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RS-274D is the recommended standard for numerically controlled machines developed by the Electronic Industry Association in the early 1960's. The RS-274D revision was approved in February, 1980. These standards provide a basis for the writing of numeric control programs.

There are a number of historical sidelights to this standard, many having to do with the original use of punched paper tape as the only data interchange medium. The 64-character EIA-244 paper tape standard is now (thankfully) obsolete, and ASCII character bit patterns are now the standard representation. This old tape standard had specific characters used for 'searching' for specific lines (program blocks) on the tape, 'rewinding' the tape, etc. Ocasionally this obsolete language is still used when referring to some cnc control tasks.

The full NIST Enhanced Machine Controller is nc programmed using a variant of the RS274D language to control motion and I/O. This variant is called RS276NGC because it was developed for the Next Generation Controller, a project of the National Center for Manufacturing Science. The version of RS274 used by EMC adheres closely to the publications of the NCMS wherever those publications produce an unambiguous set. In some cases reference to other implementations of RS274 had to be made by NIST.

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This article is about the machine tool programming language. For the video recorder programming system, see Video recorder scheduling code.


G-Code, or preparatory code or function, are functions in the Numerical control programming language. The G-codes are the codes that position the tool and do the actual work, as opposed to M-codes, that manages the machine; T for tool-related codes. S and F are tool-Speed and tool-Feed, and finally D-codes for tool compensation.

The programming language of Numerical Control (NC) is sometimes informally called G-code. But in actuality, G-codes are only a part of the NC-programming language that controls NC and CNC machine tools. The term Numerical Control was coined at the MIT Servomechanisms Laboratory, and several versions of NC were and are still developed independently by CNC-machine manufacturers. The main standardized version used in the United States was settled by the Electronic Industries Alliance in the early 1960s. A final revision was approved in February 1980 as RS274D. In Europe, the ISO standard DIN 66025 is often used instead.

Due to the lack of further development, the immense variety of machine tool configurations, and little demand for interoperability, few machine tool controllers (CNCs) adhere to this standard. Extensions and variations have been added independently by manufacturers, and operators of a specific controller must be aware of differences of each manufacturers' product. When initially introduced, CAM systems were limited in the configurations of tools supported.

Today, the main manufacturers of CNC control systems are GE Fanuc Automation (joint venture of General Electric and Fanuc), Siemens, Mitsubishi, and Heidenhain, but there still exist many smaller and/or older controller systems.