On-hook transmission circuit in cable line

摘要

In an on-hook transmission circuit provided in a subscriber circuit connected between a subscriber terminal and an exchange, a constant-current source unit causes a constant current to flow in first and second wires during a period in which a control signal indicating that on-hook transmission should be turned ON is being received. A loop supervisory unit detects establishment of a loop including the first and second wires and a subscriber terminal connected thereto, and generates a scan signal used for causing the constant-current source means to stop the constant current flowing in the first and second wires after detecting the establishment of the loop.