IEC Common Data Dictionary (CDD)

The IEC's machine-readable dictionary of units, quantities, and product properties for industrial automation.

cdd.iec.ch IEC/TS 62720

How IEC CDD uses UnitsML

The IEC Common Data Dictionary (CDD) is the IEC’s central repository of standardized product ontologies, quantities, and units. Its units are defined by IEC/TS 62720 — Identification of units of measurement for computer-based processing — which assigns stable, machine-readable identifiers to every unit used in electrotechnical and industrial standards.

UnitsML provides the semantic foundation for that representation. In the industrial automation ecosystem, ECLASS provides IRDIs (International Registration Data Identifiers) based on UnitsML. These ECLASS unit identifiers have a 1:1 relationship with the units defined in IEC/TS 62720 and published in the IEC CDD. In practice, this means that a unit referenced in an ECLASS product description can be resolved to the same concept in the IEC CDD, and vice versa.

The OPC Foundation’s OPC UA standard demonstrates this in practice. OPC UA maps IEC/TS 62720 units to its own EUInformation structure, using the IEC CDD as the authoritative source. Because UnitsML aligns with IEC/TS 62720 through the ECLASS mapping, OPC UA systems can exchange unit information with UnitsML-based systems without loss of meaning.

What UnitsML gives IEC CDD

  • Semantic grounding — UnitsML’s dimensions, quantities, and systems provide the formal model that underlies IEC/TS 62720’s unit identifiers.
  • ECLASS harmonization — the 1:1 mapping between UnitsML-based ECLASS IRDIs and IEC CDD units enables cross-vocabulary data exchange.
  • SI alignment — UnitsML’s basis in the BIPM SI Brochure and Digital SI Framework keeps IEC CDD units anchored to the international measurement system.
  • Machine readability — XML-based definitions let automation systems validate and convert units programmatically.
  • Standards credibility — IEC CDD can reference a mature, NIST/OASIS/CalConnect standard rather than maintaining a parallel, proprietary unit model.

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