Serial Port?

Is there a modem involved here (PCMCIA or on-board)? You may have a conflict with IRQ of COM1 and COM3, which is often the same (IRQ4). This would be the first thing to settle, as the "device" seems to be in working order. Many laptops have a serial port (COM1), an infrared port (COM2) and possibly an built-in modem that occupies COM3.

Check BIOS settings for COM ports and your manual for anything about modems.

Regards
/RID

both the serial port and the modem are built in. Serial is COM1 and modem is COM3.

The device manager is XP lists the following:
Serial Port:
COM: COM1
IRQ: 04
Conflicts: None

Modem:
COM: COM3
IRQ: 09
Conflicts: None

The setup looks like it should work OK. Since it isn't clear what kind of "device" is involved here, and how it is supposed to be set up or respond to input from COM1, I can only suggest you first find a way of testing this port, including the connector etc. Something really simple, like from a DOS prompt, to bypass all software issues. If HW checks out OK, the program setup may need to be changed.

Regards
/RID

I know the program works fine. I wrote it :-). It works fine on all other versions of Windows and uses simple serial port I/O API that is supported by all versions of Windows. I already checked out the software end. It must be a hardware problem. What about the issue with HyperTerminal? Does this indicate a problem with Windows or the hardware?

I know that the hardware device and the software work. It must be a problem with the one of the following:
*Windows
*Serial port Interface
*Other conflicting software

Testing the serial interface with hyperterminal requires that there is a loopback of the characters you type in (they "bounce" from the device), in which case you should get single characters on screen when typing something in. (and ddoouubbllee characters if local echo is enabled...). Is your device supposed to do this? If it is, and you know it works, and you know the cable is OK, and you get nothing when typing, it is a hardware problem or a windows problem. To eliminate windows you need to test the COM port without windows interference, using something like a DOS based terminal or test program. Or you could try using the COM port in win XP for something else - try installing an external modem on the COM1. If that works, I'd say hardware is OK. You may be faced with an incompatibility issue between your software and XP.

If your program uses serial I/O by the I/O address, make sure BIOS setup shows the COM1 to be on the expected address.

Regards
/RID

I use the CreateFile() and WriteFile() API. Wasn't sure about how Hyperterminal was working, but this is a "dumb" device and has nothing but some electrical wiring in it. I have a spare serial modem around somwhere. I will try that tomorrow. If that doesn't work do you know of any DOS programs that will let me test the serial port?