Abstract: The disclosure is directed to a calling circuit which controls the connection of two different audio signal sources to a single audio signal transmission path for broadcasting the audio signals over a telephone set loudpseaker as well as controlling the energization of indicators in the telephone set and at the main control circuit. Only four conductors are used to connect the telephone set with the main control circuit, two of which are power supply conductors which may supply other circuits as well.

Abstract: A counting circuit is disclosed which generates an electrical representation corresponding to the number of pulses in a plurality of pulse trains, each pulse train containing at most ten pulses. The circuit does not start to count the number of pulses in individual pulse trains until a first pulse train with more than a preselected number of pulses is received. The circuit then counts each pulse in subsequently received pulse trains for a preselected number of pulse trains. The circuit is particularly useful in key telephone dial pulse signalling apparatus as a register circuit which is not responsive to false pulses generated by lifting the handset.

Abstract: The disclosure relates to a key telephone system intercom line circuit which uses only three leads to connect plural telephones and an intercom line control circuit. These leads comprise two talking circuit leads and one control lead. Exclusion circuits in each of the telephones are controlled by signals applied on the single control lead (1) to allow a telephone to seize the intercom line circuit when it is in an idle state, (2) to allow a called telephone (signalled via a separate pair of calling leads) to answer without being excluded and (3) to exclude all other telephones from the intercom line circuit after the calling telephone has seized the intercom line circuit.