Rockwell Automation Publication 1756-RM100F-EN-P - October 2018 125
Instruction Execution Chapter 6
Math Status Flags are Valid Only in One Rung
On 5570 and 5370 controllers, the math status flag reflects any math event that
occurs anywhere in the routine that performs the evaluation. The ability to
identify the instruction that caused the error is a challenge. Also, other
instructions that execute correctly after the offending instruction can clear any
math status flag errors.
On 5580 and 5380 controllers, the math status flag must reside after an
instruction that can set the math status flag. The math status flag reflects the
math status that occurred only on the previous instruction that can set the flag
within this rung.
This removes the ambiguity as to which instruction caused the math status flag
result. The controller does not waste CPU time generating math status flag
values if they are not examined.
This implementation affects all math status producer/consumer instructions.
Mitigation
Rearrange logic that evaluates math status flags so that the logic is on the same
rung as the flag-generating instruction, and there are no intervening
instructions that could affect the flags.
Language Affected
Ladder Logic (RLL) Yes
Structured Text (ST) No
Function Blocks (FBD) No
Sequential Function Chart (SFC) No
5580/5380 Controllers 5570/5370 Controllers
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