Key telephone system providing selection of key or pbx operational modes

Abstract

A key telephone system which provides both key and private branch exchange (PBX) operational modes utilizes a plurality of digital telephone station sets which communicate with line circuits connected to a time division multiplex (TDM) telephone switching system. A single pair of leads (tip and ring) connect each station set to the line circuits. The TDM switching system has a processor with an operating system which carries out the processes which provide for the selection of key and PBX features. The TDM switching system also provides access to trunk lines which connect the telephone station sets via the TDM switching system to other switching systems, such as the central office of the public utility telephone company. In the station sets and in the line circuits, there are provided channels for signalling data and for PCM data. The PCM data may be voice, codes, or digital data. Line interfaces are connected to the lead pairs from the station sets to line circuits and transmit and receive multibit digital messages containing signalling bits and PCM code bits. The signalling messages on one of the signalling channels represent the status of the keys, and control data for indicators (light emitting devices) and a display (LCD display on two lines) where alphanumeric characters as to call originations and the status of certain lines in the system may be displayed. These messages both signal and control the key features and the PBX features as and when selected by the keys; the signalling bits of the messages being compiled to provide the control data used in the station sets and transferred to the TDM switching system.