| 摘要 |
<p>PROBLEM TO BE SOLVED: To send a signal through a transmission line normally by selecting a 2nd voltage detection route while a very small AC signal is transmitted so as to detect on/off of a DC voltage and its polarity so as to avoid C very small AC signal from being interrupted even when the DC voltage is shut off. SOLUTION: A line controller 9 isolates a 2-wire transmission line 5 by using a transmission line isolate section 7 and a DC voltage monitor circuit 6 sends/receives a very small AC signal superimposed on a DC voltage generated by a data exchange 1 through the 2-wire transmission line 5. In this case, when the DC voltage is interrupted, the very small AC signal is interrupted depending on the forward characteristic of each diode of both photocouplers in the circuit 6 and an interrupted speech or a loss of a modem signal might be caused. Then a control section 8 detects transition to a stage of a speech or data transmission depending on a state of both the photocouplers so as to control relay contacts across the photocouplers.</p> |
