Abstract: A digital control system for a gas turbine engine comprises two identical control lanes each of which includes a digital computer, two data buses, input circuits, output circuits and an interface unit. Each interface unit is arranged to multiplex data on the buses and transmit this data serially through a connection. Each interface unit is responsive to receipt of data from the other unit to generate control signals (read, write and valid address) for the received data, so that, in addition to timing signals on a further connection data only is transferred between the lanes.