The Excellon format is widely used to drive CNC drilling and routing machines in printed circuit board (PCB) manufacturing. It is a variant of the EIA RS-274-C standard.
Contents
The name Excellon format is derived from the Excellon Automation Company, which was market leader in PCB drilling and routing machines during the 1980s, and whose proprietary format became widely used.
The IPC organization defined the IPC-NC-349 format as standard NC format. It is similar to the Excellon format.
Usage
The Excellon format was designed for and is primarily used to drive CNC drill and route machines. It is suitable for that task as it can specify machine specific information such drill feed and speed.
An example:
M48 METRIC,TZ FMAT,2 ICI,OFF T02C0.8000 T01C0.9000 T03C1.9500 T04C3.2000 % M71 G90 G93X0Y0 T02 X125730Y16193 Y16828 Y17463 ... M30The Excellon format is also used to transfer design information from CAD to CAM. It is not adequate for that task: essential information such as plating and drill span is missing. Furthermore, the Excellon output in CAD systems is often poorly implemented, resulting in poor registration between drill holes and copper layers and other problems. For data exchange between CAD/CAM systems it is more practical to describe the drill data in Gerber format as the quality of the Gerber file output software is usually better and Gerber supports attributes to transfer meta-information such as which are the via holes.
Excellon-like files
The name Excellon format or Excellon file is also commonly used for files that only vaguely follow any specification. These files contain a few Excellon commands, but follow neither the Excellon nor the IPC-NC-349 specification. It would be more appropriate to call them Excellon-like or generic NC files. Commands are not used properly, or are used in a syntactically incorrect way, and binary data objects may be included. Sometimes the historic EIA or EBCDIC character encoding is used instead of ASCII as required by the standards. Usually the header is incomplete: essential information such as the scale or the tool diameters is missing. Sometimes there is no header at all, and the file only contains tool numbers, with an unspecified diameter, and X,Y coordinates, in an unspecified unit. An example:
% T01 X006272Y001092 X006354Y001093 X006653Y001092 ... T02 X008091Y001754 X-002028 M30These files are meaningless without additional information, typically put in a free format human readable tool file. This information must be re-entered manually by the CAD/CAM operator. When critical information is missing (e.g. omitting leading or trailing zeroes without properly documenting if and which zeroes are omitted) the file is ambiguous. The user may substitute his own judgement or stop the job pending clarification.
These inadequate Excellon-like files are used all too often, resulting in unnecessary manual labor and risk of delays or errors.