1756-PM003H-EN-E, SFC and ST Programming Languages ...

4-32 Design a Sequential Function Chart
Use the Programmatic Reset Option
An optional method to programmatically turn off (clear) devices at the
end of a step is to execute all actions on the last scan of the step. This
lets you execute your normal logic as well as turn off (clear) devices
at the end of a step.
1. In the Last Scan of Active Steps property, select the
Programmatic reset option:
2. Clear the required data using any of these methods:
• To your normal logic, add logic that clears the required data.
Use the LS bit of the step or the Q bit of the action to
condition the execution of the logic.
• 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 Programmatic reset option
executes all assignments and instructions according to logic
conditions.
• The controller does not postscan the assignments or
instructions.
• When the SFC leaves the step, all data keeps its current value.
This example uses a single action to turn on and off a conveyor. The
LS bit of the step conditions the execution of the logic. See “SFC_STEP
Structure” on page 4-8.
EXAMPLE
Use the Programmatic Reset Option and the LS Bit
When the step is not on its last scan (conveyor_fwd.LS = 0), this
statement turns on conveyor_state. When conveyor_state turns
on, the conveyor turns on.
On the last scan of the step (conveyor_fwd.LS =1), this
statement turns off conveyor_state. When conveyor_state turns
off, the conveyor turns off.
Publication 1756-PM003H-EN-E (excerpt from 1756-PM001H-EN-P) - August 2005