| 摘要 |
PROBLEM TO BE SOLVED: To satisfy standards of a line-to-line voltage without increasing a ground unbalance attenuation by supplying a current to a resistor so as to compensate a line-to-line DC voltage in the case that the line-to-line DC voltage of a communication line does not satisfy a specified voltage. SOLUTION: A current detection circuit 9 outputs a control signal that informs of a dial monitor state or a high impedance communication state when a current detected from a communication line 3 is below a setting value to a current control circuit 10. A power supply voltage monitor circuit 8 monitors a voltage of a DC power supply 7 and outputs a control signal to the current control circuit 10 when the voltage is lower than a reference voltage 8a. A reference voltage I or a reference voltage II is selectively used as the reference voltage 8a. The current control circuit 10 operates a switch 12 so as to connect a constant current source 11 to a communication line 4 only when the control circuit 10 receives each control signal from the current detection circuit 9 and the power supply voltage supervisory circuit 8. As a result, the line-to-line voltage is compensated in a way that when a voltage of the DC power supply 7 decreases up to the reference voltage I, the line-to-line voltage increases and when the voltage of the DC power supply 7 increases up to the reference voltage II, the line-to-line voltage decreases. |
