| 摘要 |
PURPOSE:To ensure the channel between a trunk line and a main telephone set even at power failure and to attain mutual communication between the extensions of conventional telephone sets by constituting a simple exchange function. CONSTITUTION:When a power supply is applied to a telephone set 2 and nothing is operated, a changeover means 7 is connected to an extension connecting line 51, a control circuit 19 turns on switches 8ab-8db one by one, a DC is supplied from a DC circuit 6b and the closing of line is detected. A changeover means 7 connects the trunk line and the telephone circuit 3, and the connection between the trunk line and the telephone sets 4b-4d is performed by a trunk line connecting circuit 5, a DC supply circuit 6a, the changeover means 8ab-8de and connecting terminals T1-T3. Since the changeover means 7 is connected to the trunk line at power failure, even if the supply of power to the telephone set 2 is stopped, the call from the telephone set circuit 3 is attained. |
