| 摘要 |
PURPOSE:To attain the confirmation whether a call is enabled or disabled by connecting or disconnecting one telephone line and any one of plural telephone sets by a switch restorable automatically. CONSTITUTION:In hooking off a telephone set 2 while telephone sets 3, 4 are in an on-hook state, a dial tone of an exchange is received and a calling enable is confirmed. A DC current is supplied at the same time as the dial tone, this DC current is detected by a photocoupler of an on-hook/off-hook detecting circuit 6, a photo transistor PT is turned on, relays B, C of a switch selecting circuit 9 are operated and changeover switches 11, 12 are released. In this case, the relay A is inoperative and dialing and calling are attained while keeping a changeover switch 10 closed. On the other hand, even if the telephone sets 3, 4 are hooked off, no dial tone is listened to, and the busy state is confirmed. When the call of the telephone set 2 is finished and the telephone set hooks on, the state is restored to the initial state and all the telephone sets 2-4 ring by a bell signal of the exchange. |
