Description Methods

The STEP standard contains the description method, with which all defined data models have to be specified: EXPRESS. Around this data modelling language a series of software tools has been developed, which are based on the 10-series of the ISO documents.

ISO 10303	Name	Status: 06/01
1	Overview and fundamental principles	IS
11	EXPRESS language reference manual	IS
12	EXPRESS-I language reference manual	IS
14	EXPRESS-X language reference manual	CD

IS - International Standard
CD - Committee Draft

Latest Status see: http://www.nist.gov/sc5/soap/

For the consistent, contradiction-free and semantically explicit description of the product model in STEP, the formal description language EXPRESS and its graphical representation EXPRESS-G have been defined.

EXPRESS is a Part of STEP and has been published with the number ISO 10303-11.

The data modelling language EXPRESS is the first international standard for the specification of data models. Due to an object-oriented approach it is possible to define

		Objects (entities) with properties (attributes)
		Inheritance rules
		Integrity conditions (Local rules for objects and global rules for overall object characteristics)
		Object classes (Schemas)
		Relationships between object classes (Schema-Interoperability)

EXPRESS is a specification language (not a programming language) for the logical description of information models. EXPRESS has object-oriented characteristics as well as such defined through the entity relationship method. It enables the formal, unambiguous and complete description of a (static) product model through objects, relations and conditions.

Within the ISO 10303 (STEP) standard, all data models (ranging from the Integrated Resources to the Application Protocols) are described in EXPRESS. Since EXPRESS is also qualified for information modelling - independent from the models defined in STEP - this specification method is also applied outside the STEP world.

The following diagram exemplary shows the EXPRESS description of circles:

The EXPRESS description defines the classes Point and Circle in an object-oriented manner. If occurrences (real objects) of these class definitions are created, instances of the classes are generated. If these instances are to be exchanged, e.g. between two CAD systems, a so-called physical file (ISO 10303-21) is used as echange format.

EXPRESS as a whole is a very complex language, which is built from simple and manageable constructs.

Schema
A schema is a collection of information elements, which can be - as whole or separately - used by other schemas. Each information element has to be specified in a schema.

Entity
An entity defines an object - consisting of properties and underlying definable rules. Each entity has a unique name within the schema. An inheritance concept is supported by sub-super-relations.

Attributes
Attributes are properties of objects. They are defined by a name and a type. Additional characteristics (e.g. aggregate, significance (optinal, mandatory), derivaton rules) can also be specified.

Type
EXPRESS supports basic types (Integer, Real, String, Boolean, Logical) as well as complex data types (Enumeration, Select, Complex).

Rules
The description of conditions for the regularity of an instance, i.e. the occurrence of an entity, can be related locally to a single entity within its definition as well as globally to the totality of entities in the schema definition.

The various possibilities to specify constants, functions, procedures and executable commands extend the definition possibilities for objects.

For the description language EXPRESS, there is also a graphical notation for information modelling - EXPRESS-G.

EXPRESS-G supports a subset of EXPRESS and, by its visual representation in compact form, furthers the understanding of the data model.

The basic elements of EXPRESS-G are shown in the following overview:

EXPRESS-X is a method for the description of object mappings between defined EXPRESS schemas, which contain common semantic elements, but are defined in different ways.

Main application areas for EXPRESS-X are:

		Mappings between schemas on different levels of abstraction, which are common in the development of STEP application protocols, in which the application-oriented objects need to be mapped onto the basic model constructs in the Integrated Resources.
		Mappings of company-specific models onto standardized STEP models and vice versa ? a fundemental task for every STEP processor.
		Different views of a data model, i.e. restriction to a defined subset or the use of alternative description formats (object- or attribute names, language).