Frequently Asked Questions

Questions about STEP Application Protocols

What is an Application Protocol?
The STEP standard is divided into many Application Protocols belonging to the ISO 10303 family of standards. Each protocol defines a data exchange standard for a defined family of products at a defined stage in its life cycle. The most popular Application Protocols for CAD are AP-203 also known as ISO 10303-203, and AP-214 also known as ISO 10303-214. The STEP-NC Application Protocol has the ISO number 10303-238 or AP-238. Other Application Protocols define data exchange standards for specialized products such as ships (AP-216) and printed circuit boards (AP-210), and for different product life cycle stages such as process planning (AP-240) and coordinate measuring machine inspection (AP-219)

How is an Application Protocol defined?
Each Application Protocol includes a scope describing its purpose, an activity diagram describing the functions that an engineer needs to perform within that scope, and an Application Requirement Model (ARM) describing the information requirements of those activities. These information requirements are then mapped into the common set of Integrated Resources and the result is a data exchange standard for the activities within the scope.

How do the Application Protocols share data?
The Application Protocols frequently define similar information. The STEP Architecture supports this sharing by mapping each protocols (ARM) model into the common set of integrated resources to produce the Application Interpreted Model or (AIM). The AIM is the model implemented by data exchange systems and it is the key to data sharing because it uses the same definition for the same concept across all of the Application Protocols. Therefore, an exchange system that implements code to process this data in one context can use the same code to process the data in any other context.

What is EXPRESS?
EXPRESS is the language used to define the AIM model and most of the ARM models. EXPRESS was defined to model geometry so it has a very powerful inheritance model and the ability to describe complex mathematical constraints. This power is more than necessary for most applications, however, so EXPRESS is rarely used for applications other than product modeling.

What is a Part 21 Physical File?
Part 21 (ISO 10303-21) is an ASCII encoding for data defined by EXPRESS schemas that is used by most exchange systems today to read and write STEP data.

What is a Part 28 XML File?
Part 28 (ISO 10303-28) is an XML encoding for data defined by EXPRESS schemas that is being used by the second generation of STEP data exchange systems, including STEP-NC.

How do I learn more about STEP and EXPRESS?
For more information about the STEP standard and its implementation methods please check the STEP Tools web site STEP Standard information pages.