What is the 'digimatic indicator and multiplexer'? Are these instruments you are trying to control? If so, what is the manufacturer and model numbers? What is the type of connection (serial, GPIB, etc.)? If GPIB, are you using an NI GPIB controller and have you tried communicating in MAX? If serial, are you sure you have the correct cable type, com settings, and termination character. Have you tried using Hyperterminal to communicate?