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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 21st Nov 2018, 9:52 am   #1
M0FYA Andy
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Default Apollo Guidance Computer

Anyone interested in historic computers might enjoy these videos of an attempt to get a rare example of the Apollo Lunar Module Guidance Computer back to life -

https://www.youtube.com/watch?v=2KSahAoOLdU

https://www.youtube.com/watch?v=9Ys-VshuOLQ

Only recently posted, there must be more to come.

I found them fascinating!

Andy
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Old 21st Nov 2018, 10:20 am   #2
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Default Re: Apollo Guidance Computer

Indeed fascinating stuff. I may have posted this before but attached is an article about the LEM computer which was sent to me by a JPL colleague.
Attached Files
File Type: txt LEMComputer.txt (11.3 KB, 132 views)
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Old 21st Nov 2018, 10:59 am   #3
Craig Sawyers
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Default Re: Apollo Guidance Computer

Wow - thanks for finding that. There is a guy out there with a set of rope modules he has been checking out. Also on youtube Hopefully they can get together and run the AGC with original programmes.

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Old 21st Nov 2018, 11:05 am   #4
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Default Re: Apollo Guidance Computer

If you want a real deep dive into how the AGC worked this book gives a _lot_ of detail https://www.springer.com/us/book/9781441908766

It's quite interesting to hear how the rather odd architecture came about when they ran out of room for various functions and how some rather clunky extensions were grafted in....like nearly every computer project before or since!
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Old 21st Nov 2018, 12:07 pm   #5
Craig Sawyers
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Default Re: Apollo Guidance Computer

There is a superb Pulitzer Prize for non-fiction book called The Soul of a New Machine, by Tracey Kidder. Describes Data General's supercharged skunk works in their 1980 attempt to catch up with Digital's stranglehold on the mini-computer market with their launch of the VAX.

It even has some industrial espionage in it, when the project manager poses as a cleaner, and gets himself into the room with a VAX in it, and spends the night taking it to bits to learn its architecture.

Recruited new graduates as the project team "Because they don't understand that the project is impossible, so they will succeed where experienced designers will fail"

Still worth reading, although the technology is of course massively dated.
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Old 21st Nov 2018, 1:16 pm   #6
M0FYA Andy
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Default Re: Apollo Guidance Computer

Craig, your quote "Because they don't understand that the project is impossible, so they will succeed where experienced designers will fail" is reminiscent of what was said at the reopening of the Southern Stratford Canal by David Hutchings (the project leader) in his reopening speech - "When we started this project, all the experts told us it was impossible. Fortunately none of us were experts, so we just got on and did it".

Back on the subject of the AGC, a pal of mine who has studied the subject tells me he has never found out why the designers used 1's complement logic rather than the more standard 2's complement. 1's complement has two values for zero which can cause problems. Does anybody know the reason for this choice?

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Old 21st Nov 2018, 6:05 pm   #7
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Default Re: Apollo Guidance Computer

I not long ago re-read the Soul of a New Machine, and was not at all impressed. I first read it in the early 80's, and not since. But the concept of designing the CPU rather than throwing it together and then de-bugging obviously didn't occur to them. I thought the design team were incredibly amateurish.

As for the 1's complement, I worked for CTL for a short time and the computer, considering it was designed by Ian Barron (?, of Transputer fame) was naff. Compared with the PDP11 it was stone age, why?
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Old 21st Nov 2018, 6:12 pm   #8
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Default Re: Apollo Guidance Computer

If you do some digging on the NASA website, you should find the diagrams and mnemonic code for the AGC and lots of photos. There's also copies of the purchase orders for the parts and various correspondence between the various parties involved in its design.
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Old 22nd Nov 2018, 1:31 pm   #9
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Default Re: Apollo Guidance Computer

My understanding of why space computers appear stone-age has a lot to do with the time it took to test and certify them, and also the bizarre architectures used to make them robust enough to work in space.

I am sure a better machine could have been built out of more complex gates that were available at that time, but then it would probably have broken down en-route.

