Abstract: An automatic testing system for testing the operability of transmission line carrier units using timers that periodically activate a signal for energizing local carrier unit transmitters with a delayed energization of local carrier unit receivers, and a signal stepping circuit that connects the transmitter and receiver signal sequentially to the carrier units in a series of transmission lines. The absence of a signal being received by the local carrier unit receiver indicating a fault that is detected in latches that provide an indication of the carrier in which the fault has occurred and an indication at a remote computer when there has been at least one failure in the testing sequence, which failure indication remains activated at the end of a testing sequence to assure attention by the operator. The sequencing occurs automatically with a capacitor-delayed spike signal that provides sufficient time for the indication of the absence of a signal without requiring separate circuitry.