Duplex telephone signalling system

Claim 1

A duplex telephone signalling system comprising: plural conductors for both transmitting and receiving D.C. signals between two locations, first and second devices each having a control input terminal and an output terminal, said devices producing signals at the output terminals thereof in response to control signals at the inputs thereof, a switching device responsive to a change of state of telephone equipment at one location for producing the control signal for said first device, said first device producing a first D.C. output signal of one polarity at the output terminal thereof in response to said control signal, means coupling the control input terminal of said second device to said switching device for inverting the polarity of the control signal received by the inverting means, said second device responsive to the inverted control signal for producing a second D.C. output signal at the output terminal thereof of substantially the same absolute magnitude as, but of opposite polarity to, the first D.C. output signal, differential means having plural inputs connected to different ones of said two conductors for detecting the direction of net direct current flow in the conductors at the one location, said differential means producing D.C. outputs representing changes in current direction in said two conductors, and first and second resistors of substantially equal resistance value for connecting each of the outputs of said first and second devices to a different one of the differential means inputs, whereby said differential means rejects said first and second output signals at the one location while such output signals are transmitted by said two conductors to another location.