6 ISA-95 Data Representation Model ToC Previous Next

6.1 General ToC Previous Next

ISA-95 is an abstract specification that has no data model. The B2MML implementation further refined the ISA-95 standard to include a set of standard data types. They selected the UN/CEFACT for this data type definition. These standard data type are now the default data type of ISA-95 Models In order to support transformation between the B2MML mapping and the OPC UA mapping of the ISA-95 Information models a mapping of UN/CEFACT is required.

The UN/CEFACT core component types are a common set of types that define specific terms with semantic meaning, such as the meaning of a quantity, currency, amount, identifier, and language. The UN/CEFACT core components were defined in a core components technical specification (CCTS) developed by the ebXML project now organized by UN/CEFACT and ISO TC 154. This definition also includes supplementary components which provide metadata that refine the value domain. See CCTS for additional information.

This standard defines a data representation model that is compatible with CCTS. The value of a content component is represented by the Value Attribute of a Variable, and the supplemental components that are defined in CCTS are represented as Properties of the DataType. In addition these supplemental Properties may optionally be defined for Variables.

Most of the CDTs, defined in CCTS, are defined as CDT DataTypes in this standard but some CDTs are mapped to OPC UA Built-in DataTypes directly.

All supplemental Properties defined as part of a CDT DataType include the definition of a default value. This allows a Server to not require the addition of the optional Properties, while still allowing a client to obtain the value for the CDT DataType supplemental Properties. However, for some DataTypes or VariableTypes the supplemental Properties may have different values on instances than the default value. For example, a Unit of Measure type depends upon measurement data. In this case, the Server shall duplicate the supplemental Property from the CDT DataType and to the Variable. A client shall try to read the supplemental Property from the Variable first, if the Property does not exist on the variable than the Client shall read the supplemental Property from the CDT DataType.


Figure 16 – Basic data representation model

Third parties may define VariableTypes that rely on CDT DataTypes to enhance data representation methodology. This approach will allow the use of generic clients that support performing actions based on a type system (programming against a type system). Figure 17 illustrates a type definition based on a CDT type. This is an enhanced feature but this standard recommends using the enhanced representation instead of the basic representation model.


Figure 17 - Enhanced data representation model

Figure 18 illustrates an example of the Amount and the Measure types defined in CCTS. In this example, a value of Currency Property is commonly used in UA Server but a value of Unit Property relates to a measurement value that is temperature.


Figure 18 - Example of CDT implementation

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