To allow companies to implement STEP standards, and at the same time use XML data, to exchange information, is required to transform EXPRESS models to XML technology. Identifying that need, ISO 10303 developed its Part 28 to detail a valid morphism of the structural information contained in the models. However, EXPRESS models also contain a great deal of semantics and behavioural information which also needs to be mapped, thus enabling more reliable implementations.
The STEP standard currently defines three neutral data exchange formats - ASCII text file (STEP Part 21), programming language APIs (STEP Part 22-27, 29), and XML (STEP Part 28).
Only a few relatively specialized applications, such as STEP translators and STEP repositories, use the API programming language, called Standard Data Access Interface (SDAI). Unlike the STEP Part 21 syntax, XML data is easily extensible and is supported by numerous inexpensive and widely used software tools. Thus, from the perspective of a typical programmer, it is easier to render XML data into forms that are suitable for human perusal.
ISO10303 Part 28 Transformation from EXPRESS to XML Schema, i.e. EXP2XSD, is a morphism that converts an EXPRESS schema into XML Schema language following the ISO10303 Part 28 recommendations. The EXP2XSD morphism, despite implementing the default transformation of EXPRESS models (input) to XSD (output), also enables the configuration of the morphism according to a pre-defined structure. Thus, the user (i.e. the company implementing the STEP AP) can configure the tool to choose among certain mapping parameters and obtain a schema more adapted to the manner that its own internal system handles XML information.
Since the degree of expressiveness of the EXPRESS language is significantly higher than the one available in XSD, it is recommended the usage of a complementary morphism, i.e. the EXP2SCH, to map the semantics that are left out of the EXP2XSD transformation (see Figure below).
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