a digital bus system for the transmission of pulse code modulated speech and control data between a common multiplexer and a multiplicity of line circuits in a telecommunications system . in the past the line circuits were multiplexed directly onto a common bus so that failure of one line circuit could result in the balance of the line circuits on that bus being rendered unserviceable . in the present system each line circuit is coupled over a separate data line to a pair of multiplexers so that failure of any one line circuit does not affect the operation of the others . under normal operation one of the multiplexers functions as the primary and the other as a secondary . however , should the primary multiplexer fail , the secondary will take over upon actuation of a single control signal from a controller .