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• An example of a proprietary format is the CADL file produced by Cadkey for transferring geometric entities through the use of an ASCII file. Such entities may be lines, points, arcs, splines, etc.

DG:18.2.2 Standard Formats

•A device independent format is based on public standards that are controlled by non-commercial bodies.

• examples of these include

STEP (Standard for Exchange of Product Data) SET (Standard d’Exchange et de Transfer) PDES (Product Data Exchange Specification) IGES (Initial Graphics Exchange Specification)

DG:18.2.2.1 - IGES

•IGES was first developed by the National Bureau of Standards, Boeing Corporation, and General Electric Corporation and published by the National Bureau of Standards in 1980.

•Initially the only data that could be exchanged were basic entity types such as points, lines, arcs, and circles.

•September 1981, IGES version 2.0 was approved by the American National Standards Institute as ANSI standard Y14.26M for CAD/CAM communications.

•More recent versions of IGES contain the ability to transfer constructive solid geometry and boundary representations of solid models.

•The IGES can be in either binary or ASCII format. In ASCII the file is easy to read and modify by hand but it tends to be larger.

•There are two formats

-a fixed 80 character line length format

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-a compressed line length free format.

•In the fixed formatted data, there are several sections that must be entered in a highly structured approach. Data must be right justified within the specified column locations, and these locations must occupy the columns between 1 and 80, inclusive. The following is a brief outline of some rules that must be followed in creating the IGES file.

1. Blanks: Blanks are only significant in string constants. A numeric filed of blanks is considered to denote the default value for that field. No blanks are allowed between the beginning of a numeric constant and the end of that constant. Leading blanks in numeric constants are ignored. Blanks between the end of any constant and the delimiter following the constant are not allowed.

2. Numeric Constants: Embedded commas in numeric constants are not permitted. The absolute magnitude of a numeric constant may not exceed the value of

2( N – 1) – 1 , where N is the number of bits used to represent an integer of a real value. Real constants may be double or single precision. Valid integer constants

3. String Constants: A string constant is defined as an arbitrary sequence of ASCII characters. Blanks, commas, and numbers are treated as characters within the string. String constants are represented in Hollerith notation. This form consists of a nonzero integer constant (i.e. the number of characters in the string), followed immediately by the letter H, followed by the character string. Valid string constants can be expressed as:

3Hr45 12H Hello There 9Htime;.erw

4.Sequence numbers: A sequence number is a string of up to seven digits and is used for indexing the lines within the various sections of the IGES file. Sequence numbers begin ar 1, (i.e. 0000001) in each section and continue sequentially without interruption to the end of that section. This is primarily used to indicate the number of physical lines of the particular section, or may be thought of as section line counters. Sequence numbers must be used in each section of the IGES file. These numbers are right justified in the columns 74 to 80. Sequence numbers may have leading zeros (0) or blanks.

•An IGES file consists of six (6) sub-sections that must appear as follows:

1.flag section

2.start section

3.global section

4.directory entry section