| 摘要 |
PROBLEM TO BE SOLVED: To provide a communication device which can reduce the consumption of a line current and has no risk of causing malfunction of an exchange. SOLUTION: Photocouplers A1 and B1 for line polarity detection which are connected in parallel and have mutually opposite polarities are switched over at a constant cycle by a pulse generating circuit (OSC) 121. At the start of transmission, the output states (combination of pulses or high-level outputs) of the photocouplers A1 and B1 detected through buffers 122 and 123 are stored as line polarity data in a RAM and then the stored line polarity data are compared with the line polarity data outputted by the photocouplers A1 and B1 to detect whether or not the output change of the side which has bee in the high level at the start of the transmission is present, thereby discriminating the change of the line polarity.
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