COPYRIGHT 1996
Last modified:
This material is presented only as a description of how I go about working on a system. It is not meant to encourage unqualified personnel to attempt computer repair. In any event, you assume all responsibility and liability for injury or damage.
The first thing you need to do is find out what COM ports are already defined on your computer. Turn the machine on and watch for a listing of the system specs, just before DOS boots. If your machine does not have this, and you've got WINDOWS 3.X, use Microsoft Diagnostics. Type "msd" from the DOS prompt to run it. It lists the COM ports, but sequentially - NOT IN THE PROPER ORDER. For instance, if you have COM 1 and COM 4, MSD will show COM 1 and COM 2. You will see COM 4 will be incorrectly identified as COM 2. To be sure, compare the memory addresses listed above.
Part A: Internal Modems
The whole trick to this is that your computer has only four possible COM ports, but IN MOST
CASES they share interrupts 3 and 4.
| COM PORT | MEMORY ADDRESS | INTERRUPT |
|---|---|---|
| COM1 | 03F8 | IRQ: 4 |
| COM2 | 02F8 | IRQ: 3 |
| COM3 | 03E8 | IRQ: 4 |
| COM4 | 02E8 | IRQ: 3 |
You can't have two devices using the same interrupt. Even if it does not cause an immediate and obvious problem (system halt, for instance), interrupt conflict will be a source of possible nagging problems and performance degradation.
More modern internal modems will also allow you to assign interrupts 5 and 7. For most systems this is not much of a help. The sound card Bogarts IRQ 5 and the printer ports grabs IRQ 7. You may be able to reset the sound card, but this is often a real pain. You have to change the jumper on the card. Next change the settings in the config.sys and/or autoexec.bat. Then you've got to redefine the driver in WINDOWS. Then your system might not work, because the new IRQ on the sound card louses something else up! (It's amazing that all PC techs are not heavy drug users!)
Sys Check 2.4, a shareware specification identification program lists all available COM ports, and much, much more.
The simple case is your system is just currently using COM 1. The default setting for an internal modem is COM 2, interrupt 3. Check the jumpers on the modem to make sure of this. With the POWER OFF, remove the case cover from your computer. Remove one of the covers from an unused slot. It's a good idea to position the modem in a "lonely" slot - one as far away from other cards as possible. Do try to avoid putting the modem right next to the video adapter. Fasten all screws. INTERNET HERE I COME!
If the computer already has a COM 2, then you've got to turn it off. If the COM 2 is located on the system board, you might be able to disable it in the CMOS SETUP. If this serial port is on an I/O card (IDE adapter or memory expansion, for example), or an older motherboard, then you must use jumpers or DIP switches to change the settings. I hope you've got the manuals! If you don't, first, WRITE DOWN ALL THE CURRENT SETTINGS. It's very disappointing to start making changes only to have NOTHING work! Next, get your favorite beverage and turn on the radio. You'll have to use trial and error to scope out how to disable COM 2. Happy hunting! ALL CHANGES TO THE JUMPERS OR DIP SWITCHES ARE DONE WITH THE POWER OFF!!!!
Once you've finally got rid of COM 2, just install the modem as described above. If the modem does not work as the second serial port, then use COM 4.
PART B: EXTERNAL MODEMS
This is usually much easier. Just plug it in, like plugging in a toaster! If you've only got one COM
port
on your
system
and you are already using it for you mouse, you are going to have to get a serial card. Be certain that
the card has a 16550A UART. Use MSD or SYSCHK to find out what sort of UARTS are in the
system. If the chips are not 16550s, then you will be better off disabling the current COM ports and
getting a card with the more modern component. If you can swing it, you might want to buy a
communications accelerator card. Without a 16550, a 14.4 modem might not work. A 28.8
probably WILL not work. Accelerator cards are available that allow a large number of modems to be
hooked up to a single PC, as in a BBS application.
If your current COM port uses a 25 pin port, it's OK to use an adapter to make it work with a 9 pin cable
DO make sure that all settings are correct in your communications program.
And, by the way, especially for all you AOL users out there, if your modem settings, internal or external, are by a DIP switch or a jumpers (NOT by software), the settings can't change unless the jumpers or switches are physically monkeyed! For some reason or other, unknown to all thinking people, whenever somebody calls AOL tech "support" with the sob story that they logged on yesterday, but can't get on today, the user is directed to change the jumpers. DON'T DO IT! If your modem worked yesterday, don't "adjust" it today! You may have a wrong setting in software. There might be some problem with the phone lines, or even AOL's system.
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