| 摘要 |
PURPOSE:To eliminate influence of a loop resistance and to detect an answer from a called subscriber securely by providing a control line other than a calling line and by further providing an outflow and an inflow source for currents. CONSTITUTION:The internal calling operation of a key telephone set 1 turns on an SCR1. Consequently, a transistor (TR) Q11 turns on while a TRQ10 turns off, and a contact 1p is still closed, thereby connecting a calling network to extension channels P1 and P2. If a key telephone set (not shown in the figure) is provided at a called extension side, it is connected to the extension channels P1 and P2 as well as the calling operation of the key telephone set 1. Currents flowing through the paths from the key telephone sets 1 and 2 join together on a line P3 to reach -V via TRQn3 and Rn7. The current value of the confluence is nearly equal to that of a constant current In determined by ZDn1 and the Rn7 in a key telephone set (n). Therefore, the 3rd key telephone set, even if intend to interrupt the extension calling, fails. |
