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

I cannot get serial devices to work properly on my system

Explanation: You cannot get serial devices to work properly through your COM ports. This is usually seen as devices that do not register on the system or cannot be seen by the software that uses them.

Diagnosis: There are several common causes of this problem. First is physical misconfiguration, such as loose or incorrectly connected cables. This is usually seen when the PC is first put together. Another common problem is incorrectly configured software that may not be looking for the devices on the ports. Resource conflicts can also cause trouble.


  • First of all, make sure that you are testing the port with a simple device that you know works. For example, a plain serial mouse is the best device to use for testing a COM port since it is easy to know if it is working properly or not. Test the device on another system to rule out the device as a problem.
  • Test the COM1 port first, since it is the least likely one to have a configuration problem or resource conflict. If COM1 is working and the others aren't, this may be due to a resource conflict, although it could just be that the software setup for the other port(s) is incorrect.
  • Check your BIOS settings to ensure that the COM port is enabled. If it is disabled at the BIOS level then of course, it will not work.
  • Check the setup of the software being used to drive the device. Some devices will only work properly if they are set to the correct parameters such as speed (baud rate), stop bits, parity, etc. Use simple software when testing ports, to eliminate complicating factors.
  • Check the external connections to make sure they are not loose. Check the cables running from the port to the motherboard headers to make sure they are tight as well.
  • If you just installed a new system, or especially if you just upgraded your motherboard in an existing system, and you are using an AT or baby AT style motherboard, you should have used the serial port cables that came with the board to connect to the serial connectors on the back of the system case. These cables are not universal. If you upgrade your motherboard and try to save time by leaving the old cables in place and connecting them to the COM1 and COM2 port headers on the motherboard, your ports may not work.
  • There may be a resource conflict with the COM port. In particular, if you are trying to use devices on COM1 and COM3 simultaneously, or COM2 and COM4 simultaneously, and you didn't change the default IRQ settings for COM3 or COM4, then you probably have a conflict because the pairs I just mentioned share an IRQ setting by default. See here for more.
  • In order to avoid resource conflicts, it often makes sense for the resource settings of the COM3 and COM4 ports to be changed from the default standards. However, when you change the IRQ setting for a COM port, you sometimes have to tell your software what you have done, or it will not work properly. Check the settings for your communications software and make sure that there isn't a specific setting for resources like IRQs. If there is, make sure they are set to the correct values for your setup.

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