Abstract: A data transmission equipment, known as a primary digital multiplex (PDMX) consists of a bus structure formed by parallel lines on a printed circuit board, with the circuit units (1,2,3,4,5) on printed circuit cards set at right angles to the bus structure. The circuit units include a control unit (1,2) also called a core facilities unit, which acts as an interface to one or more 2Mb/sec. trunks, and so-called tributary units (3,4,5) which give access to subscriber and other outlets.Various networks using one or more such PDMX equipment are described.The signalling and control arrangements for the equipment use a signalling processing unit (2) and a tone processor (6). The signalling processing unit monitors the bus for signals and line conditions and issues signals appropriately. The tone processor functions in a generally similar manner.Note that PDMX is one of STC PLC Trade Marks.

Abstract: An electronic switching network for use as a transit exchange, interconnects PCM digital trunks, and handles large numbers, e.g. 1,000 inlets and 1,000 outlets of such trunks. The switching network is fully electronic, using integrated circuit units. To provide a self-checking function, the intelligence arriving via the trunks is converted (2) into a redundant line code, e.g. 3B-6B, which has spare code combinations which are used to provide an auxiliary data channel through the switch matrix (3). The converter (2) applies to that data channel the inlet port number for a connection via the switch matrix (3). At the outlet side another converter (4) converts back to the line code (e.g. HDB-3) used over the trunks, and also extracts the inlet port number. This is then compared with what it should be, and the result of this comparison indicates whether the switch is functioning correctly. Thus the checking occurs while the switch is handling traffic.