[ The PC Guide | Troubleshooting and Repair Guide | The
Troubleshooting Expert | Troubleshooting Specific Components
| Troubleshooting the Motherboard and System Devices | System Bus, Resources and Expansion Cards ]
I think I have a resource conflict in my system; what can I do about this?
Explanation: It is suspected that the system may have a resource conflict. This means that two
different devices are both trying to use a system resource like an interrupt request line,
DMA channel or I/O address. The two devices will conflict and cause either one or both to
malfunction. A list of typical
symptoms of resource conflicts can be found here.
Diagnosis: Resource conflicts are one of the most common problems with PCs,
especially with those who upgrade or add equipment to their PCs. Since the ancient
architecture of the PC has resulted in a great variety of internal devices and expansion
cards having to share a limited amount of resources, devices will often "step on each
others' toes". The problem is almost always misconfiguration; in rare cases you will
not be able to use two devices in the same system if they cannot find a way to cooperate
by configuring themselves to use available resources.
- Read this section of the Reference Guide,
which contains much more information on resource conflicts.
- Do not try to "share" resources. Some people will say that this is possible to
do, and technically it is, but it is a headache that is not worth dealing with in my
opinion. Windows 95 is particularly unforgiving about trying to share resources.
- If you suspect a conflict with a specific device, and you are running Windows 95, go
into the Device Manager. Click on the device with the problem (which may show with a
yellow exclamation-mark-in-a-circle next to it) and select "Properties". Click
on the "Resources" tab and the system will often tell you what the conflict is.
- Sometimes, folks think they have a resource conflict because the Windows 95 Device
Manager shows a PCI device on the same IRQ as another device called "IRQ Holder for
PCI Steering". This is in fact not a resource conflict; IRQ steering is a feature of
Windows 95 that is designed to help avoid resource conflicts. If you think you
have a resource conflict you may indeed have one, but this isn't it.
- Use a diagnostic tool such as Norton Diagnostics. There
is a test in this package that looks for the IRQ usage of various devices and will
sometimes highlight conflicts (though it is not perfect by any means).
- Catalog the resource usage of all of the devices in your PC. This is the best way to
determine what resources are being used by what. You may find the device resource summary sheet I
have included here to be helpful with this. If you find any devices that are trying to use the same resources, try to change the
configuration of one of them.
- Check resource-related BIOS settings
to ensure that they are correct.
- Watch out for PCI devices using IRQs. The PCI bus uses its own interrupt scheme
but PCI devices also "map" to regular IRQs when needed. Many PCI video cards,
for example, use an IRQ, typically 9, 10, 11 or 12. Make sure that this does not cause any
conflicts with other devices.
- If you are using IRQ9 for any device, make sure you are not using IRQ2 on any other
device. They are the same
- If you are trying to use the COM1 port and the COM3 port at the same time, or the COM2
port and the COM4 at the same time, you will run into a conflict if you leave these ports
at their default IRQ settings. Each of these
two pairs uses the same IRQ number. To use COM1 with COM3, or COM2 with COM4, you must
change the IRQ number that one of the pair is using so that it does not conflict with the
- If you add a modem to your system, and you have a built in COM2 port (which most do) you
will see a conflict unless you change the modem's settings, because most of them default
to use COM2. If you just change the modem from COM2 to COM4, then the problem above will
result unless you also change the IRQ of the modem to another number.
- If you are using a sound card and a second parallel port, you will probably have a
conflict, because both devices try to use IRQ5 by default. One or the other must be
changed. (Also watch out for
the first parallel port accidentally being set to IRQ5).
- If you are using a secondary IDE controller, then IRQ number 15 is normally used by that
controller and cannot be used by other devices.
- DMA conflicts are commonly caused when enabling ECP parallel ports.
They use a DMA channel while other modes of operation of the parallel port do not.
- If you are using a network card, beware of I/O address conflicts. Many network cards use
a full 32 bytes of I/O address space,
and can conflict with other devices. They also sometimes try to use IRQs that are commonly
used by other system devices such as video cards or hard disk controllers.
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