Rockwell Automation 1756-SPESMNRMXT GuardLogix Controllers User Manual - Manuel (Page 91)

Rockwell Automation Publication 1756-UM020I-EN-P - August 2012 91
Develop Safety Applications Chapter 6
The safety task should be 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 at
which 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 Controller Systems Safety Reference Manual, publication
1756-RM093 , 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 102 for information on safety tag mapping.
• Safety output tag (output and produced) values are updated at the
conclusion of safety task execution.