Rockwell Automation Logix5000 Controllers Sequential Function Charts Programming Manual - Manual (Page 41)

Design a sequential function chart Chapter 1
Important: The postscan of an action actually occurs when the action goes from active to inactive. Depending on
the qualifier of the action, the postscan could occur before or after the last scan of the step.
As a general rule, the postscan executes instructions as if all conditions are FALSE.
For example, the Output Energize (OTE) instruction clears its data during
postscan.
Follow these steps to automatically turn off (clear) devices at the end of a step.
1. In the Last Scan of Active Steps property, select the Automatic reset
option.
2. To turn off a device at the end of the step, control the state of the device
with an assignment or instruction.
• [:=] assignment (non-retentive assignment)
• Output Energize (OTE) instruction in a subroutine
Some instructions do not follow the general rule during postscan. For a
description of how a specific instruction executes during postscan, see these
publications.
• Logix5000 Controllers General Instructions Reference Manual , publication
1756-RM003
• Logix5000 Controllers Process and Drives Instructions Reference Manual ,
publication 1756-PM006
• Logix5000 Controllers Motion Instruction Set Reference Manual ,
publication MOTION-RM002
Here is an example that uses a non-retentive assignment to control a conveyor. It
turns on a conveyor at the start of a step and automatically turns off the conveyor
when the step is done.
Example
Automatically Clear Data
• This action turns on the conveyor. When conveyor_state turns on, the conveyor turns on.
• When the SFC leaves the step, conveyor_state turns off. This turns off the conveyor.
Rockwell Automation Publication 1756-PM006F-EN-P - October 2014 41