Telephone system

Claim 1

A telephone system of the kind which has at least three telephone instruments and provision for speech connections to be set up automatically by operation of manually operated controls associated with the telephone instruments between any of said instruments, and, selectively, either another of said instruments or a line to a remote exchange, said telephone system comprising (A) a central unit to which said telephone instruments are connected each by only (I) a respective first pair of wires for carrying two-way speech signals and (II) a respective second pair of wires for carrying control and indicating signals between the central unit and the telephone instrument, (B) the central unit being arranged to operate in response to control signals received over the second pair of wires to connect the first pair of wires to any one of the telephone instruments selectively to either an intercom highway or to an exchange line, (C) wherein to enable a further telephone instrument to be connected into the system over a single pair of wires and, at least for some facilities of the system, to emulate a telephone instrument having two pairs of connecting wires as aforesaid, (I) an adaptor unit is connected, adjacent the central unit, between the single pair of wires which are terminated by the further telephone instrument and first and second further pairs of wires which are connected to the central unit, (II) said first and second further pairs of wires corresponding to the previously mentioned first and second pairs of wires and being arranged respectively to carry speech signals and control signals, (a) said adaptor unit comprising loop detection means to supply an off-hook control signal to the second further pair of wires in response to the single pair of wires being looped when the further telephone instrument is off-hook and (b) means to supply a calling signal to the single pair of wires in response to the central unit supplying a signal over the second further pair of wires to signify that the further telephone instrument is to be called.