IO-Link Sensor Is Online but the Value Is Wrong? Open the IODD
An online IO-Link sensor can deliver valid bytes that the PLC decodes incorrectly. The IODD and matching process-data documentation explain the field layout, units, and representation. Prove that mapping before applying an improvised scaling factor.
Original payload example; actual bit allocation comes from the device documentation.
What is happening?
IO-Link process data can contain several measurements and status bits in one payload. Field positions, signedness, bit lengths, and scaling depend on the device and selected configuration. A familiar integer-looking value is not enough evidence that the entire received word represents one measurement.
Check these five things
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Confirm device identity and the IODD revision against the installed sensor.
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Read the configured process-data layout, including any selectable variants.
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Separate measurement bits from status or switching bits exactly as documented.
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Apply the specified signedness, scale, and units in the PLC mapping.
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Compare several known values with the sensor's own indication during an approved test.
Worked example
An illustrative payload contains a measurement field and two status bits. Treating the whole word as one unsigned measurement makes the displayed value jump when a status bit changes. Correct field extraction resolves the interpretation without recalibrating the sensor.
Quick diagnostic reference
| Mapping item | Check |
|---|---|
| Field location | Byte and bit boundaries |
| Representation | Signedness and scale |
| Status | Quality and switching bits |
The mistake to avoid
Do not assume two sensors from the same vendor share the same process-data format. Use each product's documented mapping.
How to verify the fix
Record raw payloads and decoded values at multiple points, including a status transition, to demonstrate the mapping is correct.
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
ifm: understanding the IODD and process-data layout. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.