IRQ Settings & Conflicts

What is an IRQ and why 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. You may need to change your IRQ settings if you are using a system with ISA Devices.

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.
IRQ Settings

Here’s a handy list of Default IRQ Assignments:

IRQ 1 Keyboard. Can never be used
IRQ 2 Points cascades signals to IRQ’s 9-15
IRQ 3 COM port Can be used if not used for COM port
IRQ 4 COM port Can be used if not used for COM port
IRQ 5 Parallel port Can be used if not used for Parallel port
IRQ 6 Floppy disk drive Can never be used
IRQ 7 Parallel port Can be used if not used for Parallel port
IRQ 8 Real Time Clock Can never be used
IRQ 9 Redirect to IRQ 2. Can be used
IRQ 10 Unassigned Can be used
IRQ 11 Unassigned Can be used
IRQ 12 PS/2 Mouse Can be used if no PS/2 mouse
IRQ 13 Math Coprocessor. Can never be used.
IRQ 14 Primary IDE controller Can never be used
IRQ 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:

For older machines:
– 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.
– if using windows you should now check your device in the device manager. depending on the device you can set IRQ settings within the properties of the device. Of course any changes made wont take effect until a system restart.

You’re now an expert in IRQ settings! If conflicts persist, consider contacting an experienced provider of Industrial Computers, as they can properly configure ISA Slot devices into a cost-effective system.

Tagged on: