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Rockwell Automation Publication 1756-RM100F-EN-P - October 2018
Chapter 6
Instruction Execution
AVE and STD Instruction Accuracy
The internal data type used for internal calculations of AVE and STD
instruction now has greater precision. This improves the accuracy of the results
that the AVE and STD instructions generate.
Mitigation
N/A
BTD, FAL, FSC, and CMP No Longer Generate Math Status
The BTD, FAL, FSC, and CMP instructions no longer impact math status
flags (S:Z, S:N, S:V) because they do not write a value to a discrete destination.
If the minor overflow reporting feature is enabled, BTD, FSC, and CMP
instructions report this kind of fault. The FAL no longer generates a minor
fault on overflow because the ER bit is set and the operation is aborted.
This removes the expectation that math status has a value for BTD, FAL, FSC,
and CMP instructions.
Mitigation
N/A
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
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
There are several situations in which math status flags
are generated but you cannot determine exactly where
they were generated.
One case is with the FAL instruction. If the expression is
repeated 10 times, you cannot tell which instance caused
the flag.
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