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XML study notes Copyright Ann M Wrightson 1999
This set of XML study notes was prepared to support class-based and independent study at the School of Computing and Mathematics at the University of Huddersfield. These notes are open access; registered students should ensure that they also make full use of the specific material for their modules (password access).
STEP is concerned with product data exchange, that is, the exchange of technical specifications and other information, between companies involved in developing, manufacturing and supporting complex products. XML is a significant new force in this domain, with its rapidly growing infrastructure providing new opportunities for open information exchange between business partners in complex manufacturing projects.
Introduction to STEP
XML, SGML and STEP
STEP (STandard for Exchange of Product model data, ISO 10303) is an international standard which provides a common system independent language for exchanging technical information concerning manufactured products.
Ever since computers were first used to support engineering design, in the 1950s, there has been a need for companies who are working together to develop a product, to exchange technical data about that product. This exchanged data needs to be in a form that engineers working with the respective computer systems can use with confidence, knowing that it has been transferred accurately.
In the early days of product data exchange, conversion programs were developed to exchange drawings, and associated data such as measurements, between specific CAD systems. With growing experience, it became clear that instead of converting product data directly from one system to another, it would be better to have a single common language understood by each system: a neutral data format.
An important benefit of using a neutral data format is that whenever another system needs to be accommodated, only two conversions (one to, and one from, the neutral data format) are needed to enable the new system to exchange data with all the systems which already use the neutral data format. Without a neutral data format, then if 3 systems are already in use, then it is necessary to write 6 additional conversion programs to accommodate a fourth - and 8 to accommodate a fifth! Another benefit, is that the CAD user community can put pressure on system vendors to support the neutral data format, rather than relying on vendors releasing (potentially commerically sensitive) data to support specific conversions.
Early development of neutral data formats concentrated on the exchange of geometric shape and surface information between CAD systems. By 1980, several vendor-specific data formats were being used to a greater or lesser extent as neutral formats, and there had also been considerable work, both in Europe and in the United States, on the development of standardised neutral data formats to meet specific conversion requirements within particular industry sectors.
Work towards what is now ISO 10303 (STEP) was formally begun by the International Organisation for Standardisation in December 1983. The countries involved initially were the United States, France, United Kingdom and Germany; others joined in the work soon after. ISO Technical Committee 184, Sub-committee 4 (ISO TC184 SC4), Industrial Automation Systems, has been working on it steadily ever since, developing a standardised framework for the exchange of product data, for a wide range of products and industries.
STEP contains comprehensive support for geometric information, like its predecessors, but it also contains much more. STEP from the outset had the ambitious objective of "the creation of a standard that enables the capture of information comprising a computerised product model in a neutral form without loss of completeness and integrity, throughout the life cycle of the product." (ISO TC184 SC4: STEP requirements document). Initially it was hoped that the standard would be complete by 1990; although that target was soon found to be over-optimistic, STEP data exchanges are now happening on real projects, and it is gaining wide recognition as an essential enabling standard.
As you would expect for such a complex subject, STEP is quite a complex standard. It has many parts; some are already published, some are still under development.
The parts of STEP fall into six main groups:
Description methods are the underpinning of the STEP standard. EXPRESS, the data modelling language which is employed throughout STEP, is described here, together with other definitions and techniques used throughout the standard.
Integrated information resources are information models which are used and reused in the more application specific parts of the standard. Much of STEP support for geometric modelling is specified here, including wire frame modelling, surfaces, & solid geometry; there is also general support for product configuration, engineering analysis, and the design process. In general, if a modelling concept is needed in more than one place in STEP, it will be specified within an integrated information resource.
Application protocols are more specific information models which describe individual product data applications. They describe not only what data is used in modelling a particular type of product information, but also precisely how the data is used in the model. Application protocols provide the level of detail which experience has shown to be necessary to ensure faithful exchange of real information about complex products.
Implementation methods describe precise mappings from the information level specifications in the above parts of STEP, to actual representations of that information (for example clear text encoding), and actual ways of accessing STEP information (for example bindings to programming languages).
The conformance methodology group specifies methods of testing for STEP conformance, and also specific test data and criteria that are used to assess the conformance of a STEP software product to an application protocol. Each application protocol has a corresponding abstract test suite. This is important because it puts the criteria for conformance to STEP fully in the public domain .
