| 摘要 |
PURPOSE:To decrease the number of wires of a main trunk circuit, by detecting a current flowing in a channel and displaying busy. CONSTITUTION:When the user selects the No.1 main trunk, depresses a button LK1, hooks off a handset, then hook switches HS1, HS2 are closed and a channel circuit network 3 is connected with telephone lines R, T. Since a current flows through a resistor R2 and control lines H,L from a power supply E2, a transistor (TR)2 keeps on-state. Succedingly, a control circuit 4 keeps a TR1 at off-state. Thus, the control lines H, L display the state of a telephone set 2 to a main trunk circuit 1. In this case, a driver TR5 is also conductive, resulting that an LED1 illuminates and it is displayed that the No.1 main trunk is busy. |
