Many folks would like to see us back on the Moon and developing its resources.

Wednesday, October 01, 2008

Apollo Guidance Computer (AGC) - fly me to the Moon

Apollo Guidance Computer (AGC) - fly me to the Moon

David said that in England there was a video on the discovery channel
called Moon Machines.
One of the machines was designed by MIT, a small silicon computer that
would take the astronauts to the Moon.
They mentioned the 1201 and 1202 errors due to having the radar turned
on at the wrong time during the first landing.

I saw a TV show on similar or maybe the same subject some time back.
It showed the making of the Apollo Guidance Computer (AGC).
They showed the machines that were made to wire the logic circuits for
the firmware that let women pass a shuttle cock through to wire wrap
around the proper post to make the connections. Each card was then
placed on a tester that called up the logic it was supposed to solve.
Remember this is back in the 60's.
- LRK -


The Apollo Guidance Computer (AGC) was the first recognizably modern
embedded system, used in real-time by astronaut pilots to collect and
provide flight information, and to automatically control all of the
navigational functions of the Apollo spacecraft. It was developed in
the early 1960s for the Apollo program by the MIT Instrumentation
Laboratory under Charles Stark Draper, with hardware design led by
Eldon C. Hall. Based upon MIT documents, early architectural work
seems to have come from J.H. Laning Jr., Albert Hopkins, Ramon Alonso,
and Hugh Blair-Smith. The actual flight hardware was fabricated by
Raytheon, whose Herb Thaler was also on the architectural team.

It is interesting what was done with so little memory and storage for firmware.

I pulled a UNIVAC Digital Trainer out of salvage when I was at the
Navy training center just north of Memphis Tenn.
It took me about six months of playing with the circuit cards and
logic diagrams in the evenings to get it to work. - UNIVAC 422 TRNG COMP - BRL Report 1964

I think it had about 512 (15 bit) words of memory and was made up of
little circuit cards that were NOR gates made up of a transistor, some
diodes and some resistors. When I left my assignment there I gave it
back to salvage and had tears in my eyes.

When I got out of the Navy in 1983 I went to work supporting the
Pioneer programs at NASA Ames and they had Sigma V computers that were
still using transistors and diodes to make NOR gates to then make half
adders, that combined made full adders and WOW, you could add 1 to 1
and get 10 (in binary addition).


SDS Sigma-5 was a 32-bit computer that was introduced by the Xerox
company in 1965. (Scientific Data Systems [SDS] was a company Xerox
had acquired in 1969.) This was a reduced-capability version of the
Sigma 7 computer. It was commercially retired in the 1970s after Xerox
left the Mainframe computer manufacturing business.

Here we are now in 2008 and you can try your skill and landing.

Thanks for looking up with me.

Larry Kellogg

Web Site:
RSS link:

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space
flight systems,
with a view to their practical solution.


Block I Apollo Guidance Computer (AGC):
How to build one in your basement

Computers in Spaceflight: The NASA Experience
- Chapter Two -
- Computers On Board The Apollo Spacecraft -
The Apollo guidance computer: Hardware

[34] The Apollo Guidance Computer was fairly compact for a computer of
its time. The CM housed the computer in a lower equipment bay, near
the navigator's station. Block II measured 24 by 12.5 by 6 inches,
weighed 70.1 pounds, and required 70 watts at 28 volts DC. The machine
in the lunar module was identical.

Crew members could communicate with either computer using display and
keyboard units (DSKY, pronounced "disky"). Two DSKYs were in the CM,
one on the main control panel and one near the optical instruments at
the navigator's station. In addition, a "mark" button was at the
navigator's station to signal the computer when a star fix was being
taken. A single DSKY was in the lunar module. The DSKYs were 8 by 8 by
7 inches and weighed 17.5 pounds. As well as the DSKYs, the computer
directly hooked to the inertial measurement unit and, in the CM, to
the optical units.

The choice of a 1 6-bit word size was a careful one. Many scientific
computers of the time used 24-bit or longer word lengths and, in
general, the longer the word the better the precision of the
calculations. MIT considered the following factors in deciding the
word length: (a) precision desired for navigation variables, (b) range
of input variables, and (c) the instruction word format. Advantages of
a shorter word are simpler circuits and higher speeds, and greater
precision could be obtained by using multiple words. A single
precision word of data consisted of 14 bits, with the other 2 bits as
a sign bit (with a one indicating negative) and a parity bit (odd
parity). Two [35] adjacent words yielded "double precision" and three
adjacent, "triple precision." To store a three-dimensional vector
required three double precision words . Data storage was as fractions
(all numbers were less than one). An instruction word used bits 15-13
(they were numbered descending left to right) as an octal operation
code. The address used bits 12-1. Direct addressing was limited, so a
"bank register" scheme (discussed below) existed to make it possible
to address the entire memory.

The Apollo computer had a simple packaging system. The computer
circuits were in two trays consisting of 24 modules. Each module had
two groups of 60 flat packs with 72-pin connectors. The flatpacks each
held two logic gates. Tray A held the logic circuits, interfaces, and
the power supply, and tray B had the memory, memory electronics,
analog alarm devices, and the clock, which had a speed of one
megahertz. All units of the computer were hermetically sealed. The
memory in Block II consisted of a segment of erasable core and six
modules of core rope fixed memory. Both types are discussed fully


National Aeronautics and Space Administration

NASA History Office

NASA Contractor Report 182505 Computers in Spaceflight The NASA
Experience James E. Tomayko Wichita State University Wichita, Kansas




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