| 摘要 |
PURPOSE:To simplify the constitution of the whole device by detecting a non- ringing call signal and a ringing call signal through a single detecting circuit. CONSTITUTION:When subscriber's loops 1a, 1b are started from a central station and a non-ringing call signal arrives at the loops 1a, 1b, the non-ringing call signal is converted into a square wave having the repeating frequency by a binary-coding circuit 12. When a ringing call signal arrives at the loops 1a, 1b, the signal turns on phototransistors in a photocoupler 8 alternately and a square wave having multiplied repeating frequency is generated from the collector side. Since the output of a time constant circuit 9 is turned to the high level by the square wave, an output is generated from a gate circuit 13. If a self- telephone set 3 is in the on-hook status, current does not flow into the coupler 8 and the output of the gate circuit 13 is kept at the low level. If the telephone set 3 is turned to the off-hook state, the output of the circuit 13 is turned to the high level.
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