Interface circuit for telephone line to equipment signal coupling

Abstract

A telephone line to equipment interface circuit, characterized by a high degree of input impedance isolation, is provided for coupling supervisory and information signals from the line to signal utilizing equipment, such as a control unit of a key telephone system. The interface circuit incorporates first and second operational amplifier comparators connected in opposed phase relationship to the tip and ring conductors of a telephone line. The inputs of the comparators are biased so as to produce output signals that change in a predetermined and symmetrical fashion, similar to full wave rectification, in response to normal and reverse polarity supervisory signals applied to the telephone line by a Central Office, PBX, ESS or other signal originating equipment. The output signals produced by the first and second comparators are applied to separate inputs of a logic circuit, formed by a third operational amplifier comparator, which is biased to perform an AND logic function. The AND logic output signal from the third comparator is indicative of supervisory and information signal conditions on the telephone line, including on-hook (idle) and off-hook (busy) ringing, and a special hold condition represented by a hold signal generated for local supervisory purposes by the above-mentioned key telephone system.