Rockwell Automation Publication 1756-RM012B-EN-P - April 2018 39
Characteristics of Safety Tags, the Safety Task, and Safety Programs Chapter 5
Safety Task Execution Details
The safety task executes in the same manner as standard periodic tasks, with
the following exceptions:
• Safety input tags and safety-consumed tags are updated only at the
beginning of safety task execution. This process means that even though
the I/O RPI can be faster than the safety task period, the data in the
Safety Input tag only updates once at the beginning of each safety task
execution. Safety input and consumed packets that arrive after the start
of the safety task are buffered until the next execution of the safety task.
• Time is frozen at the start of safety task execution. As a result, timer-
related instructions, such as TON and TOF, are not updated during a
safety-task execution. They keep accurate time from one task execution
to another, but the accumulated time is not changed during safety task
execution.
• For standard tags that are mapped to safety tags, the standard tag values
are copied to the safety tags at the start of the safety task.
– The standard tag is free to continue changing.
– User code can change the safety tag within the safety task, but the
change is not reflected back to the standard tag.
• Safety output tag values can be changed during the safety task scan by
the safety application code of the user; the final value is transmitted to
safety modules at the end of the safety task scan. Likewise, safety
produced values are transmitted to consuming safety controllers at the
end of the safety task scan.
ATTENTION: This behavior differs from standard Logix task
execution.
IMPORTANT The addition of more mapped tags can increase the scan time.
To Next Page
Loading...
Need help?
General
