DE2413502

To enable selective voice communication and data transmission between calling and called subscribers of a PCM telephone network served by a central office, the latter includes a processor which intervenes during data transmission but is bypassed during voice communication while preserving the information required for instantaneous resumption of data transmission. The processor includes two buffer registers MCD1, MCD2 for incoming and outgoing message signals and two service registers MS1, MS2 for ancillary information, each register having a multiplicity of stages which are allocated to respective channels of a PCM frame and are scanned simultaneously with corresponding stages of the other registers. Calling and called subscribers are assigned separate line links giving access to respective channels via corresponding register stages, with transfer of message signals from a stage of register MCD1 (temporarily assigned to one subscriber) to a stage of register MCD2 (temporarily assigned to the other subscriber) under the control of a programmer during data transmission. This transfer is discontinued during voice communication while the contents of the associated stages of the service registers MS1, MS2 remain intact.