| 摘要 |
PURPOSE:To decrease the capacity of a backup power supply at a district where power failure hardly takes palce, by supplying power from a backup device only to a part having a call just before the power failure. CONSTITUTION:If the power supply for a button telephone set is failed druring communication between a main trunk 21 and a button telephone set 31, the supply of power from a power supply 17 to a power supply terminal + of an integrated circuit 71 is stopped. Since a latch circuit 1111, an OR gate 121 energized from a backup power supply 16 keep the state just before the power failure, the integrated circuit 71 is energized from the backup power supply 16 and the channel keeps the operating state before the power supply. On the other hand, if power failure takes place when all transmission gates 611, 612, 621, 622 of the integrated circuit 71 are opened, since the output of the gate 121 is at low level, a transistor 131 is nonconductive and no power is supplied to the integrated circuit 71 from the backup power supply 16. |
