Abstract: A telephone ringing and tone generating circuit including circuitry for generating a plurality of tones, circuitry for generating a plurality of ringing currents and circuitry for generating a plurality of power voltages. Dual sources of each of the generating circuitry are provided. A timing circuit is provided to provide cadence (timing) signals for each of the tones, currents and voltages. Switching devices corresponding to each of the dual types of generating circuitry are operable to switch from one of the dual circuits to the other. Monitoring circuits are provided to monitor the outputs and on detection of a fault in one type of output to switch from the corresponding on line generating circuitry to the back-up.

Abstract: An electronic telephone system has a plurality of extensions connected radially to a central equipment by respective cables each providing separate speech and signalling paths. Each of the signalling paths is provided by a balanced conductor pair terminated by a matching impedance at the extension unit. The conductor pairs are connected to a common control unit at the central equipment, which scans the extensions in a time-division multiplex mode, signalling data being transmitted over the conductor pair from the addressed extension to the control unit in a first period of the time slot allotted to the extension, and signalling data being transmitted from the control unit to the addressed extension in a second period of the allotted time slot. The system is also served by a number of trunks, which are scanned by the common control unit, the trunks being interfaced with the central equipment by interface units which include protective circuitry and logic control circuits.