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STEP File Anatomy

AP 210 data is exchanged using ISO 10303-21 (Part 21) files. This lesson covers the anatomy of these files.

File Structure

Every STEP file has two main sections:

ISO-10303-21;
HEADER;
  FILE_DESCRIPTION(('...'), '2;1');
  FILE_NAME('example.stp', '2024-01-01T00:00:00',
    ('Author'), ('Organization'), 'preprocessor', 'processor', 'AP210');
  FILE_SCHEMA(('AUTOMOTIVE_DESIGN'));
ENDSEC;

DATA;
  ...
ENDSEC;

END-ISO-10303-21;

Header Section

The HEADER section contains metadata:

  • FILE_DESCRIPTION - Description and implementation level

  • FILE_NAME - Filename, timestamp, author, organization, preprocessor, processor

  • FILE_SCHEMA - The EXPRESS schema(s) used in the data section

Data Section

The DATA section contains the actual product data as entity instances.

Each instance has:

  • A numeric identifier (e.g., #100)

  • An entity type name (e.g., PRODUCT)

  • Attribute values in parentheses

Example:

#1 = PRODUCT('PCA-001', 'Main Board', 'Printed circuit assembly', (#2));
#2 = PRODUCT_CONTEXT('electrical design', #3, 'mechanical');
#3 = APPLICATION_CONTEXT(
  'electronic assembly, interconnect and packaging design');

Entity References

Entities reference each other by their numeric identifiers:

  • #1 references #2 in its context attribute

  • #2 references #3 in its frame_of_reference attribute

  • This creates a directed graph of entities

Common Entity Patterns

Product Definition Chain

product → product_definition_formation → product_definition → ...
  → shape_definition_representation → representation

Assembly Structure

product → product_definition_relationship → next_assembly_usage_occurrence
  → item_defined_transformation (placement)

Shape Representation

representation → representation_item → geometric_representation_item
  → (point, direction, curve, surface, solid)

Reading Strategy

When reading a STEP file:

  1. Start with the APPLICATION_CONTEXT - Usually the first entity

  2. Find the top-level PRODUCT - The main assembly

  3. Follow the product definition chain - To get to the shape and structure

  4. Trace relationships - Follow references to build understanding

  5. Focus on what you need - Don’t try to read everything at once