Use the automatic reset option – Rockwell Automation Logix5000 Controllers Sequential Function Charts Programming Manual User Manual
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Chapter 1 Design a sequential function chart
When the action is not on its last scan (conveyor_start.Q =1), this statement turns on conveyor_state. When
conveyor_state turns on, the conveyor turns on.
On the last scan of the action (conveyor_start.Q =0), this statement turns off conveyor_state. When
conveyor_state turns off, the conveyor turns off.
You can also use a P0 Pulse (Falling Edge) action to clear data (see
on
). This example uses an action to turn on a
conveyor at the start of a step. A different action turns off the conveyor at the end
of the step.
Example
This action turns on the conveyor. When conveyor_state turns on, the conveyor turns on.
Before the SFC leaves the step, the P0 action turns off the conveyor. On the last scan of the step,
conveyor_state turns off. This turns off the conveyor.
Use the automatic reset option
Automatic reset provides a system-defined cleanup of actions (known as postscan)
when they are shut down when any of the following occur.
• transition out of the associated step
• reset of a stored action
• reset of an SFC routine
Postscan is similar to prescan in that most instructions are executed as if they are
FALSE. Some instructions have specific postscan behavior.
• In RLL, OTE instructions are turned off and non-retentive timers are reset.
• In structured text, the destination of a non-retentive assignment "[:=]" is
cleared.
• A JSR instruction invokes its subroutine but parameters are not passed and
the logic in the subroutine is executed in postscan mode.
• An Add-On Instruction executes its logic routine in postscan mode and
then executes its postscan logic in normal mode (if a postscan routine is
configured).
• Any nested SFC (SFC that an action calls as a subroutine) is reset.
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Rockwell Automation Publication 1756-PM006F-EN-P - October 2014