Port system for interfacing digital PBX to digital transmission facility

摘要

A port system interfaces digital PBX's to any transmission facility having DSX-1 communications ports via a microprocessor controlled PBX port board connected between PBX switch fabric and a first DSX-1 communications port. A channel service unit (CSU) module coupled directly to the port board for communications with DS1 transmission lines defines a second DSX-1 communications port which is coupled to the first DSX-1 communications port. The CSU module also includes a T1 transceiver and line interface circuitry for coupling the CSU module to a DS1 transmission line. The combination of the port board and CSU module provide full functionality of conventional CSU's while the port board alone permits direct coupling of transmission facilities utilizing a DSX-1 communications port. For remote maintenance, a microprocessor of the port board is coupled to a control processor of a PBX for control of the port board microprocessor via the PBX control processor. Wiring test circuitry is provided in the CSU module to detect faults in the wiring between a CSU module and a DS1 transmission line. The CSU module includes a transmitter transformer having a center-tapped secondary winding and a receiver transformer having a center-tapped primary winding. A current limited current source is selectively connected between the center taps of the transmitter and receiver transformers such that, when connected, one or more transformers in a CSU/DS1 interface circuit which is faulty will saturate leading to failure of transmission tests and hence proper detection of the fault.