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.