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Windows Port Troubleshooting

Perhaps the most commonly encountered problem with Intel/Windows boxes are the communications ports. CPUs get faster and expand their op codes, channel processors become more speedy and complex, but we are still stuck with the same interrupt and I/O structure that existed in the early 1980s.

Each communication port must have a unique address. The port address is much like a postal mail slot, where each person has their own box number. If two persons would be assigned the same address they would become confused as messages never made it to them and others that did were not intended so. There are plenty of addresses availiable for ports. It is the software that limits the selection to a few predefined addresses. The trick is to choose an address that is not in use by another device.

Unlike port addresses, interrupt request numbers are few and they may sometimes be used by more than one device. Rather than the processor constantly asking each device "is there anything to do?," these special interrupt lines tell the CPU they need attention. Short programs called device drivers are run immediately when these interrupts are asserted. Each program is associated with one of the unique interrupt number. Well written device drivers can co-exist with others allowing more than one device to assert the same number and run all of the programs. In general, it is better to assign each device driver a unique number rather than finding out the hard way that two are not compatible.

Armed now with the two goals, unique port and interrupt numbers, you are now the expert troubleshooter. This document is a review of the tools and settings required to correct and adjust communication port settings. I can't give you an exact formula. Write down all of your discoveries in a well labeled notebook and it will come in handy in the future. Most documentation sets suck, but if you are lucky some of the information will be in your computer manual.

Disclaimer: In using this information you agree not to sue me because you or I or both are stupid. The Images and trademarks shown in this doc are copyright(c)Microsoft people.

Microsoft Diagnostics (Non-Plug and Play Devices)

This tool has some value in determining the nature of your hardware. It can detect hard addressed ports and devices. It will lie if the device is plug and play enabled.

If you cannot find MSD.EXE on your system, download a copy from Microsoft.

Exit windows, and enter the command MSD. If it is not in your path, change to the directory listed above by the find command and re-enter the MSD command.