Getting Started for STEP Implementers

Estimated Time: 10 minutes

This guide is a super-intro for implementers who have CAD experience but are new to the STEP standard. It aims to help you grasp the basics of STEP quickly.

🎯 What You'll Learn in 10 Minutes

What STEP is (from an implementer's perspective)
First step: Reading a STEP file
STEP File Basics: Understanding syntax and structure fundamentals
Recommended tools and resources
What to read next

1. What is STEP? (Implementer's Perspective)

History & Background

Who created it: Developed by ISO TC 184/SC 4 (Industrial data).
Historical context: Development started in 1984 as a successor to IGES (Initial Graphics Exchange Specification), aiming to overcome its limitations in data consistency and completeness.
Why it was created: To provide a single, vendor-neutral, and computer-interpretable international standard that covers the entire life cycle of a product, from design to manufacturing and disposal. The first version was published in 1994.

Core Mental Model

Before diving into the syntax, it is helpful to understand the relationship between the file structure and the data objects.

Core Properties

STEP (Standard for the Exchange of Product model data, ISO 10303) is:

A text-based CAD data exchange format.
A way to represent 3D geometry using B-rep (Boundary Representation).
Capable of storing not just geometry, but also management information (Products, Assemblies) and PMI (Product and Manufacturing Information/Tolerances).

Why Choose STEP? (Strengths & Weaknesses)

✅ Strengths

International Standard: Widely supported by all major CAD vendors (ISO 10303).
High Precision: Uses B-rep (mathematically exact formulas) instead of mesh approximation.
Rich Data: Supports assemblies, colors, layers, and semantic PMI (AP242).
Long-term Archiving: Supported by LOTAR for 50+ year data preservation.

❌ Weaknesses

Large File Size: Being a text-based (ASCII) format, files can be significantly larger than proprietary binary formats.
Lack of Design History: Does not preserve the "feature tree" or parametric constraints from the source CAD (e.g., "Extrude 10mm").
Implementation Complexity: The standard is massive (hundreds of parts), making full implementation difficult.

TIP

Alternative formats? For cases where you need both high-fidelity visualization and exact geometry (B-rep) but don't want to use a raw STEP file, 3D PDF (PRC format) is a common alternative. See the Format Comparison for details.

What is the Part 21 Format?

Part 21 is the standard for saving STEP data as a text file.

Official Name and Origins

Official Name: ISO 10303-21 "Clear text encoding of the exchange structure"
Established by: ISO (International Organization for Standardization)
First Published: 1994
Latest Version: ISO 10303-21:2016

Why "Part 21"?

The STEP standard (ISO 10303) is a massive collection of hundreds of "parts":

Relationship Map of Major Parts

List of Major Parts

Parts 1-20s: Fundamentals & Shared Specifications

Part 11: EXPRESS Language (Data modeling language)
Part 21: Clear text encoding (ASCII/Clear text) ← Most Important!
Part 28: XML representation

Parts 40-50s: Integrated Resources

Part 41: Fundamentals of product description and support
Part 42: Geometric and topological representation
Part 43: Representation structures
Part 44: Product structure configuration
Part 45: Materials and others

Parts 100s: Integrated Application Resources

Part 101: Draughting
Part 104: Finite element analysis
Part 105: Kinematics

Parts 200s: Application Protocols (AP) - Most used by implementers

Part 203: AP203 - Configuration controlled 3D design
Part 214: AP214 - Core data for automotive design
Part 238: AP238 - Application protocol for computer controlled controllers
Part 242: AP242 - Managed model-based 3D engineering (Latest & Most Important)

Parts 500s+: Abstract Test Suites, etc.

Essential Parts for Implementers

In practice, only these five are essential:

Part 21 (Text format) ← Reading and writing files
Part 11 (EXPRESS) ← Understanding the schema
Part 203, 214, 242 (APs) ← Deciding which standard to use

Part 21 simply means the "21st part" of the standard, defining the encoding method (how data is saved to a file).

Relationship with Other Formats

Standard	File Format	Characteristics	CAD Support
Part 21	`.stp`, `.step` (ASCII)	Human-readable, most common	✅ Almost all CAD
Part 28	`.stpx`, `.stpZ` (XML)	XML-friendly, redundant	❌ Very limited
-	(Binary Format)	No official binary format	-

IMPORTANT

CAD Support Status

Part 21: Supported by almost all CAD software including SolidWorks, CATIA, NX, Creo, Inventor, and Fusion360.
Part 28: Only supported by specific tools. Major CAD software often does not support it.
- ✅ Supported: Siemens NX, Kubotek Kosmos, STEP Tools
- ❌ Not Supported: SolidWorks, Creo, Inventor, Fusion360, FreeCAD
In professional practice, Part 21 (.step) is the standard.

