Allen-Bradley LOGIX 5000 Reference Manual page 462

Chapter 7 Equipment Phase Instructions
462
Major/Minor Faults
A major fault will occur if:
Instruction is called from outside an equipment phase
program.
If an Add-On Instruction used a PSC instruction and a non-equipment phase
program calls the Add-On Instruction, Logix Designer gives a warning. Check the
Add-On Instruction for this instruction and disallow it. See Index Through Arrays
for operand-related faults.
Execution
In structured text, instructions execute each time they are scanned. To limit the
scan of an instruction, use a qualifier of an SFC action, a structured text construct,
or both.
Condition/State Action Taken
Prescan No action taken.
Postscan No action taken.
EnableIn is false No action taken.
EnableIn is true The instruction executes.
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
Fault type Fault code
4 91