A telephone which can receive and transmit voice and asynchronous data simultaneously is provided. A signalling circuit provides signals indicating the request for a communication link or requesting a communication link to a predetermined location. A transceiver circuit synchronously receives and transmits frames of digital data bits representing data, voice and signalling information. A voice digitizer circuit converts the received frames of digital bits to an analog output and converts a received voice signal to a digital voice output. An interface circuit is used to interface between an asynchronous data source and the synchronous transceiver circuit. The telephone is line powered by utilizing D.C. isolating transformers to power both voice and data circuitry.

A digital interface for PCM data, allowing bidirectional data exchange between a processing unit and a PCM channel, the data being transmitted in records each divided into a plurality of groups (bytes) transmitted at successive periods in homologous time intervals. This interface comprises a reception memory receiving from the PCM channel data groups to be stored, a control processor connected to the reception memory and receiving therefrom complete data records, a transmission memory, receiving from the control processor data records to be transmitted on the PCM channel. The reception and transmission memories are divided into a plurality of buffers storing data records received or to be transmitted in preset time intervals. A timing phase sequencer is connected to the memories and to a protocol controller for detecting a data record reception beginning and correctly storing data groups of a same record in the reception memory and controlling transmission of a data record in data groups from the transmission memory towards the PCM channel in preset time intervals.

Each station of a switched telephone network voiceband data communication system includes a data set and a telephone set. The data set includes primary signal processing circuitry which performs the real-time signal processing of the data set under the control of a microprocessor-based primary controller. The latter also controls connection of the data set and telephone set to, and disconnection of same from, the telephone line. A portion of the frequency spectrum of the telephone line is allocated to a secondary channel over which a secondary controller within each data set communicates test, health and other information to a counterpart within the other data set. Line functions, such as the switching of a station into and out of the data state, talk state and disconnect state, are not executed at least until a line control message indicative of the line function in question has been communicated to the data set within the other station via the secondary channel. This allows appropriate steps to be taken at one or both stations to avoid such potential problems as the non-disconnection of unattended telephones and the premature re-enabling of disabled echo suppressors.

A key telephone system uses four wires for transmission of voice and audio signals to and from the telephone stations and other components which are part of the system. A plurality of telephone stations are provided, each having a two wire audio input port, a separate two wire audio output port, and a data port. Communcation paths are established through a crosspoint matrix. The matrix includes a first group of addressable crosspoint switches for coupling selected transmit signal paths to selected first link points. A second group of such switches is provided for coupling selected receive signal paths to selected second link points. A third group of such switches couples selected first link points to selected second link points. A link searching routine determines which links are available for effecting communication paths. A minimizing function shares communication paths among common signals when possible.

A digital isolation apparatus for coupling between a telecommunication network and a digital signal processing device. A CODEC is arranged to couple to the telecommunication network, convert a first set of analog signals from the telecommunication network to a first set of digital signals, and convert a second set of digital signals to a second set of analog signals to the telecommunication network. A digital isolation device, arranged to connect between the CODEC and the digital signal processing device, pass the first set of digital signals from the CODEC to the digital signal processing device, and pass the second set of digital signals from the digital signal processing device to the CODEC. A power supply assembly supplies power to the CODEC via the digital isolation device.