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:: So the trail from EIA RS-274 to X-code to MIT is as follows:
:: RS-274-C, which was approved in April 1974, unified two different EIA standard: RS-273 (Straight Cut) and RS-274 (Contour Cut), both of which had initial publication in January 1963 (see Library of Congress
:: The EIA RS standards, which governed members of the EIA, were essentially copies of earlier AIA National Aerospace Standards: NAS 943 (Straight Cut) and NAS 955 (Contour Cut). NAS 943 was approved in 1960 (as per the AIA Annual Report, available online). The EIA governed Electronics manufacturers, which the AIA governed Aerospace manufacturers.
:: Those standards are predominately based on the two leading Numerically Controlled mills during the APT work: the Giddings & Lewis mill (trademarked as the Numericord) which used a combination of General Electric and Concord Controls controllers. The second system was the Kearney & Trecker mill, which used a Bendix controller. Both machines had detailed papers published at the Eastern Joint Computer Conference in December 1957 (proceedings available online). The Bendix solution used "R-codes" to describe operational modes (essentially G-codes) and another set to describe auxiliary on/off functions (essentially M-codes). The Giddings & Lewis system used "X-codes" which also described operational modes and auxiliary on/off functions. These are the X-codes described in the APT flowcharts. The assertion that these were the two leading NC mills in the mid-50s can be found in <i>Forces of Production</i> by David F. Noble. The same book contains significant content as to why Bendix became the preferred solution during the standardization process, which was driven as a requirement by the US Air Force.
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