| 摘要 |
PURPOSE:To prevent the catching of the trunk line during the service interruption and while the extension is busy, by connecting in series the switching circuit which works with closure of the nonlock type trunk line button to the talking loop and then giving a by-pass to the switching circuit when the extension is busy. CONSTITUTION:When the trunk line button LK is pushed with off-hook of the TEL, the SCR-Th is turned on by the trunk line loop current. And the relay RL1 having the capacitor C2 works to supply the current to the gate of the Th. Thus the Th is turned on at the dial make time although it is turned off at the dial break time to continue catching of the trunk line. At the same time, the D2 is provided across the serial circuit containing the reverse current blocking diode D1, the Th and the RL1 each in order to give a by-pass to the serial circuit while the extension is busy. And in case the service interruption occurs while the extension is busy, the contact of the trunk line relay A is reset to be switched to the side of the trunk line. Thus the Th is never turned on unless the LK is pushed. As a result, the trunk line is never caught. |
