Telephone line seizure circuit

摘要

A telephone system including circuitry for monitoring the "on hook" and "off hook" conditions of telephone handsets coupled to a system controller and disconnecting all off hook handsets from the telephone line. With the detection of an alarm condition at a security alarm system that requires a communication link, line seizure is effected via an SCR controlled seizure circuit serially connected between each distributed handset and the telephone line. The seizure circuit normally conducts when a connected telephone is off hook. Upon detecting an alarm, a line monitor momentarily shorts the telephone line to drive any conducting line seizure circuits into a non-conducting condition that is maintained sufficiently long to permit the system controller to gain control of the line. During controller communications, the handsets are also isolated from the telephone line. A local phone is also coupled to permit reprogramming if an inoperative line is detected. A speech interface facilitates system programming via a central station.