By Ray Geroski

For the most part, IRQ conflicts can be easily resolved by assigning values manually to competing devices. This might not always be possible, however. Though Windows does a fairly good job of allocating resources, the hardware often dictates what it can do. An easily overlooked component of installing expansion cards is the IRQ sharing of specific slots on the motherboard. As one user posting in our Technical Q&A forum discovered, installing cards in the wrong slots can make it impossible to use the devices because of the resulting IRQ conflicts. Yes, for many add-in cards, some PCI slots are actually better than others. Many motherboard manufacturers have designated specific slots for specific types of cards.

Information provided by TR members can help you resolve such issues or even avoid them altogether. An important first step is being familiar with the information provided by the motherboard manufacturer.

After installing two PCI cards in a system running Win98SE, Regarchrm discovered that neither device would function properly.

“I recently installed a PCI USB card and have installed a PCI internal US Robotics modem,” wrote Regarchrm. “I have an HP scanner connected to the USB port. I receive [an] error that [the] scanner can’t be initiated and the modem says the port is open and can’t connect.” Upon investigating, Regarchrm found that both devices were using the same IRQ. When Regarchrm attempted to change the settings, the system stated that they could not be altered. Reinstalling the devices also failed to resolve the issue.

“I’ve uninstalled and reinstalled both the modem and USB port several times and still have conflict, although Windows says that there is no conflict,” said Regarchrm. “I believe that I need to change the IRQ setting of either the modem or USB port so that they do not have the same IRQ setting (IRQ10). Windows will not allow me to change the resource settings.”

TR members presented good troubleshooting solutions, including one that specifically addressed the underlying cause in this case.

Fred07, for example, suggested that the IRQs could be assigned in the system BIOS rather than through Windows. “My bios allows for IRQ settings and keeps Win out of it,” wrote Fred07. Though this was not an option for Regarchrm, it’s a good point to consider if IRQ conflicts occur.

The cause of the problem, as D_V Ant pointed out, is likely that specific PCI slots share IRQs. Other slots are designed for use by devices that don’t allow IRQ sharing. “Sometimes,” wrote D_V Ant, “the only way to separate them is to try each card in a different slot and check BIOS to see which IRQ was assigned. Some PCs have one slot that is not shared.”

Regarchrm was likely installing two IRQ-dependent devices in slots that share IRQs. The motherboard user’s manual should specify which slots share IRQs and usually recommends which slots to use for devices such as modems and sound cards. The manual for the Abit KX7-333, for example, warns users that PCI slot 5 shares IRQ signals with other devices and that a device may not function properly in the slot if it can’t share IRQs.

It’s a good idea to consult the motherboard manual before installing add-in cards to avoid such problems. Installing a new card may require that you do some rearranging to ensure that all devices can operate without conflicts.

The feedback of TR members on Regarchrm’s problem illustrates that two options available to resolve conflicts are to see if the system BIOS allows IRQ assignments and to consult the motherboard and device documentation for details about IRQ sharing.