The communications system of the present invention defines a complete feedback path consisting of first and second transceivers and an appropriate signal path. Each transceiver comprises a detector and a signal source capable of generating a signal at one of two frequencies. The detector in each of the transceivers is a two state device tuned to respond only to a predetermined desired difference frequency. If the desired difference frequency signal is present, the detector provides and output signal in a first state; if the desired difference signal is not present, the detector provides an output signal which is in a second state. The signal source can be a voltage controlled oscillator, a pair of single frequency oscillators, a synthesizer, or any signal source capable of being electrically switched between two frequencies. Data can be exchanged between the transceiver pair only when the desired difference frequency is present.
Thomas J. Warnagiris