| 摘要 |
An interface for connecting a PBX line (18) to a central office trunk (15) and for providing a supervisory signal (28) and accepting PBX hook switch and dial pulse inputs (30), being readily adaptable to ground start trunks and loop start trunks. A pair of transistor switches (55, 56) sharing a common base drive (57) is biased by a differential bias means (59, 60, 61) to operate a ground start isolated switch (32, 35) and an isolated loop closing switch (36, 37). The supervisory signal is controlled by a comparator (45) which senses grounding of central office tip (16) to establish the supervisory signal and is subsequently responsive only to the presence of current in the central office loop detected by a relay (65) buried in a coupling transformer.
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