A time-division signaling system employing a transmitter and a receiver which communicate in recurrent operating cycles over a common transmission medium. The transmitter is contructed to transmit during an operating cycle time-spaced interleaved indexing and signaling voltage pulses which differ in respective maximum voltage levels. Discrimination between these two different kinds of pulses at the receiver is made on the basis of a comparison of their respective maximum voltage levels with a reference voltage level which has been generated by preceding pulses. The system includes circuitry which assures synchronization between the transmitter and receiver; and a modification of the system further includes circuitry which prevents the receiver from producing a responsive output to any signaling pulse which is received at the time that the transmitter and the receiver are out of synchronization.