Design a sequential function chart
40 Rockwell Automation Publication 1756-PM006I-EN-P - February 2018
Characteristic During the last scan of a step, this option does
Don’t scan Programmatic reset Automatic reset
Execution actions Only P and P0 actions execute. They execute
according to their logic.
All actions execute according to their logic.
P and P0 actions execute according to
their logic.
• All other actions execute in Postscan
mode.
• On the next scan of the routine, the P and
P0 actions execute in Postscan mode.
Retention of data values All data keeps its current values. All data keeps its current values.
Data reverts to its values for postscan.
• Tags to the left of [:=] assignments clear
to zero.
Method for clearing data Use P and P0 actions. Use either of these.
• Status bits of the step or action to
condition logic
• P and P0 actions
Use either of these.
• [:=] assignment (non-retentive
assignment)
• Instructions that clear their data during
postscan
Reset of a nested SFC A nested SFCs remains at its current step. A nested SFCs remains at its current step.
For the Restart Position property, if you
choose the Restart at initial step option,
then these occur.
• A nested SFC resets to its initial step
• The X bit of a stop element in a nested SFC
clears to zero
The default option for handling the last scan of a step is Don’t scan. With this
option, all data keeps its current values when the SFC leaves a step. This requires
you to use additional assignments or instructions to clear any data that you want
to turn off at the end of a step.
Follow these steps to turn off a device at the end of a step.
1. Make sure that the Last Scan of Active Steps property is set to the Don’t
scan option (default).
2. Use a P0 Pulse (Falling Edge) action to clear the required data. Make sure
that the P0 action or actions are last in the order of actions for the step.
During the last scan of the step, the Don’t scan option executes only P and P0
actions. The assignments and instructions of the actions execute according to their
logic conditions.
• The controller does not execute a postscan of assignments or instructions.
Use the Don’t Scan option
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