Watchdog Time

For the safety task, the watchdog time in Logix SIS impacts the controller reaction time and execution of the safety task during a loss of redundancy. For information about the watchdog time for the safety task, see Configure Logix SIS Requirements.

For standard tasks, the watchdog time in redundancy applications must be longer than tasks in non-redundancy applications because more time is required to conduct crossloads and synchronization.

The way programs execute in the event of a switchover is another reason for a longer watchdog time. A program can execute a second time after a switchover. This action depends on when the switchover occurs in the task or program and in the task crossload and synchronization.

If a program executes a second time, the length of time that is required for the program scan increases. However, the watchdog timer is not reset and continues to countdown from the beginning of the task that the previous primary controller started. Therefore, the watchdog timer must be configured to account for the potential of additional program scans.

If there is a watchdog timeout, a major fault (type 6, code 1) results. If this fault occurs after a switchover, the control system fails-to-safe or to the configured hold state.