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RS-274D is the 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.

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. Others are methods for 'searching' for specific lines (program blocks) on the tape, 'rewinding' the tape, etc.

The basic unit of the program is the 'block', which is seen in printed form as a 'line' of text. These lines usually start with a number, as N0001 X123.

Each block can contain one or more 'words', which consist of a letter, describing a setting to be made, or a function to be performed, followed by a numeric field, supplying a value to that function. An example would be X10.001, which by itself indicates the X axis should move to a position of 10.001 user units, which would normally be inches or mm. Various words can be combined to specify multi-axis moves, or perform special functions.

The common axes are normally named :

A     - Angular axis around X axis
B     - Angular axis around Y axis
C     - Angular axis around Z axis
U     - Secondary axis parallel to X
V     - Secondary axis parallel to Y
W    - Secondary axis parallel to Z
X     - Primary Linear Axis
Y     - Primary Linear Axis
Z     - Primary Linear Axis

Control words are :

F     - Feed Rate
G     - Preparatory Functions
M     - Miscellaneous Function
S     - Spindle Speed
T      - Tool Function

Preparatory (G) Functions :

G00   - Positioning
G01   - Linear Interpolation
G02   - Circular (Clockwise) Interpolation
G03   - Circular (Counterclockwise) Interpolation

G04   - Dwell (not modal)

G17   - XY plane
G18   - ZX plane
G19   - YZ plane

G33   - Threadcutting, constant lead
G34   - Threadcutting, Increasing Lead
G35   - Threadcutting, Decreasing Lead

G40   - Cancel Cutter compensation
G41   - Cutter Compensation, Tool Left of Path
G42   - Cutter Compensation, Tool Right of Path
               How to use Cutter Diameter Compensation
G43	- Tool Length Offset
               How to use Tool Length Offset
G49	- Cancel Tool Length Offset

G70   - Inch Programming
G71   - metric programming

Miscellaneous (M) Functions :

M00   - Program Stop
M01   - Optional Program Stop
M02   - End of Program

M03   - Spindle CW
M04   - Spindle CCW
M05   - Spindle Stop

M06   - Tool Change
M07   - Flood Coolant On
M08   - Mist Coolant On
M09   - Coolant Off