Abstract: An optocoupled line circuit is arranged with a diode bridge providing uniform direction current flow so as to eliminate the mechanical switch hook contact of the prior art. A zener diode is used to bridge a clipped version of AC ringing current across the optocoupler when the circuit is in the on-hook condition. Current presented to the optocoupler from a line control device allows current to flow from the telephone line through the line interface circuit.

Abstract: There is disclosed a system and method of providing electrical isolation between a telephone line and a connected device, such as a computer. In one embodiment the telephone line is 2-wire and the connected device is 4-wire and includes a feedback elimination circuit. A pair of controlled CTR opto diodes are used in the communication path to effect electrical isolation. The opto diodes are used in conjunction with an electronic inductor constructed using a pair of cascoded darlington transistors to control the telephone line voltage and current and the circuit is designed to maintain the optocouplers within their linear operating range.