Category: PLC Basics
PLC State Number Is Invalid? Your CASE Needs a Defined Fallback
An unexpected state value should produce a defined response.
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One FOR Loop Makes the PLC Unresponsive? Check the Worst-Case Bound
A loop that looks short can perform too much work in one task execution.
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PLC Position Wraps Below Zero? Check Negative Remainder Rules
A remainder operator does not necessarily turn a negative value into a positive cyclic position.
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PLC Division Faults Only With No Product? Check the Zero Denominator
An empty batch can invalidate a calculation that works throughout production.
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PLC Value Is Neither Above Nor Below the Limit? Check for NaN
Ordinary range comparisons are not a complete validity test for floating-point data.
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PLC REAL Total Stops Growing? The Increment May Be Too Small
A floating-point total can stop responding to small additions while remaining far below its maximum magnitude.
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PLC Timer Runs a Thousand Times Too Long? Inspect Duration Conversion
A duration value needs both a numerical representation and a unit.
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One Missing Pair of Parentheses Changes PLC Logic? Prove the Truth Table
Mixed AND and OR expressions can mean something different from the sentence an engineer intended.
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Your PLC Guard Condition Is True but the Expression Still Faults? Check Evaluation
A guard expression protects a later operation only if evaluation rules actually skip that operation.
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PLC Enumeration Starts in the Wrong Mode? Define Its Initial Member
An enumeration improves readability but does not remove initialization requirements.
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