Detail: Part 21 vs Part 28 Comparison

Note: Over 99% of STEP files are in Part 21 format. Part 28 is mainly for academic or research purposes and is rarely used in actual CAD data exchange.

Basic Structure of Part 21 Format

The most common STEP files (.stp / .step) use the Part 21 format and have the following structure:

step

ISO-10303-21;                  ← Part 21 format declaration
HEADER;                         ← File metadata
  FILE_DESCRIPTION(...);
  FILE_NAME(...);
  FILE_SCHEMA(...);            ← AP version (Crucial!)
ENDSEC;
DATA;                           ← Actual data
  #10 = PRODUCT(...);          ← Entity instances
  #20 = PRODUCT_DEFINITION_FORMATION(...);
  ...
ENDSEC;
END-ISO-10303-21;

Three Key Concepts

① Entity

The "type" of data. Equivalent to a "class" in object-oriented programming.

Examples: PRODUCT, SHAPE_REPRESENTATION, ADVANCED_FACE
Always written in uppercase.

② Instance

Concrete data for an entity. Identified by #number.

step

#10 = PRODUCT('Part_A', 'Part_A', 'Description', (#20));

#10 = Instance ID
PRODUCT = Entity type
'Part_A', ... = Attribute values

③ Reference

Referencing other instances using their #number.

step

#10 = PRODUCT(..., (#20));  ← References #20
#20 = PRODUCT_CONTEXT(...);

Parsing Tips:

Instance IDs start from 1 and are unique within the file.
Forward references are allowed (an ID can be referenced before it is defined).
Using a hash map (dictionary) is efficient for resolving references.

2. First Step: Reading a STEP file

Step 1: Open with a Text Editor

Since STEP files are plain text, you can open them with Notepad or VS Code.

Prepare any .step file.
Open it in your text editor.
Inspect the HEADER section.

Step 2: Identify the AP Version

Check the FILE_SCHEMA in the HEADER section:

step

FILE_SCHEMA(('AP214_AUTOMOTIVE_DESIGN { ... }'));

→ This is an AP214 file.

step

FILE_SCHEMA(('AP242_MANAGED_MODEL_BASED_3D_ENGINEERING_MIM_LF { ... }'));

→ This is an AP242 file.

Why is this important?

Different APs use different entities.
Parsers need to load the schema corresponding to the AP.

Step 3: Find the PRODUCT Entity

Search for PRODUCT( in the DATA section:

step

#10 = PRODUCT('Part_A','Part_A','Simple part',(#20));

This is the top-level information for the part.

Following the Related Entities:

PRODUCT (#10)
  ↓ (via PRODUCT_DEFINITION_FORMATION)
PRODUCT_DEFINITION (#50)
  ↓ (via PRODUCT_DEFINITION_SHAPE)
SHAPE_REPRESENTATION (#90)
  ↓
REPRESENTATION_ITEM (#100-#500)  ← Geometry data is here

Visual Traversal Path:

Traversal Steps:

Start with PRODUCT (#10) - the top-level part information
Follow PRODUCT_DEFINITION_FORMATION to manage versions
Follow PRODUCT_DEFINITION to get design context
Follow PRODUCT_DEFINITION_SHAPE as the bridge to geometry
Follow SHAPE_REPRESENTATION to reach the geometry container
Extract REPRESENTATION_ITEM instances (faces, edges, vertices)

See the Data Model Map for details.

Step 4: How to Reach Geometry Data (Overview)

In an actual implementation:

Pseudocode (Python-style):

python

# 1. Get the PRODUCT instance
product = find_entity(step_file, 'PRODUCT')

# 2. Follow to PRODUCT_DEFINITION
product_def = traverse(product, 'PRODUCT_DEFINITION')

# 3. Get the SHAPE_REPRESENTATION
shape_rep = traverse(product_def, 'SHAPE_REPRESENTATION')

# 4. Get geometry elements (like FACES)
faces = filter_items(shape_rep.items, 'ADVANCED_FACE')

Parser Processing Flow:

Why Two-Pass Processing?

Pass 1: Build instance map (all #ID = ENTITY(...) entries)
Pass 2: Resolve references (replace #ID with actual instance objects)
Forward references require this approach - you cannot resolve references until all instances are parsed.

See Data Model Map for detailed implementation examples.

