| 摘要 |
PURPOSE:To discriminate and detect an incomming call and a polarity inversion by inserting the light emitting side elements of two photocouplers in one line of a channel in parallel connection each in opposite polarity, and making use of the logic of the output of the light receiving side elements of side two photocouplers. CONSTITUTION:When a call arrives, call signal is transmitted from a trunk, and an A.C. voltage is generated between the A.C. signals (-) and (-) of a bridge BR1. By such a voltage, the light emitting side elements PC1(A) and PC1(B) of the two photocouplers emit light alternately in accordance with the frequency of the call signal. In case of a trunk call from a telephone set TEL, a D.C. voltage to make a resistor R1 side come positive is generated between the A.C. terminals (-) and (-) of the bridge BR1. By such a voltage, the light emitting side element PC2(A) of one the photocouplers emits light. If the callee answers for the trunk call from the telephoneset TEL, the firection of the current reverses, the light emitting side element PC1(A) emits light, and that PC2(A) of the photocoupler stops the light-emitting. As a result, both the detection of a call-arrival and that of polarity invertion can be executed.
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