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

I can use COM1 on my system for my mouse without any problems, but when I try to use COM2 I experience errors, lockups or other problems

Explanation: COM1 is the only serial port that is working properly on the system. Other COM ports do not seem to work.

Diagnosis: This is most likely a resource conflict of some sort, especially with a modem. COM1 usually works more often than COM2 because many modems come preconfigured with their own COM2 on board, which usually conflicts with the built-in COM2 on the motherboard unless you remember to disable it. If you are using Plug and Play under Windows 95, PnP may not be doing its job properly.


  • If you have just installed a modem into your system, check to see if it is working. If it isn't, you probably have a resource conflict with COM2. Check to see if the modem is configured to use COM2 by default. If it is, you need to either reconfigure it to use a different COM port (and IRQ channel), or disable COM2 in the PC. Disable COM2 using the correct BIOS setting (for built-in COM ports on the motherboard) or using jumpers (for COM ports on an I/O controller card.)
  • Check the internal and external cables for the COM2 port. It's possible that they are loose or connected incorrectly (although it's unusual for one port to not work for this reason when another does).
  • Check for resource conflicts elsewhere in the system.

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