4. Recommended Tools

STEP Viewers (Free)

Tool	Features	URL
FreeCAD	Open-source CAD. Can read and edit STEP.	https://www.freecad.org/
OpenCascade	CAD library. C++/Python.	https://www.opencascade.com/
3D-Tool Free Viewer	Lightweight viewer for Windows.	https://www.3d-tool.com/

Validators

STEP Tools (Paid): High-performance commercial validator.
CAx-IF Recommended Test Cases: https://www.cax-if.org/

Parser Libraries

Language	Libraries / Tools
Python	`ifcopenshell` (for IFC but useful reference), `pythonOCC` (OpenCascade wrapper)
C++	OpenCascade, STEP Tools SDK
JavaScript/TypeScript	Mostly custom implementations (Part 21 parsers are relatively simple).

5. What to Read Next

After understanding the basics, proceed in this order:

① Deepen Your Understanding

STEP File Basics - Syntax and structure fundamentals.
Glossary - Memorize the Top 5 important terms.
FAQ - Resolve common questions.

② Select an AP

Which AP should I use?
Capability Matrix

③ Start Implementation

STEP File Walkthrough - Understand real files line-by-line.
Data Model Map - Entity hierarchy.
Common Pitfalls - Implementation warnings.

💡 Pre-Implementation Checklist

Before you start coding, ensure you:

[ ] Understand the basic structure (HEADER + DATA) of a STEP file.
[ ] Grasp the concepts of Entity, Instance, and Reference.
[ ] Chose the appropriate AP for your project.
[ ] Understand the syntax of Part 21 (basic parsing methods).
[ ] Obtained sample STEP files for testing.
[ ] Set up a STEP viewer to open actual files.

📚 Further Learning

Official Resources

ISO 10303 Standards: Official specifications (Paid).
CAx-IF Recommended Practices: Implementation guidelines (Free).

Communities

CAx-IF: https://www.cax-if.org/
STEP modularization: https://www.stepmod.org/

Recommended Learning Path

📚 Next Steps

FAQ - Resolve common questions.
Which AP should I use? - Select the AP for your use case.

Back to README

Getting Started for STEP Implementers ​

🎯 What You'll Learn in 10 Minutes ​

1. What is STEP? (Implementer's Perspective) ​

History & Background ​

Core Mental Model ​

Core Properties ​

Why Choose STEP? (Strengths & Weaknesses) ​

✅ Strengths ​

❌ Weaknesses ​

What is the Part 21 Format? ​

Official Name and Origins ​

Why "Part 21"? ​

Relationship Map of Major Parts ​

List of Major Parts ​

Essential Parts for Implementers ​

Relationship with Other Formats ​

Basic Structure of Part 21 Format ​

Three Key Concepts ​

① Entity ​

② Instance ​

③ Reference ​

2. First Step: Reading a STEP file ​

Step 1: Open with a Text Editor ​

Step 2: Identify the AP Version ​

Step 3: Find the PRODUCT Entity ​

Step 4: How to Reach Geometry Data (Overview) ​

4. Recommended Tools ​

STEP Viewers (Free) ​

Validators ​

Parser Libraries ​

5. What to Read Next ​

① Deepen Your Understanding ​

② Select an AP ​

③ Start Implementation ​

💡 Pre-Implementation Checklist ​

📚 Further Learning ​

Official Resources ​

Communities ​

Recommended Learning Path ​

📚 Next Steps ​

Getting Started for STEP Implementers

🎯 What You'll Learn in 10 Minutes

1. What is STEP? (Implementer's Perspective)

History & Background

Core Mental Model

Core Properties

Why Choose STEP? (Strengths & Weaknesses)

✅ Strengths

❌ Weaknesses

What is the Part 21 Format?

Official Name and Origins

Why "Part 21"?

Relationship Map of Major Parts

List of Major Parts

Essential Parts for Implementers

Relationship with Other Formats

Basic Structure of Part 21 Format

Three Key Concepts

① Entity

② Instance

③ Reference

2. First Step: Reading a STEP file

Step 1: Open with a Text Editor

Step 2: Identify the AP Version

Step 3: Find the PRODUCT Entity

Step 4: How to Reach Geometry Data (Overview)

4. Recommended Tools

STEP Viewers (Free)

Validators

Parser Libraries

5. What to Read Next

① Deepen Your Understanding

② Select an AP

③ Start Implementation

💡 Pre-Implementation Checklist

📚 Further Learning

Official Resources

Communities

Recommended Learning Path

📚 Next Steps