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Top 10 frequently asked question

COM Port and IRQ Conflicts

There are several steps that you can take to ensure that you are not conflicting with another device or port.

Determine how many serial ports are on the machine without the modem installed. If there are two (2), make note of the communications ports that are taken. If you have both COM1 and COM2 before you install the modem, you will not be able to install your internal modem on any of the standard serial ports. The reason for this is because COM1 and COM2's port addresses are taken. If you install the modem card on those slots it will cause a memory conflict between the existing communications port and the newly installed modem. You are also not able to install the modem on COM3 or COM4 because they share the IRQ (Interrupt Request) levels with COM1 and COM2.

There are two solutions to this problem. The first, which is the more difficult, requires you to have the manual or documentation for your computer or peripheral card in which the existing serial ports reside. These ports can be found directly on the motherboard, on an I/O card, or Super IDE Controller (combination hard drive/floppy drive/serial port/parallel port card). You must move the jumper switches for a communications port not used to the 'DISABLE' position. This will allow you to place the internal modem card on the communications port that you just disabled.

The second method is to move the modem to a communications port address and interrupt level that is not being used by another device or serial port. In order to do this, you must refer to the Quick Start Guide, Page 12. On this page you will find the switch settings for the internal modem. In the above scenario, if you have both COM1 and COM2 present on the system, you can place the switches to either COM3 or COM4. The next step is to determine the IRQ level the modem is able to use. Available interrupt levels are 3, 4, 5 and 7. Again, if COM1 and COM2 exist on the system, IRQ levels 3 and 4 are both being used (even if a device is not currently present on the communications port). If you have a parallel port (printer port), then IRQ7 is being used. The recommended IRQ level in this case would be IRQ5, since most systems do not use IRQ5 by default. Some devices, such as a sound card, network card, second parallel port, CD-ROM interface cards, tape backup interface cards, and SCSI hard drive controller cards can take IRQ5, so if you own one of these cards, check which IRQ level that card is using before moving the modem to IRQ5. If IRQ5 is taken by one of those cards, you can elect to move that card to another IRQ level, and place your modem on IRQ5, or follow the first solution as outlined above in disabling a serial port.

After you have determined where your modem will be placed, the next step is to physically install it into the system. Once installed, you must configure the software to the communications port and IRQ level in which you have placed the modem. Normally there is an advanced installation or port assignment screen to change the address and IRQ level of a serial port. In Windows, it is found in the Main icon, Control Panel, Ports, and then in the 'Advanced' section of the selected communications port. (Note: This change applies to all Windows based telecommunications applications. Once configured here, select the communications port in the application without looking for an IRQ level to change). Refer to your software manual if you are having trouble locating the serial port configuration screen. Below are the default addresses and IRQ levels for COM1-COM4:

COM Port Address Interrupt

COM1 03F8 IRQ4

COM2 02F8 IRQ3

COM3 03E8 IRQ4

COM4 02E8 IRQ3

********************************************************************** AVOIDING INTERRUPT REQUEST AND COMMUNICATION PORT CONFLICTS For IBM PC AT compatible computers **********************************************************************

Each serial device attached to your computer needs its own, unique communications port (COM port) and interrupt request line (IRQ). Your internal modem, a serial device, has been factory set to operate using COM2 and IRQ3. Unfortunately, there may be another device using either COM2 and/or IRQ3. If any other device is using the same COM port or IRQ as your modem, it will cause a conflict, and can result in lock-ups or loss of data.


A serial interface card or external COM port is the most common source of an IRQ or COM conflict.


Most computers come with two external COM ports labeled COM1 and COM2. If you have an external COM2 connector (look on the back of your computer), you will get a conflict from it when trying to install your internal modem using COM2. The conflict occurs even though there may be nothing connected to it. This is because the external connector uses a serial interface card that reserves COM2 and IRQ3 for this external serial connector.


If you do not plan to use the external COM2 connector, most computers will let you disable it (consult your computer's user's guide or its manufacturer's technical support). Your internal modem may then be installed using its factory settings: COM2 and IRQ3.


Sound cards, network cards, SCSI/IDE, and other serial interface cards use COM ports and IRQs. You may have installed such a card in your computer's expansion slots to run an external hard drive or CD ROM or to connect to your network. Some video cards use an IRQ, too.

You need to determine the IRQ and COM port is used by each serial interface card you may have installed. One way to find out is to open your computer, take out the expansion card, and examine its jumper settings (be sure the computer is turned off before removing any expansion cards). Another, simpler way is to use Microsoft's Diagnostics program.


Microsoft Diagnostics is a program included with most versions of MS-DOS that tells you what is installed on each COM port and IRQ. Simply type MSD at the C:\ prompt in DOS (exit Windows) and look under "COM Ports." If COM3 or COM4 is available, it will be labeled "N/A."


Sometimes it is necessary to change the IRQ setting on an expansion card to free an IRQ for the modem. For example, if your sound card is using IRQ5, you can reinstall it using IRQ11, and thus free IRQ5 for use by your modem. Note that most serial devices, including your modem, are incapable of operating at IRQs above 7.


Your modem can be set for IRQ 3, 4, 5 or 7. The IRQs available for your modem to use, however, depends on your computer setup.

Each IRQ on the computer is assigned a "peripheral device" or other function (you can consult your computer documentation to see what these default settings are, or use MSD as explained above). The typical assignments for the first eleven IRQs are listed below:

IRQ ---- Assignment

0 ---- System Timer

1 ---- Keyboard

2 ---- Cascade Input (mandatory function)

3 ---- COM2

4 ---- COM1

5 ---- Parallel Port (LPT2)

6 ---- Disk Controller (all internal drives) floppy

7 ---- Parallel Port 1 (LPT1)

8 ---- Realtime clock interrupt

9 ---- Reserved

10 ---- Reserved

11 ---- Reserved

As you can see from above, there are no default assignments for COM3 and COM4. In order to use COM3 or COM4, you must assign it to an IRQ. This means "sharing" that IRQ with its default assignment. This is possible ONLY if the default assignment is NOT in use. For example, you can assign COM3 to IRQ5 if you are not using LPT2 for a second printer or other external parallel device. You CANNOT assign COM4 to IRQ2, because IRQ2 is being used by the computer for a mandatory function. If you have a mouse attached to COM1, IRQ4 is in use by COM1 and cannot be used for COM3.

If you plan to use COM3 or COM4, you must assign these COM ports to an IRQ through Windows or your DOS communications software. Before running your software, however, you must first re-install your modem with the right IRQ and COM port.


Begin by turning off your computer.

COM port and IRQ settings are changed using switches located on the bracket of the modem. Set the switches according to the Quick Start Guide, Page 12. Turn on your computer.


After you have assigned the right IRQ and COM port settings to your modem, you must change the settings in your software to match.


If your communications program is running under Microsoft's Windows, you must first assign COM3 or COM4 to the IRQ you selected above through the Windows Control Panel. Use the following steps:

1. In the Main menu of Program Manager, open up Control Panel.

2. Double-click on the Ports icon.

3. Use the mouse to select the COM port that you are using for your modem.

4. Click on "Settings. . .", then, "Advanced."

5. Select the IRQ you want to use for your modem.

When you run your communications software, be sure to select the appropriate COM port. Consult your software's documentation.


If you are running a DOS communications program, you must change the IRQ and COM settings in your communications software. Consult your software's documentation.


If you are having trouble assigning IRQs, disabling COM2, or changing settings on your communications software, contact either your computer or software manufacturer's technical support.

If, after reading through this and trying the recommended solutions, you are still unable to isolate or resolve a problem, send me e-mail.

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