Calculation unit to plurality of redundant computers.

At least two redundant calculators (2a,2b) are each capable of performing identical calculating tasks. The output of each calculator is directed to a comparator (4) and to a multiplexer (5) which is controlled by the comparator (4) and directs the output from one of the calculators to the output (6). Control units (8a,8b,10a,10b) check the operation of each calculator. The comparator (4) can direct the output of the calculator to be correct by the control units (8a,8b,10a,10b) to the output (6) in the event that the comparator detects a difference between the outputs from the two calculators.