Fault Handling During Prescan And Postscan - Rockwell Automation Logix 5000 Programming Manual

Fault handling during
prescan and postscan
• The Add-On Instruction stops running at the instruction that caused
the fault. This means that the remainder of the scan mode routine does
not run.
• If the fault clears, execution resumes at the instruction following the
top-level Add-On Instruction invocation. For example, assume the
Add-On Instruction myAoi in Example 2 invokes a nested Add-On
Instruction myNested, which invokes another nested Add-On
Instruction inner. Furthermore, assume that an instruction inside of
inner causes a fault. If the fault clears, execution resumes with the
MOV instruction (the remainder of inner does not execute; the
remainder of myNested does not execute; and the remainder of myAoi
does not execute.)
• During prescan:
• The Logic routine runs (in prescan mode).
• The Prescan routine runs (in normal scan mode).
• During postscan:
• The Logic routine runs (in postscan mode).
• The Postscan routine runs (in normal scan mode).
If a fault occurs while processing the Logic routine, the Add-On Instruction
aborts (the remainder of the Logic routine does not run and the pre-scan and
post-scan routines do not run). If the fault clears, execution resumes at the
instruction following the top-level Add-On Instruction invocation.
See also
Create a fault routine for a program
The behavior of each instruction varies depending on the mode in which it
runs–true, false, prescan, or postscan. For details about what a specific
instruction does in each mode, see the
Instructions Reference
• Prescan provides a system-defined initialization of the user program
when the controller switches from program mode to run mode.
• Postscan provides a system-defined re-initialization of the logic
invoked from an SFC action, when the action shuts down (if SFCs are
configured for Automatic Reset).
If an array index is out of range during prescan, the controller could generate
a major fault. There are a number of ways this could happen: the controller
loses power, encounters a major fault, or the project is saved while online.
Because the user program, during prescan and postscan, cannot assign values
to tags, the only way to correct these issues is to manually initialize the index
variables using the Logix Designer application or to write a fault handler to
ignore the array faults during prescan. To reduce the need for manual
Rockwell Automation Publication 1756-PM014N-EN-P - March 2022
on page 13
Logix 5000 Controllers General
Manual, publication number 1756-RM003.
Chapter 1 Major Faults
11