I have an application that writes to a serial port device using the CreateFile and WriteFile functions. The device simply takes the pulse from the serial cable and when it recieves a pulse it takes some electricity from the power source. It then simply lights a LED. So when it's recieving data the LED is on. When it's not, the LED is off. The LED is just a test before the real functionality is implemented. My software works successfully on Windows 95, 98, Me, and 2K, but not Windows XP. I tried removing my device and replacing it with an old serial modem. When I started sending data the modem lights lit up. I know that data is being transmitted. The only difference was that I was using a modem instead of the device and I was using the modem serial cable (the cable for my project is attached to the device so I can't use it for the modem). Can XP be incompatible with my device? I've noticed several posts on message boards and I think there is someone in the EE hardware group who may be having a similar problem, but i'm not sure. I have found no answers. MSDN reports nothing about this.