Rockwell Automation Publication 1756-RM100F-EN-P - October 2018 133
Instruction Execution Chapter 6
Store NAN in an Integer
When writing/propagating NAN values 5580 and 5380 controllers offer
standardized results.
This implementation affects all instructions that can produce a Floating Point
value and store in an integer location.
Mitigation
Revise your application if it was checking for the special value of '-1' to indicate
a '-NAN' result.
Compare NAN Values
NAN does not compare true with ANY value (even another NAN). Any
EQU, GEQ, GRT, LEQ, or LES comparison with at least one NAN input is
always false, and any NEQ input with at least one NAN input is always true.
Now offers standardized results when you use NAN in comparisons.
This implementation affects these instructions: CMP, EQU, GEQ, GRT,
LEQ, LES, NEQ.
Mitigation
Revise any logic that relies on the old, non-standard result from an NAN
comparison. Also, it is now easier to test for a NAN result. This value is the
only value that provides a true result for a 'NEQ TagA TagA' comparison.
Language Affected
Ladder Logic (RLL) Yes
Struc tured Text (ST) Yes
Function Blocks (FBD) Yes
Sequential Function Chart (SFC) Yes
(1)
(1) Only affects the embedded Structured Text.
5580/5380 Controllers 5570/5370 Controllers
Writing +/- NAN to an integer always results in the
value 0 be stored.
Writing NAN to an integer results in either -1 or 0
depending on the sign bit for NAN.
Language Affected
Ladder Logic (RLL) Yes
Structured Text (ST) No
Function Blocks (FBD) Yes
Sequential Function Chart (SFC) No
5580/5380 Controllers 5570/5370 Controllers
All compares with NAN are false except for NEQ. NAN values compare as equals and their signs are
accounted for.
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