Rockwell Automation Publication 1769-UM022C-EN-P - June 2018 143
Develop Safety Applications Chapter 9
The safety task period is limited to a maximum of 500 ms and cannot be
modified online. Be sure that the safety task has enough time to finish logic
execution before it is triggered again. If a safety task watchdog timeout occurs,
a nonrecoverable safety fault is generated in the safety controller.
The safety task period directly affects system reaction time.
The GuardLogix 5570 and Compact GuardLogix 5370 Controller Systems
Safety Reference Manual, publication 1756-RM099
, provides detailed
information on calculating system reaction time.
Safety Task Execution
The safety task executes in the same manner as a standard periodic task, with
the following exceptions:
• The safety task does not begin executing until the primary controller
and safety partner establish their control partnership. (Standard tasks
begin executing as soon as the controller transitions to Run mode.)
• All safety input tags (inputs, consumed, and mapped) are updated and
frozen at the beginning of safety task execution.
See page 156
for information on safety tag mapping.
• Safety output tag (output and produced) values are updated at the
conclusion of safety task execution.
Safety Programs
Safety programs have all the attributes of standard programs, except that they
can only be scheduled in the safety task and can only contain safety
components. Safety programs can only contain safety routines. One safety
routine must be designated as the main routine, and another safety routine can
be designated as the fault routine.
Safety programs cannot contain standard routines or standard tags.
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