44 Rockwell Automation Publication 1756-RM093J-EN-P - April 2018
Chapter 5 Characteristics of Safety Tags, the Safety Task, and Safety Programs
• The safety task responds to mode changes (for example, Run to Program
or Program to Run) at timed intervals. As a result, the safety task may take
more than one task period, but always less than two, to make a mode
transition.
Use of Human-to-machine
Interfaces
Follow these precautions and guidelines for using HMI devices in SIL-rated
GuardLogix systems.
Precautions
You must exercise precautions and implement specific techniques on HMI
devices. These precautions include, but are not restricted to the following:
• Limited access and security
• Specifications, testing, and validation
• Restrictions on data and access
• Limits on data and parameters
For more information on how HMI devices fit into a typical SIL loop, see
Figure 1 on page 13
.
Use sound techniques in the application software within the HMI and controller.
While safety-unlocked and without a safety task signature, the controller
prevents simultaneous write access to safety memory from the safety task
and communication commands. As a result, the safety task can be held off
until a communication update completes. The time required for the update
varies by tag size. Therefore, safety connection and/or safety watchdog
timeouts could occur. (For example, if you make online edits when the safety
task rate is set to 1 ms, a safety watchdog timeout could occur.)
To compensate for the hold-off time due to a communication update, add 2
ms to the safety watchdog time.
When the controller is safety-locked or a safety task signature exists, the
situation described in this note cannot occur.
The safety task signature is required to operate in a SIL3 safety function.
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