A fault tolerant data processing system for controlling a real time process. The system is tolerant to systematic faults in its software. The system includes a data processor unit with a program memory and a data memory and input and output units where program software residing in the program memory can be executed on the data processor unit. The system includes a normal-mode software program residing in the program memory, and a backup-mode software program also residing in the program memory arranged to perform the same or similar function of the normal mode software program but being differently implemented than the normal-mode software program, and where a trigger signal received by the data processor unit can switch execution control such that the normal-mode software program stops executing and the backup-mode software program starts executing.