TELEPHONE INTERFACE CIRCUIT

摘要

2101059 9310620 PCTABS00022 A circuit for interfacing a host telephone system having a trunk line pair (20) with a central office line pair (30) and an additional circuit line pair (32) includes a loop current monitor circuit (12) for monitoring the host telephone system trunk line pair which is normally connected to the central office line pair. The loop current monitor provides a status signal output indicating the presence or absence of loop current. A relay controller (14) receives the status signal and provides a relay control signal and a hold signal. A relay (22) selectively connects the host telephone system trunk line pair to the additional circuit line pair in response to the relay control signal when loop current is detected after the host telephone goes off-hook. A hold termination circuit receives the hold signal and traps any loop current signals which may be detectable over the central office line pair. The relay controller circuit is active to generate the relay control signal only for an initial time period after the host telephone system goes off-hook.