Editing F-14 CADC

{{Short description|Early flight control computer using MOS}}
[[File:Garrett F-14A dual channel CADC.png|thumb|250px|A Garrett-manufactured F-14A dual channel central air data computer (CADC)]]

The '''F-14's Central Air Data Computer''', also abbreviated as '''CADC''', computes altitude, [[vertical speed]], [[air speed]], and [[mach number]] from sensor inputs such as [[Pitot-static_system#Pitot_pressure|pitot]] and [[Pitot-static_system#Static_pressure|static pressure]] and temperature.<ref>{{cite book|title=Dictionary of Military and Associated Terms|date=Oct 1, 1987|publisher=DIANE Publishing|isbn=9780941375108|page=63}}</ref> From 1968 to 1970, the first CADC to use custom digital [[integrated circuit]]s was developed for the [[Grumman_F-14_Tomcat|F-14]].<ref name=holt>{{cite book |first1=Raymond |last1=Holt |first2=Leo |last2=Sorge |title=The Accidental Engineer |date=2017 |publisher=Lulu.com |isbn=9781387313488 |page=36 |url=https://books.google.com/books?id=_r47DwAAQBAJ&pg=PA15 |accessdate=2 June 2020 |archive-date=26 December 2020 |archive-url=https://web.archive.org/web/20201226121448/https://books.google.com/books?id=_r47DwAAQBAJ&pg=PA15 |url-status=live }}</ref> 

==History==
The CADC was a multi-chip integrated flight control system developed by [[Garrett AiResearch]] and used in early versions of the [[US Navy]]'s [[F-14 Tomcat]] fighter. It is notable for early use of [[metal–oxide–semiconductor|MOS]] custom integrated circuits and has been claimed as the first [[microprocessor]] [[chipset]].<ref name=holt/> The first commercial microprocessor chip was the contemporary [[Intel 4004]]. The 4004 did not have nearly the computing power or interfacing capability required to perform the functions of the CADC. At the time, the best integrated circuit (chip) technology available lacked the scale (number of transistors per chip) necessary to build a single-chip microprocessor for a flight control system.

[[File:F-14 Tomcat preparing to refuel.jpg|thumb|250px|The MP944 chip set was the core of the CADC used to control the swing wings of the [[F-14 Tomcat]] naval interceptor.]]

The CADC was designed and built by a team led by Steve Geller and [[Ray Holt (computer scientist)|Ray Holt]], and supported by the startup [[American Microsystems]]. Design work started in 1968 and was completed in June 1970, beating a number of [[electromechanical]] systems that had also been designed for the F-14.  It was classified by the Navy<ref>{{cite book |first1=Sudhir |last1=Dixit |first2=Ramjee |last2=Prasad |title=Human Bond Communication: The Holy Grail of Holistic Communication and Immersive Experience |date=2017 |publisher=9781119341338 |page=211 |isbn=9781119341338 |url=https://books.google.com/books?id=UW4lDgAAQBAJ&dq=%22ray+holt%22+%22microprocessor%22+classified&pg=PA211 |accessdate=2 June 2020 |archive-date=26 December 2020 |archive-url=https://web.archive.org/web/20201226121449/https://books.google.com/books?id=UW4lDgAAQBAJ&pg=PA211&dq=%22ray+holt%22+%22microprocessor%22+classified |url-status=live }}</ref> until 1998. Ray Holt's story of this design and development is presented in his autobiography ''The Accidental Engineer''.<ref name=holt/>

In 1971, Holt wrote an article about the system for ''Computer Design'' magazine.<ref name=":0">[https://firstmicroprocessor.com/wp-content/uploads/2020/02/ap1-26-97.pdf 1971 paper on the CADC (which was classified and never published)]</ref> The Navy [[Classified information in the United States|classified]] it, and released it in 1998.

==Components==
The CADC consisted of an [[analog-to-digital converter]], several [[quartz]] pressure sensors, and a number of MOS-based [[microchips]]. Inputs to the system included the primary flight controls, a number of switches, static and dynamic air pressure (for calculating stall points and aircraft speed) and a temperature gauge. The outputs controlled the wing sweep and the maneuver flaps and slats and limited allowable control inputs.<ref>{{cite book |title=NATOPS Flight Manual Navy Model F-14D Aircraft |date=15 Jan 2004 |publisher=Department of the Navy |pages=11-1,20-6,14-42 |url=https://info.publicintelligence.net/F14AAD-1.pdf}}</ref>

The CADC's MP944 chip set ran at 375 kHz, executing 9375 instructions per second and was based on a 20-bit [[Fixed-point arithmetic|fixed-point]]-fraction [[two's complement]] number system. The complete 28-chip system enabled by 74,442 transistors<ref>{{Cite web |title=Ray Holt and the history of MP944/Cadc @ Rome Technopole, 2017 - YouTube |url=https://www.youtube.com/watch?v=3GROYRkWvxc&feature=youtu.be&t=1343 |url-status=live |archive-url=https://web.archive.org/web/20201226121449/https://www.youtube.com/watch?v=3GROYRkWvxc&feature=youtu.be&t=1343 |archive-date=2020-12-26 |access-date=2020-11-06 |website=www.youtube.com}}</ref> used the following 6 unique [[dual in-line package]] (DIP) chips:
{| class="wikitable"
|+
!Chip name
!Abbreviation
!Pin count
!Purpose
!Number of chips used in the system
|-
|[[Read-only memory]]
|ROM
|14
|provides instructions and [[Constant (computer programming)|constants]]
|19
|-
|Steering logic unit
|SLU
|28
|decodes instructions and routes data into a computation unit
|3
|-
|Parallel multiplier unit
|PMU
|28
|computation unit
|1
|-
|Parallel divider unit
|PDU
|28
|computation unit
|1
|-
|Special logic function
|SLF
|28
|computation unit
|1
|-
|Random-access storage
|RAS
|14
|stores data from its computation unit
|3
|}
The system arranges these chips into 3 modules. Each module consists of a set of ROMs which [[Serial communication|serially]] send [[Microcode|microinstructions]] and constants to that module's SLU, which routes data inputs to that module's computation unit (either a PMU, a PDU, or a SLF), whose results are written into that module's RAS and routed via the SLUs to any module. Each module forms its own [[Pipeline (computing)|pipeline]] and can be used without the others. This made it easy to expand the system with additional modules. Multiple pipelines worked at the same time, a [[parallel computing]] technique called "pipeline concurrency". The ROM stores 128 [[Word (computer architecture)|words]] of 20-bits each. A register counter in ROM can be reset, step through the words in sequence, accept a retain address command and hold the present address, and accept a numerical input for address modifying or loading.<ref name=":0" />

==References==
{{Reflist}}

===Further reading===
*[https://www.wired.com/story/secret-history-of-the-first-microprocessor-f-14/ The Secret History of the First Microprocessor, the F-14, and Me]
*[https://www.youtube.com/watch?v=YpruA5mC7wg The World's First Microprocessor: F-14 Central Air Data Computer]

==External links==
*[https://firstmicroprocessor.com/ First Microprocessor]

{{AlliedSignal}}

[[Category:Avionics computers]]
[[Category:Computer-related introductions in 1970]]
[[Category:Flight control systems]]
[[Category:Military computers]]