Rockwell Automation Publication 1756-RM100F-EN-P - October 2018 123
Instruction Execution Chapter 6
Subscript Expressions
Subscript expressions are treated separately from instructions. If an overflow
occurs during their evaluation, the overflow can be recorded as a minor
overflow fault but always generates a major fault. The major fault indicates that
an out-of-range condition exists.
Subscript expressions behave much more predictably:
• REAL operands/operators are no longer permitted.
• Calculation of subscript expressions no longer silently produces
invalid results.
• Overflow conditions are detected and produce a Major
Recoverable Fault.
Mitigation
Review all subscript expressions in your application to make sure they cannot
produce an overflow result, for example, a divide-by-zero. Update any major
fault recovery logic in your application to handle this new fault appropriately.
Language Affected
Ladder Logic (RLL) Yes
Struc tured Text (ST) Yes
(1)
(1) Structured Text does not update math flags but
REALs in the expression and the major fault is
not permitted if the calculated index that is out
of range affects ST.
Function Blocks (FBD) No
Sequential Function Chart (SFC) No
5580/5380 Controllers 5570/5370 Controllers
In these controllers, math errors in subscript calculations
do not impact math status flags in any way. Overflow
minor faults for subscript expressions are reported if
enabled.
A major fault is generated if an overflow occurs to
indicate that the index was not computed normally.
In these controllers, a subscript calculation changes the
value of a math status flag. The change makes it
impossible to identify if the instructions or evaluation of
a subscript expression used with the instruction during
operand address processing caused the actual error.
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