| 摘要 |
<p>PURPOSE:To execute a subscriber line test and a channel test with high accuracy, by interrupting the flow of leakage current from a driving circuit to a test device. CONSTITUTION:In a DC polarity switching circuit C10 at the polarity switching of feeding current, a control signal from input terminals T3, T4 keeps ''0'' and an output of an NOR gate G2 goes to ''1''. As a result, switches S15, S16 are closed, currents I15, I16 are applied to opening use control terminals D1-D4 of pnpn switch circuits S1-S4, to turn on npn transistors in the switch circuits S1-S4, allowing to open the pnpn switches forcedly. When at least one of control signals from input terminals T1-T4 is ''1'', the output of the NOR gate G2 goes to ''0'', then the switches S15, S16 are released, then the supply of the driving currents I15, I16 for release the switches S1-S4 can not be stopped at the subscriber line test and channel test. Thus, the driving current for release does not flow from cathode terminals K1-K4 to the channel.</p> |
