PC Serial Ports
E-6
Issue 1 January 1997
Workarounds and Solutions to the IRQ Conflict
Problem
It is important to remember that the IRQ conflict is a problem in hardware; it cannot be
resolved in software alone. Consequently, there are only three alternatives for working
around or resolving it. These are described in the following topics.
Workaround 1: Configure your serial devices such that you use only two at any one time,
and those two use serial ports with unique IRQs.
This is the simplest workaround to the IRQ conflict problem, but it does not solve the
underlying conflict. The idea is to assign your peripheral devices to your available serial
ports in such a way as to avoid using any devices simultaneously which might conflict.
For example, if you have a serial mouse on COM1 (IRQ4), a fax/modem card on COM2
(IRQ3), and your connection to the telephone system on COM3 (IRQ4), you cannot
effectively use MERLIN LEGEND Reporter since you need to use your mouse under
Windows while the call collection function is running in the background.
A better arrangement would be to move call collection to COM4 (IRQ3), which could then
be safely used with your mouse on COM1 (IRQ4). In this case, the workaround is to
avoid trying to use MERLIN LEGEND Reporter at the same time you use the fax/modem
on COM2, since the conflict now would be over IRQ3 (COM2 and COM4).
Workaround 2: Replace one or more of your serial peripherals with equivalent devices
which do not require a serial port.
The idea with this approach is to eliminate the conflict by reducing the number of
peripherals in your system which require serial ports. For example, replacing a serial
mouse with a bus mouse (that is, a mouse which requires its own add-in card) would
make another serial port available that could then be used by another device.
Given the scenario described in item (1) above (that is, a mouse on COM1, a fax/modem
card on COM2, and MERLIN LEGEND Reporter’s call collection on COM3), you might
buy a bus mouse and configure it to use, say, IRQ2 or IRQ5. This would then permit you
to move call collection onto COM1 (IRQ4), where it could then be used simultaneously
with both the mouse and the fax/modem.
Likely candidates for conversion from a serial interface to some other interface include
mice (which can be converted to bus mice) and serial printers (which can be converted to
an additional parallel printer port).
Workaround 3: If your serial port hardware permits you to select IRQs other than the
default ones (IRQ3 and IRQ4), make use of one or more unused IRQs in your system to
assign each COM port a unique IRQ.
This solution is generally not possible for built-in serial ports since these are usually “hard
wired” and cannot be changed. Although most add-in cards containing serial ports permit
you to change the IRQs assigned to them, many cards do not let you select IRQs other
than IRQ3 and IRQ4. For example, an internal modem card generally has jumpers or
switches which permit you to administer the serial interface on the card to be COM1,
