Allen-Bradley ControlLogix 5570 - Improved Math Instruction Accuracy; SQR;SQRT Adjustment

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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.