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COM Port Does Not Exist

Enabling COM Port in BIOS

You try to use a serial port with a standard serial communications program like the Software Wedge for Windows or HyperTerminal and cannot get any data to go in or out the serial port.

Make certain that you have checked the following:

  • The serial port exists and that you are connected to the correct port on the back of your computer.
  • You are also certain that you have the correct serial communications parameters selected in the program that you are using.
  • You know that the cable you are using is the correct one for the device that you are trying to interface.

A good test for all of the above is to try using a different serial port or even a different PC and if everything works on the other port then the problem may be that the serial port has been disabled in the BIOS setup for your computer.

To cure the problem you have the following choices:

  1. You can run the BIOS setup on your PC and re-enable the disabled serial port using its default settings. If you will not be using the modem at the same time as the COM port then this approach will probably work although it is not fully guaranteed. (You still may have a I/O port conflict)

    To run the BIOS setup on most computers, you press a key when the computer is booting up and a setup screen will appear (Usually the DEL key, but most PCs display a message on screen during the boot process - e.g. "Press F1 to enter setup"). The serial port settings are usually located in the section labeled "Integrated Peripherals setup". Do not forget to save the settings when you exit the BIOS setup.

  2. In some computers the BIOS setup will allow you to select the "Port Base Address" and IRQ value for each of the on-board serial ports. The easiest complete solution to the problem is to configure the disabled serial port to use a Port Base Address and IRQ that does not conflict with any of the other serial ports on the PC or the modem. For example if you could configure the port to behave as COM3 (base address 3E8) and use IRQ5, you eliminate all conflicts with the other ports (unless you have some other device installed that uses IRQ5).

  3. If you cannot use either of the above solutions then the only option left would be to enable the serial port in the BIOS setup and then either remove the modem from the system or set the jumpers on the modem to behave as a non-conflicting port (i.e. COM3 using IRQ5, etc.). Most modems that are not plug & play modems have jumpers specifically for this purpose. You may need to contact the manufacturer of your modem or at least refer to the users manual for the modem to configure it correctly for your system. In some cases the instructions for the jumper settings are printed directly on the modem circuit board so you may not even need a manual.


Categories: Troubleshooting

Last Updated: 2011.07.05

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