Problem:
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.

If you are certain that:

1. The serial port exists and that you are connected to the correct port on the back of your computer.
2. You are also certain that you have the correct serial communications parameters selected in the program that you are using.
3. 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.

Background:
Most PCs are now shipped with two on-board serial adapters and a modem. Quite often, the modem is configured as COM2, COM3 or COM4 which will normally conflict with one of the on-board serial adapters. COM2 and COM4 normally use the same IRQ (IRQ4) and COM1 and COM3 also use the same IRQ (IRQ3) therefore you cannot use COM1 and COM3 at the same time nor can you use COM2 and COM4 at the same time. Since most non plug & play modems are configured at the factory to use COM2, they will conflict with the COM2 port on most PCs.

Instead of re-configuring the modem to use a non-conflicting IRQ, what many PC manufacturers do to eliminate any conflicts with a modem is to disable either COM1 or COM2 in the BIOS setup for the computer and install the modem to use the serial port that has been disabled. From the outside, the PC appears to have COM1 and COM2 as well as a modem however the disabled COM port does not function. Unfortunately, Windows is unable to detect if a serial port has been disabled in the BIOS setup and it even allows serial communications programs to "open" a disabled serial port without any indication that the port is disabled. What is actually going on is that you are opening the modem and not the serial port. Even if there is no modem installed and a port has been disabled, Windows is still unable to detect that the port is not functioning. When you run a serial communications program and select the disabled port, everything appears to work correctly however no data can be either sent or received through the disabled port.

Solution:
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.