Abstract: A communication system for communicating digitally encoded information from a remote transmitting station to a central receiving station. The central station supplies d-c operating power to a remote station through a d-c power distribution line. A voltage regulator at the remote station receives the d-c operating power and derives therefrom a constant d-c voltage for powering the circuitry of the remote station. A digital transmitter at the remote station changes the magnitude of the current drawn through the voltage regulator, in accordance with a message to be transmitted, and thereby changes the current in the power distribution line without affecting the voltage supplied to the circuitry of the remote station. These line current changes are detected and decoded at the central station to reconstruct the transmitted message.