| 摘要 |
PURPOSE:To supply an AC signal to a talking line in compliance with the electric communication installation rules for enterprises without forming a DC loop at the line idle state by detecting whether or not a telephone talking line forms a DC loop and switching respectively high and low voltage power supplies of the talking line. CONSTITUTION:A group detection circuit 7 receiving a voltage across resistors 3,4 connecting to talking lines A,B detects whether or not the talking lines A,B form a DC loop. When the telephone set is hooked on and the talking lines A,B do not form a DC loop representing the idle state, a circuit 2 controls power changeover circuits 8,10 to switch the high and low voltage power supplies, the power supply higher than 0V and the power supply lower than -48V of the speech current power supplies are connected, it is in compliance with the electric communication installation rules for enterprises and the subscriber supplies an AC signal to the talking lines A and B without making a DC loop at terminal equipments and a feeding circuit able to make automatic metering or the like is attained. |
