PCB NC formats
PCB NC drill files convey PCB drilling and routing information. The NC formats were originally designed by CNC drill and route machine vendors as proprietary input formats for their equipment, and are known under their company name: Excellon, Hitachi, Sieb & Meyer, Posalux, etc. These formats are similar as they are based on RS-274-C and related to G-code. In 1985 IPC published a generic standard NC format, IPC-NC-349. Later XNC was designed, a simple strict subset of IPC-NC-349, designed not for driving machines but for exchanging drill information between CAD and CAM. They are collectively referred to as (PCB) NC files.  
The NC files are primarily used to drive CNC machines, and they are adequate for that task.
They are also used to exchange design information between CAD and CAM, for which they are not adequate: essential information such as plating and drill span is missing. Furthermore, the NC output in CAD systems is often poorly implemented, resulting in poor registration between drill holes and copper layers and other problems. To exchange date between CAD and CAM it is more preferred to use the Gerber format. The quality of the Gerber file output software is typically good, and Gerber supports attributes to transfer meta-information such as plating and span.  
The IPC-NC-349 format is the only IPC standard governing drill and routing formats.  XNC is a strict subset of IPC-NC-349, Excellon a big superset. Many indefinite NC files pick some elements of the IPC standard.
A digital rights managed copy of the specification is available from the IPC website, for a fee. It is targeted at input for drill/rout machines, not CAD to CAM data exchange.
The XNC format is strict subset of the IPC-NC-349 specification targeted at data exchange between CAD and CAM. The name XNC format stands for Exchange NC format. As a strict subset, it is highly compatible with existing software. Its purpose is to address the current chaos of different subsets and incomplete NC files, and define a simple common standard. The XNC subset was defined by a consortium formed by Graphicode, Ucamco, KiCad and Pentalogix.
The specification is freely available among others at the Ucamco download page. The specification is brief, detailed, easy to understand and unambiguous. It is targeted at CAD to CAM data exchange, not at input for drill/rout machines.  
M48 Start of header METRIC Metric units (mm) T01C0.6 Tool 1 has diameter 0.6mm T02C1.0 Tool 2 has diameter 1.0mm % End of header G05 Set drill mode T01 Select tool 1 X8.5Y4.8 Drill a hole of 0.6 mm at coordinates 8.5mm,4.8mm X8.55Y2.85 Drill X6.54Y2.85 Drill X6.45Y4.8 Drill T02 Select tool 2 G00X10.25Y3.825 Move to coordinates 10.25mm,3.825mm M15 Plunge rout tool down G01X6.50Y3.25 Rout to coordinates 6.5mm,3.25mm M16 Lift rout tool up M30 End of file
The name Excellon format is derived from the company Excellon Automation (not to be confused with Excellon Software), which was the market leader in PCB drilling and routing machines during the 1980s, and whose proprietary format became widely used.
There are actually two Excellon formats, the older Excellon 1 and Excellon 2. Excellon 2 is a superset of IPC-NC-349. Commands from both are sometimes confusingly mixed in the same file.
Excellon Automation stopped publishing the specification of its format, without statement on the IP and usage rights of the format. An archived copy exists.  The specification is ambiguous, leading to different interpretations, especially in the area of coordinates and units. It is targeted at input for drill/rout machines, not CAD to CAM data exchange.
Indefinite NC files
For CAD/CAM data exchange often NC files are used that only that only vaguely follow any specification. These files contain a few IPC-NC-349 commands, but follow neither the Excellon nor the IPC-NC-349 specification. 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. Usually the header is incomplete: 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. They are often called Excellon files although they are undefinite NC files.
No specification exists. It is not always clear what the purpose of these stunted files is.
% T01 X006272Y001092 X006354Y001093 X006653Y001092 ... T02 X008091Y001754 X-002028 M30
The tool diameters and how to interpret the coordinates is not specified. This file is 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, with unnecessary manual labor and risk of delays or errors. Astonishingly, such enigmatic files are still used.
- Charras, Jean-Pierre. "XNC Format: Gerber Takes Data Into the Future" (PDF). Retrieved 7 April 2019.
- "PCB Layout Data". Eurocircuits. Archived from the original on 6 December 2011. Retrieved 26 November 2011.
- ""Gerber Format Specification"" (PDF). Ucamco.
- Tavernier, Karel. "PCB Fabrication Data - A Guide - section on drill files". Retrieved 16 January 2015.
- IPC-NC-349 Computer Numerical Control Formatting for Drillers and Routers, published in 1985
- "CAD software firms develops XNC format for PCB drill date". pcdandf. Retrieved 26 February 2019.
- "XNC file format specification". Ucamco. Retrieved 26 February 2019.
- "Former Excellon format description". Archived from the original on 2007-10-30. Retrieved 2007-10-30.