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[ The PC Guide | Troubleshooting and Repair Guide | The Troubleshooting Expert | Troubleshooting Specific Components | Troubleshooting Modems | Operation and Connection Problems ]

My modem came preconfigured to use COM2, but I have the COM2 serial port built into my PC and they are conflicting

Explanation: You have installed an internal modem, which is trying to use the COM2 serial port. However, the PC itself has a built-in COM2 port and this is conflicting with the internal modem.

Diagnosis: This is a very common problem, because most motherboards today come with the COM2 port on the motherboard enabled, and most modems come preconfigured to use COM2. This may be a throwback to the days before it was common to have two COM ports in active use; I'm not sure.


  • The easiest solution to this is simply to disable the built-in COM2 port in the system BIOS setup and allow the modem to stay on COM2. This solution means you avoid reconfiguring the modem, and you avoid some of the problems associated with trying to use COM3 or COM4. The drawback of course is that you lose the use of COM2, and if you are using a serial mouse then you are out of serial ports. (One suboption here is to make use of a PS/2-style mouse if your system supports one, which does not take up a serial port.)
  • If you want to keep COM2 operational, you need to set the modem up to use a different COM port, typically COM3 or COM4. This will cause other problems however, because COM3 shares an IRQ line with COM1, and COM4 shares one with COM2. Most modems will allow you to choose an alternative IRQ for the COM port. For example, some modems will let you run them on COM3 and IRQ 7. This can be a very good solution (as long as you aren't using IRQ 7 for something else, of course.)

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