Transmission line signalling unit - has polarity-switching relays and voltage-current characteristics controls which are independent of the transmission line length

摘要

This line signalling unit has polarity-switching relays which reverse the connections to the transmission line at the beginning and the end of each transmitted signal to reduce noise. The transmission lines are connected to the unit via inductances and a capacitor, which act as an audio signal blocking filter. The circuit includes a signal generator which produces a reference voltage, a fault detecting circuit fed by a sampling circuit and a servo control circuit. The servo control provides feedback to make the filtering and the line voltage and current levels independent of the physical length of the transmission lines. The unit control circuit consists of semi-conductor logic elements, bistable multivibrators and a 14-bit counter.