| 摘要 |
PURPOSE:To surely supervise the thief of a handset by supervising the connecting state of the handset based on the presence or absence of a current conducted to a power supply circuit in switching a changeover means which switches at least one of a transmitter and a receiver to the talking circuit side and the power supply circuit side to the power supply circuit side. CONSTITUTION:A control section CNT8 has a self-diagnosing function, the potential of a resistor Ri at the handset 6 is fetched as a part of the self- diagnosis, then the presence of a current conducted to leads l1-l4 is discriminated to supervise the connecting state of the handset 6. That is, if relays RS, TS using latching relays are released, a receiver R and a transmitter T are interposed between the resistor R1 and a common circuit via the leads l1-l4 by contacts rs<1>, rs<2>, ts<1>, ts<2> and a current is conducted to a series circuit from a power supply V. The potential of the contact side rs<1> of the resistor Ri is nearly the potential of the common circuit because the receiver R and the transmitter T have a low resistance. If the connection by the leads l1-l4 is disconnected, the power supply is stopped and the potential of the resistor Ri at the contact rs<1> side is nearly a power supply voltage V and the thief of the handset 6 is detected.
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