Rockwell Automation Publication 1756-RM100F-EN-P - October 2018 111
Instruction Execution Chapter 6
Improved Math Instruction Accuracy
The implementation changed from a polynomial algorithm to an
industry-standard algorithm. The algorithm change, along with hardware
improvements, help to improve overall accuracy for the controllers.
This implementation affects these instructions: ACS/ACOS, ASN/ASIN,
ATN/ATAN, COS, LN, LOG, SIN, SQR, TAN, XPY.
Mitigation
Reverify any existing code that expects an exact result whenever the input is
close to values that produce zero, infinity, or asymptotic results.
SQR/SQRT Adjustment
The SQR/SQRT instruction now uses round-to-even type conversion of the
floating point result to integer destination. Previously, this operation truncated
the floating point result.
The new implementation results in behavior from the SQR instruction that
better matches the IEC standard math expectations.
Mitigation
N/A
Language Affected
Ladder Logic (RLL) Yes
Struc tured Text (ST) Yes
Function Blocks (FBD) Yes
Sequential Function Chart (SFC) Yes
(1)
(1) Only affects embedded Structured Text.
5580/5380 Controllers 5570/5370 Controllers
Because 2*PI is an estimated value for Rads, the result
does not equal 0.0.
Language Affected
Ladder Logic (RLL) Yes
Struc tured Text (ST) Yes
Function Blocks (FBD) Yes
Sequential Function Chart (SFC) Yes
5580/5380 Controllers 5570/5370 Controllers
Source - DINT
Dest - DINT
Source - DINT
Dest - DINT
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