| 摘要 |
PURPOSE:To eliminate the need for special constitution for detecting the polarity of a communication circuit and to miniaturize a device by deciding the change state of the polarity of the communication circuit based on the polarity data on a memory for storing the polarity of the communication circuit during communication and the output of a detecting means which detects the polarity of the communication circuit. CONSTITUTION:Photocouplers (a) and (b) detect the conduction and direction of the current of the circuit 37. When the opposite side is connected in an off-hook state (wherein the hook of a telephone set 36 is off or wherein the circuit is connected to the side of a modem 14 through a relay cm1), an LPL 1 is at a level L, but when the opposite side is disconnected from the circuit in the off-hook state, an LPL 0 is at the level L and the LPL 1 is at a level H, so that the polarity of the circuit is inverted. The polarity (levels of the LPL 0 and LPL 1) of the circuit when the opposite side is connected to the circuit in the off-hook state is stored in a RAM and it is decided frequently whether the polarity is inverted or not during transmitting operation by an interruption routine based on the polarity data stored in the RAM; when it is judged that the polarity is inverted, the transmitting operation is interrupted immediately to an error end.
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