What is EXPRESS?
EXPRESS is a data modeling language defined in ISO 10303-11. Its basic object types are called --entites-- but there are several other datatypes as well such as various aggregates, defined types, enumerations, select type and combinations of them. One of the main features of EXPRESS is that data models can be constrained by rules. A rule can be a simple expression or can expand in more complex algorithms using functions and procedures. EXPRESS data models are encapsulated in schemas. Higher-level schemas can be composed of lower level schemas.
The main notation of EXPRESS is in ASCII from, suitable for formal checking and compilation. The JSDAI Express Compiler is such a tool. In addition EXPRESS defines a graphical notation, called EXPRESS-G.
In the standard we can find the statement "EXPRESS is not a programming language". However looking to the ASCII format and all the expressions we can see several similarities to programming languages such as PASCAL. And in fact when going to validate a data set against all the rules defined in an EXPRESS schema all the rules must be executed. This is very similar to executing a programming language.
A special extension of EXPRESS is EXPRESS-X, defined in ISO 10303-14. It allows to map one data model onto another and to define views.
The first edition of EXPRESS was standardized in 1994 (ISO 10303-11:1994). The 2nd edition of EXPRESS 2003/2003 mainly introduces extensible selects and enumerations to the standard.
What is STEP?
STEP stands for "STandard for the Exchange of Product model data". It is registered as international standard ISO 10303. STEP got developed and is further extended and maintained by the Technical Committee TC184 "Industrial automation systems and integration", Sub-Committee SC4 "Industrial data".
The web-page of ISO TC184/SC4 is http://www.tc184-sc4.org/.
Where can I download ISO standards?
You cannot. ISO standards are copyrighted by the International Standard Organization. You have to buy them. Go to http://www.iso.org for more details.
What is SDAI?
SDAI (ISO 10303-22) is the Standard Data Access Interface of STEP (ISO 10303), the standard for Industrial Product Data. In other words, SDAI defines the functionality of an API (Application programmers interface) or toolbox to simplify the development for application programs.