Apollo Guidance Computer

The Apollo Guidance Computer was the first recognizably modern embedded system. It was developed by Charles Draper[?] and the MIT Instrumentation Laboratory[?], with hardware design led by Eldon C. Hall. Each flight to the moon had two of these computers. They ran the inertial guidance systems of both the command module and LEM.

The Apollo flight computer was the first to use integrated circuits. The computer consisted of roughly a thousand identical integrated circuits, each with two NAND gates. They were interconnected by a technique called wire wrap, in which the circuits are pushed into sockets, the sockets have square posts, and wire is wrapped around the posts. The edges of the posts bite the wire with tons of pressure per square inch, causing gas-tight connections that are more reliable than soldered PC boards. The wiring was then embedded in cast epoxy plastic.

The computer's RAM was magnetic core memory and ROM was core rope memory[?]. Both cycled in 12 microseconds. The word length was 15 bits, except for the accumulator, which had 16 bits (for overflows).

The instruction format was 3 bits for opcode, 12 bits for address.

The user interface was an array of numerals and a calculator-style keyboard (electronic calculators were unknown at the time - slide rules were used). Commands were entered numerically as two digit "prog", "verb" and "noun" numbers. The numerals were light-emitting diodes, arranged in an array of seven segments per numeral to display numbers. Input was by pushbuttons. This "calculator-style" interface was the first of its kind, the prototype for all similar interfaces.

It's not widely known, but the system failed during the first lunar descent, with a "1201 alarm" and "1202 alarm" (tasks not finished) because the terminal approach (docking) radar was left on and unexpected radar returns from the moon overloaded the CPU. This almost caused a mission abort. The software failed safe, shedding unnecessary tasks, and the inertial guidance tasks continued to operate reliably.

External Links

• National Air and Space Museum - Apollo Guidance Computer (http://www.nasm.si.edu/nasm/dsh/artifacts/GC-ApolloBlock1.htm)
• National Air and Space Museum - Apollo Display Keyboard (DSKY) (http://www.nasm.si.edu/nasm/dsh/artifacts/GC-ApolloDSKY.htm)