Abstract: A redundant industrial controller system has a primary controller backed up by a secondary controller and recovers from a power loss by analyzing the state memories of the controllers to reach a determination of which controller would best be suited to assume primary control status without resorting to arbitrary tie-breaking procedures or race conditions. The rules and their inputs are applied by both controllers which independently make the determination as to which controller should be the primary controller.

Abstract: An industrial controller provides a primary controller and a redundant secondary controller and allows switchover between the controllers in the event of a failure in the primary controller. The process of qualification of the secondary controller in which its programming is made to match the primary controller may be inhibited to permit the secondary controller to maintain a clean version of an upgraded program executing on the primary controller. The clean program may be reverted to in the event an upgrading of the program in the primary controller is unsuccessful. Switchover is permitted even though the qualification of the secondary controller is not enabled.