Abstract: Apparatus for sensing, communicating and displaying signal states in a multiplicity of functional units is described. A signal multiplexer is assigned to each given functional unit; the multiplexer inputs are connected to signal lines carrying the signal states of the functional unit. The outputs of the signal multiplexers are connected by time division multiplex (TDM) trunks to a group multiplexer. A multiplexer control applies first encoded selection signals to the signal multiplexers for selection of signal lines at the signal multiplexers. The selected signaling lines are interrogated, and the signal states are transmitted timewise in parallel with the signal states of other functional units over the corresponding TDM trunks to the group multiplexer. The multiplexer control applies second encoded selection signals to the group multiplexer for connecting the TDM trunks to group TDM trunks throughout the duration of a sampling phase.