Safety Task Requirements

In Logix SIS applications, the safety task for
ControlLogix®
 5590 controllers has the following requirements.
Safety Task Requirements
Parameter
Requirement
Period
Be sure that the safety task has enough time to execute its logic before it restarts:
  • If Front Port Crossload mode is enabled, enter a value of 5…500 ms.
  • If Front Port Crossload mode is disabled, enter a value of 7…500 ms.
Priority
A value of 1 is enforced to make the safety task the highest priority task.
Watchdog
Enter a value of 5…500 ms.
To avoid a fault that can shut down the safety task during a loss of redundancy, enter a value that is at least 5 ms higher than your safety task max scan time. For example, if your safety task max scan time is 5 ms, set your safety task watchdog to at least 10 ms. There is no fault to indicate that the safety task is consuming too much scan time.
IMPORTANT: When determining the safety task max scan time, we recommend that you perform several disqualifications and switchovers to get a more accurate time measurement that accounts for system dynamics.
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