Telephone circuit with signal interchange over one pair - allows three levels to be transmitted at two voltage levels using threshold detector

摘要

The telephone circuit permits the interchange of three signals at two voltage levels to take place between two equipment units using a single line pair. the first signal is sent from the first unit to the second at earth potential. The second signal is also sent at earth potential from the second to the first station. The third signal is seng from the first to the second station at a negative potential. The second signal overlaps the first and third switching signals. A detector in the second station operates as a threshold detector for the first and third signals. Two resistors in series are connected to the input of the detector. The second signal is fed to their junction point.