Subscriber telephone line loop state verification - uses detector with resistance wire coils between lines to prevent short circuit

摘要

The loop state of a subscriber telephone line is verified using coils connected between the lines and detector circuit. The use of series resistors for short circuit protection is eliminated by coils using resistance wire. A telephone line is connected (1, 2) to a subscriber line equipment and conductors (5, 6) permit connection to a switching circuit (4) via capacitors (7, 8). Coils (12, 14) of resistance wire connect a battery (10) to the conductor circuit whose loop state is sensed by a detector (20) also connected to the switching circuit. A third coil (22) with diode limiter (28) is coupled to the coils to enable tone signals to be sent to the subscriber. A fourth winding (24) in a wheatstone bridge (26) series temperature.