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.

Abstract: In a key telephone system of the type that uses the tip and ring conductors of a telephone line to transmit a hold condition control signal between stations, interstation signal-generating and detecting circuitry are provided at first and second stations, respectively, for enabling the first station to actuate, during a hold condition on the line, an audible signalling device, such as a buzzer, located at the second station. The interstation signal is generated on command by selective actuation of a hold switch button at the first station, and is transmitted over the line as an altered hold control signal, which is detected at the second station. Upon such signal detection, a buzzer is energized alerting the second station to the presence of a held call on the line.

Abstract: The exclusion circuit disclosed herein provides for line privacy in a key telephone system of the type in which a control unit provided at each of a plurality of stations can be operated to selectively connect one or more telephone lines to the phone set at the corresponding station. Within each control unit, an exclusion circuit is provided for each telephone line that extends to the control unit, and is connected so as to cooperate with a line condition sensing circuit, a line switching relay, a relay latching circuit and a manually operated line selector switch, all of which are associated with the same line as the exclusion circuit.

Abstract: To convert a multiple line, multiple telephone set installation into a key telephone system, a control unit is provided at each telephone set. The control unit includes a regulated power supply, a control circuit, pushbutton line select and hold switches, and indicator lights packaged in a compact assembly which can be readily attached to the base of a standard telephone set or mounted separately. Electrically controlled switching devices within each control circuit respond to the pushbutton line select switches to couple the telephone set to a selected one of the lines and automatically disconnect the set from the line when the call has been completed. An incoming ringing signal on any one of the telephone lines is detected by the control circuit and the ringing signal is automatically applied to the bell of the telephone set.