Telephone PBX circuit with extension line multiplexer - has converters and evaluation circuits for earthing key signals between extension and automatic exchange

摘要

The PBX circuit is used with an extension line multiplex unit. There are exchange side and automatic exchange side carrier frequency units, and loop current generators. A convertor for the earthing key signal is inserted between each telephone instrument and the corresponding carrier frequency unit and an evaluation circuit for the earthing key signal is provided between each loop current generator connected to the automatic exchange side carrier frequency unit, and the automatic exchange.