Rockwell Automation Logix5000 Controllers Major, Minor, and I/O Faults Programming Manual - Manual (Page 12)

Chapter 1 Major faults
• Prescan provides a system-defined initialization of the user program when
the controller is switched from program mode to run mode.
• Postscan provides a system-defined re-initialization of the logic invoked
from an SFC action, when the action is shut 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 intervention, the Logix Designer
application includes an internal fault handler. This handler is only used during
prescan and only clears array faults (type 4, fault codes of 20 of 83).
Tip: Array faults are not ignored during postscan because the user program controls index tag values when an action is
shut down.
12 Rockwell Automation Publication 1756-PM014G-EN-P – October 2014