| 摘要 |
PURPOSE:To prevent the reductions of transmission loss and a trunk line voltage by using an FET as a switching element in the case of detecting polarity inversion so as to bring the impedance viewed from the trunk line into a high impedance. CONSTITUTION:When the voltage of a line 1 is larger than the voltage of a line 2, a current flows in the order of line 1 rectifier circuit 3 voice line 4 voice line 5 rectifier circuit 3 line 2. Thus, a potential difference is produced between the gate and source of the FET 10 and the FET 10 is turned on, then the photocoupler of a current detection circuit 20 is operated and a detection signal is sent to a data line 9. On the other hand, since the gate voltage of the FET 11 is not larger than the source voltage in this case, the FET 11 is not turned on and a current detection circuit 19 is not operated. Since FETs 10, 11 are used to detect a trunk line voltage, the impedance viewed from the lines 1, 2 is brought into the high impedance. Thus, the polarity inversion is detected without giving effect onto the line voltage and the transmission loss is reduced, as well.
|
