Abstract: A digital switching device of the stored program type operating under the control of a central control unit has a first stage which multiplexes digital signals from a plurality of line units and separates data signals from signalling information. The data signals are fed to an input time switching stage. The input time switching stage operates under the control of the central control unit to transmit signals from an input port in a particular time slot on a highway to an output time switching stage. The output time switching stage transmits the data signals to the first stage which combines them with signalling information and after demultiplexing transmits them to appropriate ports.

Abstract: A ringing signal supply, provided at a concentrator remote from a switching office, compares a fraction of its output with a low level reference ringing signal supplied from the switching office, and uses the comparison result to control a high voltage chopper which connects one of two d.c. supply voltages via a low pass filter to the output to produce the ringing signal at a desired frequency. A d.c.-d.c. forward converter produces the two d.c. supply voltages commonly for a plurality of ringing signal supplies whenever at least one of them is to produce a ringing signal.