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All About USB

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A Complete Guide to the Universal Serial Bus

UPDATE: this document applies to USB 1.1 and USB 2.0.
For the latest information, please see USB 3.0 Explained.





WHAT IS USB ?

USB stands for ‘Universal Serial Bus’ and is the next step in creating a computer interface that really works universally. It boasts cross-platform compatibility for Macintosh, Linux/UNIX, and all versions of Windows since 98SE. The USB connector ports are available standard on virtually every computing machine manufactured in the past eight years.

WHY USE USB ?

    As computer and peripheral device manufacturers strive to create standardized products, connectivity becomes easier and easier to manage. Previous interfaces such as serial and parallel connections required a fairly advanced knowledge of configuring DIP switches and IRQ settings, while newer methods like PS2 and ADB were improved but not hassle-free, and nothing compares to the nightmare known as SCSI configuration.

USB is the first cross-platform “hot-swappable” interface- no more operating system incompatibility, no more restarting before unplugging or plugging in, no more mess.

   After USB was accepted as the new standard for computer devices, operating system designers like Microsoft and Macintosh went extra lengths to include drivers for as many devices as they could with their new systems. Thanks to their preparations, many USB devices now don’t even require installation CD’s or driver disks… just plug it in and the OS will find and install the driver for you. Even if the driver is not found on the hard drive, many USB devices also feature self-contained drivers, so they can essentially install themselves.


HOW FAST IS USB ?

USB currently exists in three stages: 1.0, 1.1, and the high-speed 2.0. The original USB rev. 1.0 was relatively slow, pulling in 1.5 Mpbs. Revision 1.1, first widespread USB release, jumped up to a hefty 12 Mbps. Looking around the peripheral market nowadays, you will notice the common advertisement of USB devices and cables as ‘High-Speed USB’, which means that these products meet the specifications of USB 2.0.

USB 2.0, by now the most common interface around, boasts theoretical speeds of up to 480Mbps, surpassing the 400Mbps of USB's rival interface, Apple's FireWire (or iLink). Real-world speed tests, however, still hold that FireWire is the fastest interface, with actual speeds generally running double that of USB 2.0.


WHAT ARE THE LIMITATIONS OF USB ?

Every product has a weakness, and even an interface as praised as USB has its downsides. One such problem is a streamline effect that occurs because of the Plug-N-Play nature of USB: devices such as keyboards and mice require constant interface with the computer, so they need to have their own dedicated USB port. They can usually be streamlined together, as with the Macintosh mice that stream through the keyboards, but they will only cause problems on hubs and other ‘sharing’ type of situations.

This is also noticed, for example, if a scanner and a printer are on a USB hub together. If the computer is sending a print job and the scanner is used, the printer will have to stop and wait for the scanner before it can resume printing. This is a big weakness compared to FireWire, which can daisy-chain devices (even without hubs) in a string up to 64 peripherals long. The easiest and cheapest solution to this problem is, when necessary, the addition of USB ports by additional PCI or PCMCIA cards.

The most aggravating limitation of USB is the length restriction. Because of the nature in which data is carried through the cable, USB has an accepted maximum length of 15 feet. Some devices, depending on power needs and data bandwidth, may be able to go beyond this, but there are no guarantees.

The only practical solution, aside from using a different interface, is the use of repeater extension cables to fill the gaps. These cables have a small booster at the end, which reads and re-sends the signal instead of just passing it along. They can be strung along in a row, and so long as there is never more than 15 feet of cable between boosters, it is possible to run USB up to an extended maximum of 80 feet. This can get expensive, however, and alternatives are most often recommended.


WHAT ARE THE DIFFERENT TYPES OF CONNECTORS ?

USB A to B Cable For the most part, there are two types of connectors on USB cables: a long, flat rectangle for plugging into computers and hubs called 'USB-A', and a smaller, square connector for plugging into devices like external CD and Hard drives, scanners, and printers called 'USB-B'.

Additionally, smaller devices such as cameras, camcorders, PDA's, and cell phones often don't have the space for a full-sized USB-B port, so they instead use a smaller trapezoidal connector called a 'Mini-B'. As if this weren't enough, Sony has also decided to make it's own USB connections, called a USB 4-pin Mini-B, which are two variations of an extremely small squarish connector found on some digital cameras.


Written by Anthony van Winkle for DataPro International Inc.
Unauthorized duplication strictly prohibited.

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