I have an option 11c with one way communication to the terminal. I can see the data coming from the machine but I can't talk to it. I took the terminal back to my office and tested it and it works fine. Does anyone know which card controls the serial port?

there should be a 3 headed cable that has 3 rs232 connectors.

Normally a modem would be on connector 0
Your local laptop or admin terminal would be on connector 1

I have seen those cables go bad, but is uncommon. Try moving the terminal to a different connector. Connector 2 usually is not programed to do much.