Rockwell Automation Logix5000 Controllers Major, Minor, and I/O Faults Programming Manual - Manual (Page 11)

Major faults Chapter 1
The logic inside an Add-On Instruction generates a fault. While the logic of an
Add-On Instruction may look like a subroutine, it is not–the Add-On Instruction
is an instruction. When a fault occurs inside an Add-On Instruction, the
remainder of the Add-On Instruction is aborted. If the fault is cleared, execution
resumes with the MOV.
Important points regarding Add-On instructions
• 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 is cleared, 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 is cleared, execution resumes with the MOV (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 is cleared, execution resumes at the instruction
following the top-level Add-On Instruction invocation.
Fault handling during prescan and postscan
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 Logix5000 Controllers General Instructions Reference
Manual , publication 1756-RM003 .
Rockwell Automation Publication 1756-PM014G-EN-P – October 2014 11