| 摘要 |
PURPOSE:To eliminate the OFF operation of a polarity-inversion signal detecting circuit by inserting two current detectors in diode bridge sides connected between the 1st and 2nd main wires at the talking circuit input sides of a telephone set and by providing a coincidence circuit for main-wire polarity output. CONSTITUTION:Between main wire terminals L1 and L2, diode bridge circuit DB is connected and in sides forming a DC loop to an office when main wire L1 of diode bridge circuit DB is held positive and L2 is held positive, the 1st and 2nd current detectors PC1 and PC2 are inserted. When hook switch contacts HS1 and HS2 are changed over with the handset off the hook, the 1st flip-flop F1 remembers that by the 1st current detector and when a called subscriber makes an answer after dial origination, the 2nd flip-flop F2 remembers that by the 2nd current detector. Then, AND gate AND1 checks identity between those and sends out an identity output. This output is unchangeable without reference to the connection direction of the main wires. |
