POLARITY DETECTING CIRCUIT FOR CENTRAL OFFICE LINE TRUNK CIRCUIT

摘要

PURPOSE:To obtain a polarity detecting circuit which has high impedance independently of the polarity of a trunk line by providing a switching means which disconnects one of a pair of diodes. CONSTITUTION:In a polarity detecting circuit 25a, a Pd relay 47 is provided and a diode 23a is provided in the opposite direction of a diode 23 and in parallel with the diode 23. When an extension designating signal is sent to a PBX from an exchange and is received by the PBX to complete reception, an extension designation reception completion signal is sent from the PBX to the exchange. That is, the extension designation reception completion signal disconnects a loop between terminals 1 and 3, and therefore, a control circuit 15 returns CM relays 5 and 7 to positions indicated by continuous lines. Since the relay 47 is connected to the position indicated by a dotted line in this state, the state where the terminal 1 and the terminal 3 are connected to a battery and the earth respectively is equivalent to a reverse bias of the diode 23a, and the circuit 25a keeps high impedance.