Contents:

• IRQS, I/O Addresses, And COM Ports
• Common Symptoms Of An IRQ Conflict
• I/O And IRQ Conflicts
• Finding A Com Port For The Modem/100 & Modem/300
• Finding A Com Port For The SatisFAXtion Modem
• Deciding Which Port To Use For The Modem

The most common problem users encounter when using a data modem is an IRQ conflict. But just what is an IRQ and why do conflicts arise? If you know the answer, skip to the next section. If you don't, read on.

The microprocessor in your computer works on only one task at a time. When a device (such as your modem) needs the processor to do some work, the device sends an interrupt request (IRQ) to get the processor's attention and an instruction telling the processor what to do. When the processor receives an interrupt request, it puts down the task it was working on, executes the new instruction, then goes back to the original task.

Most add-in devices (including the modems and faxmodems) are very selfish when it comes to their IRQ; they don't like to share. So when you have two devices using the same IRQ, one or the other (or both) of the devices may not operate reliably. The only way to ensure reliable operation of your add-in device is to give the device its own IRQ.

In addition to the IRQ, a data modem also needs a unique I/O address. The I/O address is simply a path for the data to flow in and out of the computer. Having two devices at the same I/O address yields results similar to the IRQ conflict: the device doesn't work reliably.

This brings us to COM ports, which are just specific combinations of IRQs and I/O addresses. The first PCs were given two COM ports: COM1 and COM2. COM1 uses the I/O address 3F8h and IRQ 4. COM2 uses the I/O address 2F8h and IRQ 3. As time went on, the need arose for more COM ports, so COM3 and COM4 were invented. Here's how it looked:

What's wrong with this picture? In the original configuration, COM1 and COM3 use the same IRQ (as do COM2 and COM4). It didn't take long to learn that the original configuration didn't work well in many cases, so some inventive minds created the alternate configuration which solved some problems, but created others.

So what's the bottom line? Well, many of the devices you can install in your computer require an IRQ, but there are very few IRQs to go around. Your task is to configure every device you add to your computer so that it has a unique IRQ and a unique I/O address. The more devices you add, the trickier this job can be.

The best way to avoid a conflict is to know the I/O address and/or IRQ used by every device in your computer before adding a new device. Examine each board, read the manuals, or call your dealer or the manufacturer to determine the current settings for each board. Also determine your options for reconfiguring the boards (if necessary) to avoid annoying conflicts.

Here are some of the more common symptoms you might see if you experience an IRQ conflict between your SatisFAXtion data modem and another device.

• Erratic response to AT commands.
• Modem won't echo commands back to the screen.
• Modem doesn't work.

When two devices share the same I/O address or IRQ (interrupt request), the devices may not work correctly. To eliminate the potential for conflicts, make sure each device you add to your computer occupies a unique I/O address and IRQ.

Standard COM (serial) Port Address/IRQ Settings

Here's an easy method for finding an unoccupied COM port for the faxmodem. Perform these steps BEFORE inserting the faxmodem into the computer. If you've already inserted the faxmodem, remove it before performing these steps.

• Insert the Crosstalk Communicator diskette into drive A (or drive B).
• Type A:\TESTCOM /S (or B:\TESTCOM /S ) and press ENTER.
• Write down the information on your screen. If you have a printer connected to your computer, hold down the SHIFT key and press the key labeled "Prt Sc" or "Print Scrn". This will print out the text displayed on your screen.

Here's an easy method for finding an unoccupied COM port for the data modem on the SatisFAXtion Classic, Upgraded SatisFAXtion Classic, and SatisFAXtion Modem/350.

1. Change to the SatisFAXtion software directory (usually C:\FAX).
2. Type 'SETUP' then press ENTER.
3. Run the SatisFAXtion SETUP program, select "Advanced setup", select "Setup hardware", and make sure the "Modem I/O, interrupt" is set to "off".
4. Exit SETUP, reboot the computer, then type 'TESTCOM /S' and press ENTER.

If TESTCOM reports that only COM1 is active, set the modem to COM2. If TESTCOM reports both COM1 and COM2 active, find out if there are devices using COM1 or COM2 (such as a mouse, printer, etc.). If possible, disable the unused COM port so you can set the modem that port. See the user's guide for your computer to find out how to disable a COM port.

If you're using both COM1 and COM2 or can't disable your COM ports, you may be able to use COM3 or COM4 depending on the devices attached to COM1 and COM2. A mouse and a modem don't usually share an IRQ successfully. A printer and a modem sometimes can successfully share an IRQ.

Intel internal faxmodems offer two additional configurations. If your faxmodem has switches, see the faxmodem Installation Guide for the switch settings for these configurations (these settings are not printed on the board's metal bracket).

COM3 3E8 IRQ 5 (don't use IRQ5 in a PC/XT-compatible [8088/8086-based] system)

COM4 2E8 IRQ 2 (don't use IRQ2 in an AT-compatible [286/386/486/Pentium-based] system)

If you configure your modem for COM3 IRQ5 and plan to use Windows communications software, you must configure the COM3 port in the Windows Control Panel. After opening the Control Panel, select "Ports". Next, select "COM3" then select the "Advanced" button. Change the "Interrupt request line" to 5.

NOTE: Before configuring your modem for COM3/IRQ 5 or COM4/IRQ 2, make sure your communications software supports these ports.

CAUTION: Never use IRQ 5 in 8088/8086-based computers because IRQ 5 is used by your hard drive. Never use IRQ 2 in 286/386/486-based computers because IRQ 2 is used to "cascade" the two interrupt controllers.

This applies to:

Modems and Fax Boards