38 Rockwell Automation Publication 1756-RM012B-EN-P - April 2018
Chapter 5 Characteristics of Safety Tags, the Safety Task, and Safety Programs
The Safety Task
Creation of a GuardLogix project automatically creates one safety task. The
safety task has these additional characteristics:
• GuardLogix controllers are the only controllers that support the safety
task.
• The safety task cannot be deleted.
• GuardLogix controllers support one safety task.
• Within the safety task, you can use multiple safety programs that are
composed of multiple safety routines.
• You cannot execute standard routines from within the safety task.
The safety task is a periodic task, and you must configure the period and the
priority of the safety task. The safety task can be interrupted according to the
same rules as standard tasks (including interruptions by the motion task, which
is always a higher priority than any user task).
Configuring the safety task with a higher priority (lower number) can reduce
fluctuations in execution time, which can allow a lower setting for the safety
task watchdog, which improves the reaction time of the safety system.
Safety Task Limitations
You specify both the safety task period and the safety task watchdog. The
safety task period is the time interval between successive executions of the
safety task. The safety task watchdog is the maximum time that is allowed from
the start of safety task scheduled execution to its completion.
For more information on the safety task watchdog, see Appendix C
on
page 79
.
The safety task period is limited to a maximum of 500 ms and cannot be
modified online. Make sure that the safety task has enough time to finish
before it is triggered again. Safety-task watchdog timeout, a nonrecoverable
safety fault in the GuardLogix controller, occurs if the Safety Task does not
finish before the watchdog expires.
For more information, see Chapter 7
on page 63.
IMPORTANT Only the instructions that are listed in Appendix A
on page 69 can be used in
the safety task.
IMPORTANT Large amounts of mapped safety tags or large amounts of safety produce/
consume tag data can cause fluctuations in the safety task scan time of the
controller.
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