| 摘要 |
PURPOSE:To ensure the trunk line calling by providing a means connecting a trunk line and a single telephone set at the fault of a private branch exchange or a key telephone set even when the fault takes place. CONSTITUTION:When the fault takes place in the inside of a trunk line interface equipment or an equipment at the side of a central control unit, the potential of an out-port 12 or a control line 13 goes to a low level and the driving to a relay driver 17 is stopped. As a result, relay contacts ra, rb are connected in the state of full lines and the trunk line 2 is connected to a single telephone set 3 for power failure via terminals T, R the relay contacts ra, rb and terminals TA, RA.
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