Central Air Data Computer

From Wikipedia, the free encyclopedia

The Central Air Data Computer is the integrated flight control system used in the early versions of the US Navy's F-14 Tomcat fighter. It is notable for its use of a MOS-based LSI chipset, the MP944, arguably making it a candidate for the first microprocessor design in history.

The CADC was designed and built at Garrett AiResearch by a team lead by Steve Geller and Ray Holt, and supported by the startup American Microsystems. Design work started in 1968 and was completed in June 1970, beating out a number of electromechanical systems that had also been designed for the F-14.

The CADC as a whole consisted of a 20-bit A-to-D converter, several quartz pressure sensors to detect the positions of various switches and flight controls, and the MOS-based CPU. 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 primary flight controls, wing sweep, the F-14's leading edge "glove", and the flaps.

The MP944 consisted of six chips, used in various numbers to build the CADC's CPU. They were the Parallel Multiplier Unit (PMU), the Parallel Divider Unit (PDU), the Random Access Storage (RAS), the Read Only Memory (ROM), the Special Logic Function (SLF), and the Steering Logic Unit (SLU). The complete microprocessor system used one PMU, one PDU, one SLF, 3 RASs, 2 SLUs, and 19 ROMs.

Holt wrote an introduction to the design in 1971, but the Navy classified it. Another attempt to publish the paper in 1985 also failed, and it was not until 1997 that the government finally agreed to allow it to be published. For this reason the MP944 remains fairly unknown in spite of its historical importance.

Perhaps because of the secrecy, the Intel 4004, released on 15 November 1971, is widely considered to be the first microprocessor. The 4004 was a simple 4-bit CPU, without pipelining and other advanced facilities, and hence was much less capable than the CADC. However, David A. Patterson argues that the F14 CADC should not be considered a microprocessor in the modern sense and instead a "microprogrammed special purpose computer, using a variable number of custom chips."^[1]. Russell Fish, noted Motorola designer, says that "Is the CADC designed in 1968 by Ray Holt and Steve Geller a microprocessor? The answer is "YES". "Their PDU and PMU chips are probably the world's first math coprocessors.." "Three decades later, Mr. Holt's solution must still be viewed with awe." "It is by any definition a technical tour-de-force of the first order." ^[2].

Holt went on to found Microcomputer Associates and produce several single-chip systems based on the general principles of the MP944.