On 1's complement, it has advantages in signal processing calculations because negating a number always changes the sign, and also negating is faster because there is no carry propagation.
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Old 22nd Nov 2018, 1:44 pm   #10
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Default Re: Apollo Guidance Computer

The same thing is true now. Space qualified semiconductors, which are:

1. Capable of handling launch loads, including 1000g shocks when explosive bolts are fired.
2. Are hermetic, so they neither outgas or explode when transitioning from earth to vacuum
3. Can handle the continuous ionising radiation load.
4. Are latch up free - so when hit by a gamma ray photon or high energy electron. In fact they have to be double latch-up free. So when hit twice in rapid succession they do not latch up.

100%. For years. No service visits possible. And redundancy is helpful, but not a panacea. It adds weight, which is expensive (about 2 million of most currencies per added kg).

From design to launch is typically ten years, and for interplanetary spacecraft many years thereafter. So all the superb images you see from Jupiter and Saturn are with twenty year old technology. That is just the way it is.

Craig
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Old 22nd Nov 2018, 6:56 pm   #11
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Default Re: Apollo Guidance Computer

If you want what I think is a pretty definitive contemporary view on why the AGC was designed the way it was; in particular the use of a single logic gate (3 input NOR) as the only logic element (one other IC was used - a memory sense amplifier), then this paper from MIT is probably as close as you can get:

https://www.ibiblio.org/apollo/hrst/archive/1716.pdf

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Old 22nd Nov 2018, 9:58 pm   #12
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Default Re: Apollo Guidance Computer

very interesting this post, i have a board of the Apollo Guidance computer, I will put a photo in a few days if you are interested.
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Old 22nd Nov 2018, 11:18 pm   #13
M0FYA Andy
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Default Re: Apollo Guidance Computer

I'll read that report with interest, many thanks!

Andy
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Old 25th Nov 2018, 12:38 pm   #14
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Default Re: Apollo Guidance Computer

One board similar to this is the board that I have in my collection

https://www.apolloartifacts.com/2007...aturn_v_1.html
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Old 26th Nov 2018, 1:52 pm   #15
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Default Re: Apollo Guidance Computer

Quote:
Originally Posted by M0FYA Andy View Post
Anyone interested in historic computers might enjoy these videos of an attempt to get a rare example of the Apollo Lunar Module Guidance Computer back to life -
Great stuff, Andy!

Thanks for sharing these and looking forward to more!
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Old 27th Nov 2018, 9:32 am   #16
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Default Re: Apollo Guidance Computer

I have just finished reading the documents posted / linked on this post and watched the videos.
Fascinating stuff, I await the next episode with interest.

This thread shows the power of this forum, I would never have found these items without it.
It is in sharp contrast to the AI 'bots used by the likes of the BBC iplayer and others who try and deliver "recommended for you" based on your viewing history, I have never yet found any of their recommendations of any interest, but I digress.

Peter
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Old 27th Nov 2018, 12:40 pm   #17
M0FYA Andy
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Default Re: Apollo Guidance Computer

Part 3 of the video has now been uploaded -

https://www.youtube.com/watch?v=vfUhF3BJoBc

Andy
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Old 27th Nov 2018, 7:33 pm   #18
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Default Re: Apollo Guidance Computer

It's an absorbing set of videos and what a great collaboration between these guys. Particularly as I'm reading Dan Parrys 'Moonshot' at the moment.

Andrew
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Old 27th Nov 2018, 8:56 pm   #19
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Default Re: Apollo Guidance Computer

Nice to see this. I tried to ask some simple questions about it while standing next to the hardware in Cape Canaveral or whatever name it goes by today.

Stock reply - "Book shop is in the visitor centre" - or some such.

I bought a book.

It was very interesting but it was about the design / build process of the hardware for the whole program.
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Old 28th Nov 2018, 2:10 pm   #20
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Default Re: Apollo Guidance Computer

Hi all,

If its of any interest, there's a website describing how to build your own AGC with links to circuit diagrams etc plus software simulators etc. See:-

http://klabs.org/history/build_agc/


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