Operator Enters 12.34, but the PLC Gets 12.3? Trace Precision End to End
Displayed decimal places, accepted entry precision, and stored precision can differ. Trace the number through the entire HMI-to-controller path.
Original precision examples; entry and storage behavior are component-specific.
What is happening?
Formatting changes presentation, while rounding or type conversion changes the value. A numeric field may show fewer decimals without losing stored precision, or a commit script may actively round before writing. These cases require different corrections.
Check these five things
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Record the operator entry before commit.
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Inspect the written tag value and data type.
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Check formatting separately from conversion logic.
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Review engineering-unit scaling and permitted increments.
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Test boundary and half-step values in a test project.
Worked example
A teaching screen displays 12.3 while its tag stores 12.34; that is presentation rounding. Another screen writes 123 into an integer representing tenths, losing the hundredth by design. The screen appearance alone cannot distinguish these paths.
Quick diagnostic reference
| Layer | Question |
|---|---|
| Entry | What value is accepted? |
| Conversion | What value is written? |
| Display | What precision is shown? |
The mistake to avoid
Adding decimal places to the label does not restore precision already lost during conversion. The allowed process resolution must be defined explicitly.
How to verify the fix
Compare requested, stored, and displayed values across the permitted range. Confirm invalid precision is handled clearly for the operator.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Inductive Automation: Format transforms. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.