ISO 10303-21

Filename extension .step, .stp, .p21
Magic number ISO-10303-21
Developed by ISO
Initial release 1994
Website Specification

STEP-File is the most widely used data exchange form of STEP. ISO 10303 can represent 3D objects in Computer-aided design (CAD) and related information. Due to its ASCII structure it is easy to read with typically one instance per line. The format of a STEP-File is defined in ISO 10303-21 Clear Text Encoding of the Exchange Structure.[1]

ISO 10303-21 defines the encoding mechanism on how to represent data according to a given EXPRESS schema, but not the EXPRESS schema itself. A STEP-File is also called p21-File and STEP Physical File. The file extensions .stp and .step indicates that the file contain data conforming to STEP Application Protocols while the extension .p21 should be used for all other purposes.[2]


Some details to take care of:

ISO 10303-21 Building blocks


A typical example looks like this:

/* description */ ('A minimal AP214 example with a single part'),
/* implementation_level */ '2;1');
/* name */ 'demo',
/* time_stamp */ '2003-12-27T11:57:53',
/* author */ ('Lothar Klein'),
/* organization */ ('LKSoft'),
/* preprocessor_version */ ' ',
/* originating_system */ 'IDA-STEP',
/* authorization */ ' ');
FILE_SCHEMA (('AUTOMOTIVE_DESIGN { 1 0 10303 214 2 1 1}'));
#11=PRODUCT_DEFINITION_CONTEXT('part definition',#12,'manufacturing');
#12=APPLICATION_CONTEXT('mechanical design');
#16=PRODUCT('A0001','Test Part 1','',(#18));
#20=ORGANIZATION_ROLE('id owner');

HEADER section

As seen in the above example, the file is split into two sections following the initial keyword ISO-10303-21;:

The HEADER section has a fixed structure consisting of 3 to 6 groups in the given order. Except for the data fields time_stamp and FILE_SCHEMA all fields may contain empty strings.

The last three header groups are only valid in second edition files.

DATA section

The DATA section contains application data according to one specific express schema. The encoding of this data follows some simple principles.

See also


  1. ISO 10303-21:2002 Industrial automation systems and integration -- Product data representation and exchange -- Part 21: Implementation methods: Clear text encoding of the exchange structure
  2. ISO TC184/SC4 Secretary "Cumulative list of resolutions" Resolution 583 (Stuttgart, Germany, - June 2003) "Registration of SC4 MIME-Types",
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