920M - an European contemprary of the Apollo Guidance Computer

Dr. Erik Baigar erik at baigar.de
Mon May 3 13:27:20 CDT 2021

Dear colleagues in vintage computing ;-)

In the last years there has been substantial interest in the Apollo
Guidance Computer (AGC). So maybe someone  might be interested what 
happened outside of the United States in the 1960ties: Over the last few 
years I restored a 920M computer which was (among others) used as the
guidance computer in the Europa rocket. This was the ancestor of the 
European Ariane sapce launcher:


Although not well known, this machine is a contemporary of the AGC, it
also uses integrated, SMD mounted circuits and is realized in DTL
techology (Diode Transistor Technology). Its word width is 18bit,
8k of core store where standard within a show-box sized chassis. 2us
and later down to 1us instruction time where stadnard. In contrast
to the AGC, this machine was a member of a whole series of airborne
computers - some of which are still in use TODAY (2021).

After core and the DTL chips became obsolete, this architecture even was 
re-implemented by BAE systems using the AMD29XX chipset in the same
chassis as drop-in replacement.

The 920M never had core rope memory, and for rocket guidance 8k have
been enough although adding a 2nd box extending the core was possible.
Tte architecture is able to support up to 256k words of 18 bit.

Of course, the instruction set is very archaic - no carry flag, no stack
but it served its purpose with 18 bit choosen as the perfect word length
to give reasonable resolution without need for double-word calculation in
most applications; http://www.programmer-electronic-control.de/Elliott920FactsCard.pdf

If this triggered some interest, you may watch my recently released video
on the 920M (covers applications, internals and some software is shown 


...or visit my project page regarding the work of restoring three 
different members of this computer family to working condition:


Happy computing to all of you,

     best wishes,


P.S. Just wanted to point out, that also in Europe there have been relevant
      developments regarding spaceborne computing in parallel to the Apollo
      programme! In no ways I want to diminish the achievemets of the
      Apollo development team...

                               ( o o )
| Dr. Erik Baigar                             Inertial Navigation & |
| erik at baigar.de           .oooO              Vintage Computer      |
| www.baigar.de            (   )   Oooo.      Hobbyist              |
+---------------------------\ (----(   )----------------------------+
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Dr. Erik Baigar
Scientific Lead Spectroscopy Systems
Technologie Team Spectroscopy

M?nchner Weg 1
85232 Bergkirchen

Tel.:   +49 (0) 8131 5956-40147
Fax.: ?+49 (0) 8131 5956-99
Mail: ?EBaigar at thorlabs.com
Web: ?www.thorlabs.com

General Manager: Dr. Bruno Gross
HRB No: 85345, M?nchen

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