardware)
3. RDY/BSY (hardware)
4. ETX/ACK (software)

Here is the only situation in which an improperly built cable can cause garbage data: if the devices have hardware flow control selected, but the flow control line is not connected properly, data would be clean initially, until the input buffer overflows, at which time (many) characters will be lost - you will be able to read the output, but there will be gaps in the data stream.

Any other type of garbage data will NOT be the result of a cable problem.

Problem: NO RESPONSE AT ALL

If this occurs, the following checklist will help in identifying the problem:

Are all cable plugs connected tightly?
Are cables going to the right ports?
Have operating system and software been properly installed?
Have ports been configured properly?
Is AC power on for all devices?
Did you re-power after setting switches and jumpers? If not, turn device power off, wait 30 seconds, and turn it back on.
Have switches and jumpers been set properly?
Are cables wired as shown by the pin-to-pin diagram?
Are there any broken lines? Any extra pins connected?

For printers:

Is paper properly loaded?
Is ribbon (and printwheel) installed?
Is cover closed?
Is the on-line switch set "on-line"?

NOTE: Many printers will refuse to print under one or more of the following conditions:

Cover not closed tightly.
Differences detected between printer settings and computer settings.
Out of paper.

For modems:

Is phone connection achieved and indicated by the Carrier-Detect/Ready light?
Have you pressed several returns to activate remote computer?

For terminals:

Are brightness and contrast turned up?
Is full duplex communications mode set?

Written by DataPro International Inc.
Unauthorized duplication strictly prohibited.