Rockwell Automation Publication 1756-UM022D-EN-P - May 2017 103
Develop Safety Applications Chapter 6
The safety task is a high priority. You specify the safety task period (in ms) and
the safety task watchdog (in ms). The safety task period is the period that the
safety task executes. The safety task watchdog is the maximum time allowed
from the start of safety task execution to its completion.
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
116 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.
TIP Prescan Execution
Beginning with firmware revision 30, the safety task watchdog does not trip
if the safety task prescan execution time exceeds the configured safety task
watchdog time. However, all safety I/O connections are closed and then re-
opened once the controller is in Run mode.
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