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I.R.Q.'s |
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More than anyone needs to know about them. |
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Plus what to do when you run out of them.
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IRQ3 and IRQ4
By default, IRQ3 and IRQ4 are used for your serial ports; they are also the IRQs most likely to present you with
a conflict. PC architecture allows for up to four serial ports (COM1 through COM4) but provides only two IRQs to
handle them all. By default, IRQ3 is used for COM2 and COM4, and IRQ4 is used for COM1 and COM3.
An IRQ conflict occurs when you attach a third serial device to COM3 or COM4. You'll have to resolve it manually
unless you have a Plug and Play OS that successfully assigns a different IRQ to one of the devices.
On the other hand, if you're not using any serial devices, you can probably free up IRQ3 and IRQ4 for other uses.
Most modern motherboards include onboard serial ports that by default are usually designated COM1 and COM2. IRQ3
and IRQ4 are tied to those ports. To make them available, you'll need to go into your BIOS and disable those ports.
You can probably find those settings in the Integrated Peripherals section of your BIOS Setup routine.
IRQ5
The default assignment for IRQ5 is LPT2, the second parallel port. Most systems don't have an LPT2 installed, so
IRQ5 can be freely used for some other device. You'll commonly find that Plug and Play has assigned IRQ5 to your
sound card or possibly to USB devices.
IRQ6
By default, IRQ6 is used for the Floppy Disk Controller, which is a standard function integrated onto most modern
motherboards. If you're still using a standard floppy disk drive on this machine, you'll need to leave IRQ6 alone,
but there's an important fringe benefit if you decide to upgrade this device.
You can replace a standard floppy disk drive with an LS-120 drive, which is also known as a Super Disk, from Imation.
This floppy replacement has 120-MB capacity, with the additional capability of reading and writing standard 3.5-inch
floppy disks. Most important for this discussion, the LS-120 does not use the Floppy Disk Controller.
LS-120 drives are IDE devices, similar to your hard drives and CD-ROM drives. If you install an internal LS-120
drive, you can then go into your BIOS's Integrated Peripherals section and disable the Onboard Floppy Disk Controller.
That will free up IRQ6 for use by another device.
IRQ7
By default, IRQ7 is used for LPT1, the standard parallel port. If you have a locally attached printer and can't
convert it to a USB connection, you'll probably need to leave this one alone. But there are several ways you may
be able to safely recover IRQ7 for some other device.
Most new printers today allow you to connect them to your computer either through a parallel printer cable or a
USB connection. If you have the latter option, go ahead and buy a USB cable, and you've eliminated the need for
IRQ7. You'll need to reinstall the printer as a USB device, but it's well worth that minimal effort.
If this computer is on a network and has a networked printer available and you don't have or need a local printer,
you can disable the Onboard Parallel Port in the BIOS. That will make IRQ7 available for another device.
Finally, remember that IRQs are needed by input devices to tell the BIOS the device needs attention. Since a printer
is an output device, it usually doesn't need an IRQ anyway. The only time a printer needs an IRQ is to let the
system know of an exceptional condition, such as being out of paper. If you're willing to take a chance on not
knowing about these error conditions, you may disable IRQ7 in your BIOS.
IRQ12
By default, IRQ12 is used for the PS/2 Mouse Port. If you're using a USB mouse, or even a serial mouse, you don't
need to tie up IRQ12. If you want to disable the Onboard PS/2 Mouse Port in your BIOS, though, you may not find
that setting in the same area as the other onboard devices.
On some Award/Phoenix BIOS Setup screens, the PS/2 Mouse selection is found on the BIOS Features screen instead
of Integrated Peripherals. If you see a setting on that screen that saysPS/2 Mouse Function Control, set that to
Disable to free up IRQ12.
Other IRQs you may not need
Your BIOS probably gives you the ability to reclaim two more IRQs that you may not actually be using. These settings
typically are found on the PNP/PCI Configuration screen. These settings let you enable or disable IRQs for VGA
and USB.
