| 摘要 |
PURPOSE:To attain the detection of collision between outgoing and incoming calls even after a line is switched, by providing a line polarity monitoring circuit at a position where a line from an exchange is led in from a telephone set side. CONSTITUTION:When an outgoing request comes from a terminal device, a CPU100 leads in subscriber lines L1, L2 from the exchange to the line polarity monitor circuit 102. When an incoming call comes from the exchange, since the polarity of the lines L1, L2 is inverted, an LEDD2 is not conductive, an LEDD1 is conductive to form a DC loop, and a transistor(TR)Q1 is conductive. When the CPU100 detects that the TRQ1 is conductive and there is a return signal, the CPU100 outputs a communication abandoning signal ACL to a terminal device. Thus, the terminal device discriminates the presence of an incoming call, stops the outgoing request and goes to the incoming operation mode. The CPU100 prolongs the lines L1, L2 from the exchange with a line connecting command signal CML from the terminal device to an MODEM and restores an S relay. Thus, the collision between the outgoing and incoming calls is prevented. |
