| 摘要 |
PURPOSE: To provide a small sized device not giving effect on the communication of a communication station using a signal line even when a transistor(TR) for the power supply for bus is faulty by supplying a bus current to the signal line connecting a current sensing resistor in series thereto. CONSTITUTION: A supply current IDC corresponding to a reference potential Vref at a terminal (e) is fed to a signal line L1 via current detection resistors R12, R22 in a steady state. A voltage signal corresponding to the current IDC is fed back to a feedback capacitor C21 as a potential differenceΔVab between terminals a, b of a voltage current converter 40 with respect to the resistor R12. Similarly the voltage signal corresponding to the current IDC is fed back to the feedback capacitor C21 as a potential differenceΔVgh between terminals g, h of a voltage current converter 70 with respect to the resistor R22. Thus, the potential at the terminal a, g is maintained without a change, and when TRs Q11, Q21 are not faulty, the Q21 is always in ON state and in the standby state. Through the constitution above, when a short-circuit fault takes place in the Q11, the Q21 is effectively operated and no effect is caused on communication between the signal lines L1, L2.
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