| 摘要 |
PURPOSE:To dissolve a disabled telephone line for a long time by detecting a control signal of a telephone set independently of a signal of a terminal device to line disconnection of an opposite station and allowing a modulator-demodulator itself to restore quickly the line to a telephone set side in data transmission of a public communication line. CONSTITUTION:A contact (c) of a changeover relay 2 in an NCU100 is connected to the public communication line 1 including an exchange, a telephone set 3 is connected to a contact (a) of the relay 2 and the modulator-demodulator circuit 4 in a modulator-demodulator 200 is connected to a contact (b). A terminal device (not shown) is connected to an output of the modulator-demodulator circuit 4. A level detection circuit 400 consists of a band pass filter 7 passing the control signal of the telephone set from a line side of the modulator-demodulator 200, a rectifier 8 and a comparator 9 comparing a reference voltage. An output of the detection circuit 400 resets a flip-flop 6 controlling the changeover relay 2 of a network controller 100 and the changeover relay is switched automatically to the telephone set side independently of the terminal device. |