A detailed account of the background to STEP can be found in: Product Data Exchange, by M Susan Bloor and Jon Owen, UCL Press 1995, ISBN 1-85728-279-5.
Current information on the availability and continuing development of the STEP standard can be found on the STEP website.
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The STEP and SGML standards started out as separate developments, with different visions and origins, sharing only a common concern for enabling information exchange using system-independent standardised data formats. STEP is concerned with product data, and started out from the exchange of drawings between CAD systems; SGML is concerned with structured text based information- usually documents - and grew out of requirements for data exchange and reuse in computerised typesetting and technical manuals.
There were early signs that the two areas were closely connected; for example, at one point in the early 1980s I was busy integrating drawings encoded in IGES ( a predecessor of STEP), and structured text marked up in IBM's GML (a predecessor of SGML), into a technical manual, which also included tables of dimensions and part numbers (extracted from a product information database). Over the last few years, however, the relationship between STEP and SGML has emerged as something much more fundamental than incorporating product data in a printed manual. At this time (April 1999), international experts in the two domains are working together towards full harmonisation of STEP and SGML views of structured data, and looking to take full advantage of the infrastructure for data exchange which is becoming available using XML.
With full harmonisation, then in simple terms, the power of STEP becomes available to SGML based work, and vice versa. The complementary strengths of SGML and STEP can pull together, and can also compensate for each other's weaknesses. The potential is so wide that it is impossible to say at this time which way things will go; but with growing industry support for XML providing new means for SGML structured data to be distributed on the Web, then even a cautious list has to include:
An unforeseen, but very welcome, additional benefit of the STEP/SGML harmonisation process, is the opportunity it provides for both STEP and SGML experts to see "their" familiar fundamental concepts re-formulated within a different point of view. Both sides are learning from this experience, with potential long-term benefits for both areas of standardisation.
XML is also having a profound effect on work in this area. The growing importance of XML based data exchange means that a standardised way of exchanging product information defined using EXPRESS, through the medium of XML, has become a clear requirement.
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As noted above, the growing importance of XML based data exchange means that a standardised way of exchanging product information defined using EXPRESS, through the medium of XML, has become a clear requirement. XMI, an information exchange standard recently developed by the Object Management Group, may become part of this STEP domain standard. (See XMI notes for more information.) This is a good example of the way that XML tends to break through traditional boundaries in the information systems business. As with STEP and SGML, there is potential here for concepts from STEP to feed back into future releases of XMI. (For example, the current version of XMI has very simple provision for specifying versions of data; STEP has mature well developed concepts in this area; it would be logical for a future version of XMI to use these concepts.)
Within the STEP development process, a group has been looking at product documentation for some years now. There are two main kinds of documents which are important from a STEP perspective: documents which are created during the development of a product (for example, design studies and test reports), and documents which are required for the use and maintenance of a manufactured product (for example, operations and maintenance manuals). The main emphasis to date of this work has been the first of these. (The second is gaining momentum, especially now that Web technology has matured sufficiently to provide an adequate platform for deploying interactive electronic publications. See my research pages for some work in progress in this field.)
Since much product documentation is either already handled using SGML specified structures, or has the kind of structure which sit small easily in SGML than in EXPRESS, then one of the fundamental issues for integrating product documentation into the management of product data, is the relationship between information models in STEP, and SGML. After several years of preliminary work, it is now becoming clear how useful working relationships can be set up between the fundamental underlying models of both STEP and SGML ("STEP/SGML harmonisation").
The work in progress in this area links the two domains at the level of EXPRESS schema, and SGML grove. The current goals of this work include:
Taken together, these would enable multimedia documents to be held and managed effectively within a STEP driven environment, without losing SGML driven document handling capabilities; and also enable full access to STEP driven data from an SGML driven environment.
This grove based approach comes from a technical perspective on industry requirements for interoperation between SGML and STEP based information, which emphasises links and references between existing data and documents. This perspective is valid as far as it goes, but in my opinion misses a number of issues and opportunities, especially relating to operational and maintenance documentation. See my research work in progress on STEP/SGML interoperation for discussion of these issues, and work in progress on a complementary approach.
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Ann M Wrightson, April 1999