Abstract: Terminals in a serial loop communication system monitor the loop to detect a unique polling message. Failure to detect the polling message forces the terminals into an error checking routine. The routine quiesces (i.e., forces most of the stations into a quiet state) the loop with only one active station issuing a beacon message. The active station monitors the loop and when the beacon message is detected, the station issues an error message signifying that the controller is down. The error message is used to activate a backup controller. The error routine further identifies a break or other defect in the loop if its own beacon message or any other beacon message (or a poll) is not received.