ControlLogix 5570 T06:C01: Task Watchdog Expired after program change, why?
I'm getting a Type 6 Code 1 fault on my ControlLogix 5570 controller. It started happening after I made some changes to the program, adding a new loop. What should I check first?
A Type 6 Code 1 fault, or Watchdog fault, indicates that a user task did not complete within its allotted watchdog time. This often occurs after program changes that introduce new logic or loops, causing the task to take longer to execute than expected. The controller handles these faults via the Controller Fault Handler.
Likely causes and checks
1Infinite or very long loop in the faulted task
- Points to it
- A newly added loop or existing loop in the program may not have a proper exit condition, causing it to run indefinitely or for an excessively long time. This prevents the task from completing within its watchdog period.
- Check
- Review the loops within the faulted task, especially any recent additions, to ensure they have a reliable exit condition. Look for a sudden jump in scan time rather than a gradual increase.
2Task has too much logic for its watchdog setting
- Points to it
- The amount of logic in the task, possibly increased by recent program changes, now exceeds the time allowed by its watchdog setting. The task simply takes longer to execute than the watchdog permits.
- Check
- Monitor the task's scan times (Task Properties > Monitor) and compare the maximum scan time against the watchdog setting. Identify which programs or routines within the task are consuming the most execution time.
3Higher-priority tasks are starving the faulted task
- Points to it
- Other tasks with higher priority might be running too frequently or for too long, preventing the faulted task from getting enough processor time to complete within its watchdog period. This is more likely if the faulted task itself is relatively small.
- Check
- Review the priorities, periods, and scan times of all tasks. Check for any minor faults, specifically 'MinorFaultBits bit 6', which indicates task overlap and competition for processor time.
Next check: Begin by reviewing the program changes that immediately preceded the fault, focusing on any new or modified loops in the faulted task to ensure they have proper exit conditions.
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