Allen-Bradley Micro800 General Instructions Manual page 261

Programmable controllers

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Example: Comparing Real Values using Subtraction (-) ABS, and Less than

The Real data type is not recommended when comparing values for equality

because of differences in the way numbers are rounded. Two output values may

appear equal in a Connected Components Workbench display, but will evaluate as

For example, 23.500001 compared to 23.499999 will both display as 23.5 in the

variable input display, but will not be equal in the controller.

To test whether two Real data type values are equal, you can use a Subtraction

instruction to get the difference between the values and then determine if the

difference is Less Than an established precision value. See the following LD

program example for comparing two Real data type values.

Equal (=) operator ST language example

(* ST Equivalence: *)

aresult := (10 = 25); (* aresult is FALSE *)

mresult := ('ab' = 'ab'); (* mresult is TRUE *)

Rockwell Automation Publication 2080-RM001D-EN-E - February 2015