User manual

Publication 1756-RM093F-EN-P - January 2010 49
Characteristics of Safety Tags, the Safety Task, and Safety Programs Chapter 5
• Safety input values are frozen at the start of safety task
execution. As a result, timer-related instructions, such as TON
and TOF, will not update during a single safety task execution.
They will keep accurate time from one task execution to
another, but the accumulated time will not change during safety
task execution.
• For standard tags that are mapped to safety tags, the standard
tag values are copied into safety memory at the start of the
safety task and do not change during safety task execution.
• Safety output tag (output and produced) values are updated at
the conclusion of safety task execution
• 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.
Safety Programs
A safety program has all the attributes of a standard program, except
that it can be scheduled only in the safety task. A safety program may
also define program-scoped safety tags. A safety program may be
scheduled or unscheduled.
A safety program can contain only safety components. All of the
routines in a safety program are safety routines. A safety program
cannot contain standard routines or standard tags.
ATTENTION
This behavior differs from standard Logix task execution,
but is similar to PLC or SLC behavior.
IMPORTANT
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.