Abstract: Interface circuitry is provided between a pair of lines, on the one hand, and an input to and output from other equipment on the other hand. The interface circuitry will commonly be used between `tip` and `ring` lines of a telephone connection and office telephone equipment but is not limited to the telephone art. The interface equipment provides independently controllable and adjustable impedances to `longitudinal` and transverse voltage changes on the pair of lines, means for transferring signals from the pair of lines to the output line and means for transferring signals from the input line to the pair of lines.

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.