Internal telephone system - using four wire connection in which call between two extensions also blocks any other from listening in

摘要

The private multiple extension telephone system employs a central exchange as a control and power supply unit. Two cores of the connecting cable are used for both power and speech, the remaining two cores for signalling and also for interlocking. When the speech circuits are energised, which follows the selection of an extension and subsequent acceptance by that extension of the call, a lock-out relay picks up which blocks are being used by the participating extensions to avoid selection by a third party. This can be achieved by transistor switching of power supplies in conjunction with electromagnetically operated relays in the speech circuit.