| 摘要 |
PURPOSE:To prevent the generation of a useless current by stopping the actuation of a current transmission means when it is detected that the voltage on a power supply line exceeds its threshold value. CONSTITUTION:When the power is supplied to a power supply line from a another data terminal device DTE, the power is supplied to the base of a transistor TR3 via the voltage on the power supply line. In case an FF is reset and Q' is sent to a TR1, the emitter voltage VE of the TR3 is kept at a level equivalent to the difference between a power supply VCCg and the voltage drop value Vf of a diode D2. As a result, the base-emitter voltage VBE of the TR3 is set at about 0.5V. Then the TR3 is changed to an ON state and therefore a collector current flows to increase the voltage drop of a resistance R5 and to stop the actuation of a TR2. As a result, the TR1 is turned off and the supply of power is stopped to a termination circuit 2 by the TR1. Thus the waste of the current is avoided to the VCCg from a power supply Vg.
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