User manual

48 Publication 1756-RM093F-EN-P - January 2010
Chapter 5 Characteristics of Safety Tags, the Safety Task, and Safety Programs
Safety Task Limitations
You specify both the safety task period and the safety task watchdog.
The safety task period is the period at which the safety task executes.
The safety task watchdog is the maximum time allowed from the start
of safety task scheduled execution to its completion.
For more information on the safety task watchdog, see Appendix
C,
Reaction Times
.
The safety task period is limited to a maximum of 100 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
non-recoverable safety fault in the GuardLogix controller, occurs if the
safety task is triggered while it is still executing from the previous
trigger.
See Chapter
7, Monitor Status and Handle Faults, for more
information.
Safety Task Execution Details
The safety task executes in the same manner as standard periodic
tasks, with the following exceptions:
• The safety task does not begin executing until the primary
controller and safety partner have established their control
partnership and the coordinated system time (CST) is
synchronized. However, standard tasks begin executing as soon
as the controller transitions to Run mode.
• Although the configurable range of the requested packet interval
(RPI) for safety inputs and safety consumed tags is 1…100 ms,
safety input tags and safety-consumed tags are updated only at
the beginning of safety task execution. This means that even
though the I/O RPI can be faster than the safety task period, the
data does not change during safety task execution. The data is
read only once at the beginning of the safety task execution.