DISTRIBUTED CONTROL WITH MUTUAL SPARE SWITCH OVER CAPABILITY

A distributed control system controlling a plurality of energize-to-start, energize-to-stop subprocesses is designed for on-line maintenance. One or more of the process control computers (86) forming the distributed control system (86-100) can be removed from service and the input and output connections (130) thereto switched to another of the process control computers that remains on-line. A bumpless transfer of control is achieved by reconstructing the status of the latches in the process control computer (86) removed from service that are history dependent and establishing that status in the latches of control circuits of the process control computer (96) assuming control. The status that the history dependent latches should have is derived based upon the operating status of the subprocess at the time switching is complete. Control functions dependent on the status of a latch are delayed until the reconstructed status of latches are complete.