IRQ & ISA Slots – Solving IRQ Conflicts in Industrial Computers

What is an IRQ and why is does my device have a conflict?
IRQ is an acronym for Interrupt ReQuest and refers to individually numbered channels that are used by devices to when accessing a processor. pussy 888

For instance, pressing a keyboard sends a signal to the processor via an IRQ channel (usually IRQ 1) to let it know that it needs to process the key stroke. When new hardware is configured or installed, IRQ conflicts can occur. For example, an overlap problem occurs if you have your mouse on COM 1 (IRQ4) and a modem on COM 3 (IRQ4) as the processor can’t see the both while assigned to the same IRQ.

Here’s a handy list of Default IRQ Assignments:


  • 1 Keyboard. Can never be used
  • 2 Points to IRQ 815 Can sometimes be used.
  • 3 COM port Can be used if not used for COM port
  • 4 COM port Can be used if not used for COM port
  • 5 Parallel port Can be used if not used Parallel
  • 6 Floppy disk drive Can never be used
  • 7 Parallel port Can be used if not used Parallel
  • 8 Real Time Clock Can never be used
  • 9 Redirect to IRQ 2. Can be used
  • 10 Unassigned Can be used
  • 11 Unassigned Can be used
  • 12 PS/2 Mouse Can be used if no PS/2 mouse
  • 13 Math Coprocessor. Can never be used.
  • 14 Primary IDE controller Can never be used
  • 15 Secondary IDE controller Can be used if no second Hard Drive


If you have an ISA board you must change the IRQ in the BIOS to the ISA/Legacy setting so the PCI bus does not try to use it. It’s a common mistake to use Windows XPs built-in IRQ settings by accessing Resources from the Device Manager via XP’s Control Panel, but this will NOT correctly change the motherboard’s IRQ assignments in the BIOS itself.

Changing IRQ Settings

– Access your computers BIOS (usually requires a reboot, then pressing DEL key or a Function key).
– Look for Plug N Play or PnP Configurations
– Determine if PnP is automatically or manually configured to assign IRQs. Set to Manual.
– You can now see which IRQ setting corresponds to what device and reassign them. Find your conflicting ISA device and assign it to an open IRQ using the listing above.
– Save and exit the BIOS, restart your machine.