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[ The PC Guide | Troubleshooting and Repair Guide | The Troubleshooting Expert | Troubleshooting Specific Components | Troubleshooting Peripheral I/O Ports ]

I have multiple devices using serial ports that are functional but they only work one at a time, not simultaneously

Explanation: You have a resource conflict between multiple serial devices in your system. They work when used one at a time, but there are problems (lockups, slowdowns, errors) when you try to use more than one at once. A common problem is a mouse that doesn't work when the modem is operating and vice-versa.

Diagnosis: This situation is almost always caused by having two devices set up to share an IRQ line; it is a resource conflict. By default, COM1 and COM3 share an IRQ line (IRQ 4) and COM2 and COM4 share one (IRQ 3). This is described in more detail here. If you try to use both devices at once a conflict ensues. Note that some add-in peripherals come with their own virtual "COM port". For example, an internal modem has the equivalent of a COM port built into it, which it uses to communicate with the system.

Recommendation: You should change one of the devices that is conflicting so that it uses a different IRQ setting. Try the following:

  • Change the settings for your modem (or other device) so that it is using a different interrupt request line. For example, while the standard for COM3 is to use IRQ 4, many modems let you change the IRQ to 5 or 2 (which is really 9) using either jumpers or configuration software. See here for more.
  • Remove extraneous COM ports. If you have a device using COM3 that conflicts with COM1, and if the COM2 port built into the motherboard isn't being used, you can disable it and set the COM3 device to COM2 to eliminate the problem.
  • If you are using a serial mouse and your system supports a PS/2-style mouse, change to use it. The PS/2-style mouse uses IRQ 12 and will free up a COM port for use by serial devices.

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