Remember, IRQs are used by input devices. Like a printer, a VGA adapter is primarily an output device. Unless your
VGA adapter is used for 3-D applications or other high-end applications, such as a TV tuner or DVD-ROM playback,
it probably doesn't require an IRQ.
An additional configuration issue that would require an IRQ for the VGA adapter would be a system with two video
adapters and multiple monitors. If you have that type of installation, you'll need to leave the VGA IRQ enabled.
Otherwise, try disabling this setting in the BIOS and see if the system still performs normally. You'll usually
find it does just fine without an IRQ.
If your motherboard has an onboard USB controller, as most modern motherboards do, then by default it will be assigned
an IRQ. Although my enthusiasm for USB devices should be fairly obvious by now, you don't need an IRQ for USB if
you don't have any USB devices installed. Disable this setting and reclaim another precious IRQ.
Using more than two serial devices
In some cases, you can successfully use more than two serial devices, in spite of the limitations of IRQ3 and IRQ4.
The key is to configure the devices so that no more than two are in use at the same time. Let's say you have installed
a serial mouse and a modem, and you want to add a serial scanner. At one time, a fairly common scanner was incorporated
into a keyboard, and that scanner attached to a serial port. If you can live with a simple procedural limitation,
you can make this configuration work without causing an IRQ conflict.
First, decide which COM port to use for each device. Second, choose which devices will be used simultaneously.
If you agree not to use the scanner while the modem is in use, those two devices can use the same IRQ but at different
times.
This setup will work if you attach your serial mouse to COM1, which will use IRQ4. COM3 uses IRQ4 by default, so
don't attach any device to COM3. The modem can go on COM2 and use IRQ3, and the scanner can be connected to COM4
and also use IRQ3. Even though COM2 and COM4 use the same IRQ, they use different I/O addresses, so the system
can keep the devices straight. But don't try to use the scanner and modem at the same time.
Using a PCI adapter
If, no matter what you do, all your system's IRQs are assigned to devices, there is one more way to add another
device without causing an IRQ conflict: Install a PCI adapter for the function you need. With the proper motherboard
and operating system, PCI devices can share IRQs.
Any modern (Pentium-class) motherboard will provide this capability, in conjunction with a Plug and Play OS such
as Windows 95/98/Millennium or Windows 2000. You may need to verify several BIOS settings to make it work. Most
importantly, the setting PNP OS Installed must be set to Yes. In addition, some BIOSs require you to specify which
IRQs can be shared by PCI devices. Check your motherboard documentation for all the details that could be involved
with this approach. Having said that, I am pleased to report that I've seen PCs sharing as many as four devices.
Resolving ISA conflicts
The steps listed in this article should resolve most of your IRQ problems. However, if you're still supporting
16-bit ISA cards, you may need more work to get all your devices configured. If you find yourself in this position,
make a list of all the devices that require IRQs. Then, list the range of IRQs each ISA card can support and rank
them in inverse order of flexibility. Don't worry about PCI cards—they will be at the bottom of your list.
Some of the oldest ISA expansion cards, with no plug and play capability, may require specific IRQs in order to
function properly. Some old sound cards fall into this category, and some network interface cards of this generation
have jumpers that allow you to choose between IRQ3 and IRQ10—there are no other options. These cards will be at
the top of your list.
Once you have your list prioritized, install your devices in that order—least flexible legacy ISA cards first,
then PnP ISA cards with the fewest IRQ options, PnP ISA cards with the most IRQ choices, and finally, PCI cards.
By definition, PCI devices are fully plug and play compliant, although they may still have a limited range of IRQs
from which to choose.
Conclusion
By following the suggestions in this article, you should be able to set up even the most difficult device configuration.
In a few more years, with new specifications and interfaces, we won't have to worry about running out of IRQs or
causing IRQ conflicts. In the meantime, these tips should keep you going.