Numeric Comparisons¶

Numbers in PLC code look simple, but their type matters. Integer values behave differently from floating-point values.

Integers¶

Integer comparisons are exact:

xboCounterReached := (inCounter = 10);

This is safe because Int, UInt, DInt, and similar integer types represent whole numbers.

REAL Values¶

REAL values should normally be compared with a tolerance.

_reAbsValue := ABS(reMeasuredValue);
xboValueIsNearZero := (_reAbsValue < 0.001);

Avoid this:

xboValueIsZero := (reMeasuredValue = 0.0);

Analog scaling, filtering, and calculations often produce small rounding differences. Comparing a REAL to exact zero can create conditions that almost never become true.

Range Checks¶

For analog process values, write comparisons as ranges where possible.

xboTemperatureInRange :=
    (reTemperatureDegC >= reMinimumTemperatureDegC)
    AND (reTemperatureDegC <= reMaximumTemperatureDegC);

For live control logic, prefer named limits over magic numbers.

xboSpeedNearStandstill := (ABS(reActualSpeedRpm) < reStandstillToleranceRpm);

This tells the next engineer that the tolerance is a design decision, not a random constant.

Numeric Checklist¶

Use exact equality for integers when exact equality is the real condition.
Use tolerances for REAL near-zero and near-target checks.
Put units in variable names when units matter.
Use named limits and tolerances instead of unexplained constants.
Be careful when converting between integer telegram values and engineering units.