Abstract: A bi-directional digital communication unit for requesting the performance of services by transmitting data to a central computer complex for purposes of processing the data to utilize it to initiate the performance of the requested services and for receiving data from the central computer complex for purposes of determining whether the request for the performed services was received and the performance of the requested services was initiated. Data is transmitted from the central computer to the communication unit on a carrier wave at a first low frequency F.sub.1 and data is transmitted from the communication unit to the central computer at a second low frequency F.sub.2. The data received by the system, on the carrier wave having frequency F.sub.1, is demodulated and applied to valid address check means to determine if the central computer has transmitted a valid address to the unit.