Resolving IRQ and COM Port Conflicts

What are IRQ's and COM Ports?

You can think of the word IRQ as being an abbreviation for Interrupt Request. An IRQ is a request to the processor to get the attention needed to interface an device into the system. The system acknowledges 16 IRQS, numbered 0 through 15. To use certain devices, the system requires an open IRQ. If two devices try to use the same IRQ, the system or one of the devices will likely freeze.

COM Ports (short for Communication Ports) are modular (serial) channels to the system, requiring the use of an IRQ. A COM Port is an extension of an IRQ to a device that is not plugged directly into the bus. Examples of devices that utilize COM ports are mice, modems, scanners, and serial printers. The system will acknowldge up to 4 COM Ports: COM1, COM2, COM3, and COM4. Each COM port, when in use, requires its own IRQ. You don't need to enable any COM ports (technically), but typically systems have 2 in use: mouse (usually on COM1) and modem (Usually on COM2 or COM4). Serial ports end in either 9-pin or 25-pin connectors which can easily be interconverted.

IRQ Settings

Since the number of available IRQs is limited, they become a valuable resource. Here is a list of the IRQs and their uses.

IRQ 0: System timer interrupt from TIMER-0. No user-definable options.
IRQ 1: Keyboard
IRQ 2: Cascade for IRQs 8-15. This is like an overlap IRQ for the higher-numbered IRQs. IRQ 2 is often a link from IRQ 9. For example, a SCSI controller set to IRQ9 would cascade to IRQ 2.
IRQ 3: Available. This IRQ should be used for COM2/COM4. Typically, modems work well on IRQ 3. Two devices can share the same IRQ if they have different COM ports. For example, you can have an internal modem set on COM2/IRQ 3, and an external radio-modem or scanner on COM4/IRQ 3. You will not encounter any IRQ conflict unless you try to operate both devices simultaneously. For this reason, do not share the IRQ you use for the mouse, since the mouse is used so often.
IRQ 4: Available. IRQ 4 usually services COM1/COM3, and provides the standard serial mouse connection. As stated above, it is not a good idea to have the mouse use a shared IRQ.
IRQ 5: Available. This this the most common IRQ for a secondary printer port. This is also the default IRQ for many sound cards, including the SoundBlaster series. If a network card needs a low IRQ, 5 will usually do.
IRQ 6: Floppy Drive controller. No user-definable options.
IRQ 7: Primary parallel (printer) port. It is not a good idea to set use IRQ 7 for anything except parallel.
IRQ 8: Real Time Clock. No user-definable options.
IRQ 9: Available. This IRQ overlaps to 2. IRQ 9 is a good IRQ to use for SCSI host adapters, network cards, and sound cards that can't use IRQ5 for some reason. SCSI cards should use IRQ 9 or 11.
IRQ 10: Available. This is a good IRQ for a network card, since most devices can't use it.
IRQ 11: Available. This is the 2nd choice IRQ for a SCSI controller.
IRQ 12: Available. Similar to 10,11. This is the IRQ used for a Bus Mouse (which comes in handy).
IRQ 13: Math Coprocessor. No user-definable optinos.
IRQ 14: Hard disk controller (IDE). Each IDE channel requires its own IRQ. You can control 2 IDE devices with 1 IRQ.
IRQ 15: Secondary IDE channel. If you disable the secondary channel, this becomes available.

The trick is to balance your IRQs so that the do not overlap. Prioritize you devices by how picky they are. For instance, a COM port will need to use IRQ 3 or 4 (see COM ports below). You can not put a modem on IRQ 5, since it requires a COM port; just as your serial mouse does. Devices that do not do well on high (above 7) IRQs, eg sound cards, should be filled in next. The last things to worry about are devices that have their own ROM to store IRQ and base IO addresses, like network and SCSI cards. Network and SCSI cards will be able to use the high IRQs with no problem. These defaults usually work fine:









Mouse           COM1    IRQ4







Modem           COM2    IRQ3







Sound Card      no COM  IRQ5







Printer         no COM  IRQ7







SCSI Card       no COM  IRQ9







Netwrok Card    no COM  IRQ10







Bus Mouse       no COM  IRQ12







Secondary IDE   no COM  IRQ15







Extra Serial    COM4    IRQ3 (do not use this at the same time as COM2)

This is a very basic list, but it can be helpful. The number of possibilities is countless, so you will need to piece your own puzzle together. It is easier than you think after you try it.

The best advice I can give for avoiding conficts is to disable everything that you do not use. If you use an internal modem, disable COM2 on you controller. If you use a bus mouse, disbale your COM1. If you do not use your 2nd IDE channel, disable it. If you only use a printer on your parallel port, use it in Standard mode, rather than Enhanced. Enhanced Mode will not let you print any faster; it is for parallel devices (ZIP drives, CD-ROMs, LapLink) that use high bandwith. If you are careful, you should not need to disable unused ports, but it will give you more room to maneuver.

COM Port Settings

COM ports are esy now that you understand IRQs. Here are the possible configurations for COM Ports:









COM     Interrupt       Address







---     ---------       -------







COM1    IRQ4            3f8h







COM2    IRQ3            2f8h







COM3    IRQ4            3e8h







COM4    IRQ3            3f8h

Although there are 4 COM ports, there are only 2 IRQs for them. In essence, you can have 4 serial devices attached, but use only 2 at a time. And since your mouse is usually in use, it is not a good idea to use anything that shares an IRQ with the mouse. That only leaves us with three simple configurations:









COM1    IRQ4







COM2    IRQ3







COM4    IRQ3

If you need more devices, you can use a Bus Mouse to free up COM1 / COM3, or buy a Multiport Board which can handle from 2 to 128 ports. Modems will work fine on any COM port. Internal modems are usually defaulted to COM4 to avoid a conflict with the existing controller. This works fine, but you do have an extra COM port enable that is not in use (probably). Most software is defaulted to COM2, so what I usually do with internal modems is to disable the second COM port on the controller and use the modem at COM2. External modems are set by configuring your COM port on the controller. Whatever that is set to, your modem wil use. This is usually defaulted to COM2.

Parallel Port Settings

Parallel port settings are similar to COM port settings. A PC can use up to 3 parallel devices. Here are the default settings:









LPT Port        IRQ     Address







--------        ---     -------







LPT1:            7      278h







LPT2:            5      378h







LPT3:                   3bch

You can share IRQs for parallel ports as well, but once again not simultaneously. Most computers will only need 1 Parallel port.