| 摘要 |
Apparatus for producing a reversible local battery current to provide metering pulses on a two-wire telephone trunk circuit. A DC current supply produces a current of one polarity which is increased from zero to 20 milliamps over a period of 75 milliseconds by charging a capacitor, and decreased from 20 milliamps to zero also over a period of 75 milliseconds by discharging the capacitor. The DC current supply is coupled to the trunk circuit by way of a polarity reversing bridge. In response to a control signal initiating a metering pulse, control circuitry causes the capacitor to discharge, gradually reducing the DC current through the trunk circuit to zero. When the current reaches zero, the control circuitry causes the polarity reversing bridge to reverse the connections of the DC current supply to the trunk circuit. At the same time the capacitor charges, gradually increasing the DC current through the trunk circuit. Since the polarity of the bridge has been reversed, current flow in the trunk circuit, and also the potential, is opposite to the original direction. Thus, current flow in the trunk circuit is varied gradually from 20 milliamps in one direction to 20 milliamps in the opposite direction. When the control signal terminates, the apparatus operates to change the current flow in the trunk circuit gradually to 20 milliamps in the original direction.
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