Rockwell Automation Logix5000 Controllers Add-on Instructions Programming Manual - Manual (Page 51)

Defining Add-On Instructions Chapter 2
Prescan routine
When the controller transitions from Program mode to Run mode or when the
controller powers up in Run mode, all logic within the controller is executed in
Prescan mode. During this scan, each instruction may initialize itself and some
instructions also initialize any tags they may reference. For most instructions,
Prescan mode is synonymous with scanning false. For example, an OTE
instruction clears its output bit when executed during Prescan mode. For others,
special initialization may be done, such as an ONS instruction setting its storage
bit during Prescan mode. During Prescan mode, all instructions evaluate false so
conditional logic does not execute.
The optional Prescan routine for an Add-On Instruction provides a way for an
Add-On Instruction to define additional behavior for Prescan mode. When a
Prescan routine is defined and enabled, the Prescan routine executes normally
after the primary logic routine executes in Prescan mode. This is useful when you
want to initialize tag values to some known or predefined state prior to execution.
For example, setting a PID instruction to Manual mode with a 0% output prior to
its first execution or to initialize some coefficient values in your Add-On
Instruction.
When an Add-On Instruction executes in Prescan mode, any required parameters
have their data passed.
• Values are passed to Input parameters from their arguments in the
instruction call.
• Values are passed out of Output parameters to their arguments defined in
the instruction call.
These values are passed even when the rung condition is false in Ladder Diagram
or when the instruction call is in a false conditional statement in Structured Text.
When Function Block Diagram routines execute, the data values are copied to all
wired inputs and from all wired outputs, whether or not the parameters are
required.
Follow these steps to create a Prescan routine.
1. In the Controller Organizer, right-click an instruction and select Open
Definition.
2. Click the Scan Modes tab.
Rock well Automati on Publication 1756- PM010F-EN-P - Oct ober 2014 51