Configuration Control for Product Documentation
A Way of Integrating STEP & SGML
Page 5
ISO 10303 - STEP (Standard for the Exchange Product Model data) is a
compound standard. Each module of this component standard represents a standard
in its own right.
- The various modules of STEP can be grouped in three orthogonal domains:
Figure 5. STEP data
type architecture
- methodologies and language specifications. EXPRESS (ISO 10303-11) is an
example within this group of STEP standards [ISO1]
- industry standard data models, called Application Protocols (APs).
- implementation methods. These groups of standards specify methods of
instantiation for STEP models for the purpose of data exchange. STEP currently
addresses two principle means of data exchange: data exchange on the basis of
transferring physical files, and an independent data access method, the Standard
Data Access Interface (SDAI) [ISO2, ISO3].
Looking at STEP from a metadata architectural view, shows that it provides a
3-level architecture, the language EXPRESS, the information models, and the
business data as instances of EXPRESS defined information models. While the
development of STEP information models is focused on the exchange of product
data, EXPRESS can be used also for the development of information models, which
allow data sharing and exchange. The relationship of the above three layers are
illustrated in Figure 5. STEP data type architecture.
ISO 8879 - SGML The Standard Generalized Markup Language provides a
standard declarative language for writing document type definitions (DTDs). In
contrast to the STEP standardisation effort, the SGML standard does not aim at
standardizing at the application level. That is SGML does not look at
standardizing at the DTD level, but rather provides an enabling standard
for industry specific agreements, which may develop into de-facto standards
(and occasionally de jura). Nevertheless, all SGML encoded user data
(document instances) have syntactical structures, which must conform to
specified DTDs. For example, an SGML-based authoring environment will ensure
that documents produced are consistent with the application's DTD.
Figure 6. SGML data
type architecture.
In summary, the approach for neutral information within SGML is similar to
the approach in STEP (as illustrated in Figure 6. SGML data type
architecture..)
- SGML represents a declarative language for declaring a document model and
can be considered to be the same level of the metadata architecture as EXPRESS.
- Application dependent de-facto industry standard document models,
written in SGML are referred to as Document Type Definitions (DTDs).
- The user document, which conforms to a DTD, is referred to as a SGML
instance.
Figure 7. Levels of STEP &
SGML interoperability
Figure 7. Levels of STEP & SGML interoperability shows how the
levels of STEP and SGML correlate and that the interoperability between them has
to be considered at three different levels.
- Comparison of the semantics of the EXPRESS language with the semantics of
the SGML language, particularly focusing on the mechanisms for declaring data
structures for the representation of information. The concepts of Entity &
Attribute in EXPRESS as well as Element & Attribute in
SGML have to be compared and analyzed as a prerequisite for mapping declarative
structures and translation/transformation of data. If an integrated STEP and
SGML environment is desired, both the EXPRESS and SGML Language may be used to
enable mappings between information models in EXPRESS and document type
definitions in SGML.
- An analysis of information models in EXPRESS and a comparison of document
type definitions in SGML will be needed to established the mappings of the
existing application base. Bi-directional model mappings may be desired in a
truly interoperable environment (the overall goals of the following sections).
For example, the creation of an SGML annotated information object from an
underlying STEP based implementation, respectively the storage of SGML based
publishing structures in a STEP based implementation. The process to compare
models is referred to as mapping. It is the intention of this White
Paper to initialize discussions regarding the need and strategies for mappings
of different models and applications.
- NOTE: The need for model mapping is well identified within the STEP
community and each Application Protocol (AP) includes mapping tables which
specify the mapping of a requirements model to an interpreted model - see for
example [AP221-N362]. Furthermore, based on the needs of STEP in its own
context, mapping languages are under development, such as EXPRESS-M/X
[EXPRESSM]. In this context, the scope for mapping algorithms may be extended to
include different declarative languages to specify the source and target schema.
- An analysis of the types of data that will require conversion will also be
needed. As with information models, bi-directional conversion may be necessary
where product data is converted from a STEP implementation (file or database)
into SGML instances and visa versa. Three types of conversion have been
identified: translation, a conversion process that maintains the same
structure of the data, although the external nomenclature, e.g., markup, may be
different. Transformation, where the structure of the source will be
changed into another structure, without necessarily losing information.
And, algorithmic conversion, in which the actual value of the data is
changed according to a pre-defined algorithm, e.g., to convert a temperature
measurement from degrees Centigrade into degrees Fahrenheit. (This last level of
data conversion is not addressed in this document but is an active part of the
T14 activities.)
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