NWA-PCUG Newsletter Article
Hi Diddley Dee it is USB for me.
by John R. Clark, Webmaster, April 2000
(click to email author)

When I started programming computers there was only one computer class at the university and it had a prerequisite requiring you to be an upper division math major. In those days all work on the computer was done by manufacturing reps who wore ties, white shirts, suits and wingtip shoes. They carried a fitted leather tool case and none else got to peek inside the box. By the time the personal computers arrived, I had enough academic rank to commandeer the lab tech, give him my keys, tell him my office computer needs work and I want it fixed by the time I got back from lecture. With a background like that; dip switches; IRQs, ISA, MCA, and EISA busses, etc are all magic.

However now there is a new way to add new equipment to your computer -- the USB buss. All you have to do is just plug it in. You don't have to open the case, find a slot, fumble fingers around hooking up cables etc. All you do is plug it in and the software does the rest.

This new buss is available on most all new computers and is supported by Windows 98 and some later versions of Windows 95. Besides the ease of hooking up a new device, there are several other advantages.

1. The device is kept external to the computer case and powered by its own power supply. This keeps the heat it generates out of the box and reduces the demands on the power supply of the computer.

2. There are only 4 wires in a Universal Serial Buss (USB) connection; Power, Ground, D+ and D-. Other hookups like serial has 9 wires, parallel has 25, and SCSI has 31 wires, and these devices should all require a complete power down when a connection is made.

3. The bits/second transfer is a bit faster on USB. The speed is signaled by the voltage on the data wires. High on D+ can deliver 12 Mbits/sec while a device that signals high on D- the speed is about 1.5 Mbits/sec. The conventional serial buss speed is about 100 Kbits/sec and a parallel is close to 2.5 Mbits/sec.

There are three types of data transfer:
  • Isochronous or real time -- no interrupts such as a video camera.
  • Interrupt -- such as from a keyboard or a mouse.
  • Bulk -- lots of data no big rush such as a scanner or printer.

    Each device that is plugged in is given an address and about each millisecond the processor inquires via a token it sends out to all devices. If the device with the token address has some information to transfer, it sends or receives it when the processor signals it.

    Up to 127 different devices may be attached to one processor system. However this should be done via powered hubs and not chained directly to the processor. So far all by myself I have hooked up a printer, a zip drive, a photo scanner, a page scanner, and a CDRW without any problems other than finding out my hub has only 3 working ports. I would recommend that all club members consider this type of connection on the next device purchase made.

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