| 摘要 |
PURPOSE:To simplify the circuit by connecting a polarity inversion detecting circuit in parallell with a telephone set to use an incoming collision preventing circuit and a called party response circuit in common, thereby reducing the DC voltage drop of a talking circuit. CONSTITUTION:The polarity inversion detecting circuit 6 is connected in parallel with a telephone set 2 to surpervise trunk lines in polarity. In assuming the polarity of the trunk line L1 as positive and the L2 as negative, a current flows to a light emitting diode of a photocoupler PC4, and when an incoming call arrives within a prescribed time, the polarity of the trunk line is inverted and no talking is conducted because it is judged as the arrival of the incoming signal. When no incoming signal reaches within a prescribed time, the trunk line is acquired by opening an st contact and closing contact h1, h2 to transmit an opposite subscriber number. When a called party responds, the polarity of the trunk line is inverted and the polarity of the output of photocouplers PC4 and PC5 is inverted. In selecting resistance values as R1>R2, then the photocouplers are activated stably. |
