Abstract: A processing system for use in an aircraft display system, or other applications, comprises two parallel lanes. Nominally-identical input signals from sensors are supplied to the input of a first processor in each lane which produces display driving signals that are supplied to respective displays. The display driving signals are also supplied to a second processor in the same lane and to a third processor in the other lane. The second and third processors perform inverse functions on the display driving signals to produce, at their outputs, signals nominally-identical to the input signals to the system. Each lane includes comparators which provide outputs in respect of the difference between the two inverted signals and the input signal. A monitor responds to the comparator outputs to indicate malfunction.