Abstract: A method and means for fault testing the transmit and receive sections of repeaters which are connected in a pulse code modulation (PCM) transmission system, from a single end of said system, as described wherein testing between a first and second station is effectuated by sending between the first and second stations, a spaced trio pulse train with bipolar violations which are converted at the second station to a spaced trio pulse train without bipolar violations and then returned to the first station. When spaced trio bipolar signals are sent from the first to the second station, the second station converts these to a spaced trio bipolar signal pulse train with violations and sends these back to the first station. The testing circuits in the outgoing and incoming lines are only operative in the presence of a bipolar pulse train with violations.