| 摘要 |
There is disclosed a circuit for monitoring a capacity of a subscriber line to which a capacitor equivalently representative of a telephone set is connected. This monitoring circuit has a test sequence for the subscriber line capacity. The test sequence comprises a first step to connect the subscriber line to a test circuit, and second step to apply a backward voltage to the subscriber line, thereby to discharge an electric charge stored in the equivalent capacitor. The test sequence further comprises a third step to apply a forward voltage to the subscriber line through a reference capacitor, thereby to charge the equivalent capacitor to evaluate the capacity value of the equivalent capacitor on the basis of a voltage appearing across the equivalent capacitor, thus to effect a capacitive test of the subscriber line. Accordingly, where the telephone set includes a diode bridge in an interface of the subscriber line, diodes constituting the diode bridge can become conductive so as to enable the formation of a current charging path at the time of the execution of the third step. Thus, this makes it possible to conduct a capacitive test of not only ordinary telephone set but also telephone sets of the type including the diode bridge in its subscriber line interface.
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