
Restoring YC's Xerox Alto, Day 6: Fixed a chip, data read from disk - mr_golyadkin
http://www.righto.com/2016/09/restoring-ycombinators-xerox-alto-day-6.html
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abraae
So impressed with the progress.

When I was a lad I spent 2 and a half days without sleep debugging an
intermittent logic problem in an IBM 3033, a machine the size of a room with
massive looms of trilead cables running between the frames. I remember the
terror of having to do some bodgy fix like the one described when the guys
file the notch out of the extender card, while an angry data processing
manager a few decades my senior cursed and glowered behind me and the bank's
check processing center stood idle. Perhaps the scariest experience of my
life.

Hats off to anyone who fixes old computer hardware.

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digi_owl
Seems to me that these times its worse.

With hardware you have a known good state to test against.

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abraae
I prefer software! With hardware you have to think about ringing circuits and
intermittent faults that you eventually find are due to the cleaner bringing
in a vacuum cleaner with bad motor brushes that injects electrical noise into
the mains. Most software bugs are nothing compared to tracking down that sort
of thing.

~~~
jasoncchild
It's not black magic; systematic troubleshooting is the key to either
environment.

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heywire
I just wanted to say thanks for the work that has gone into, not only the
restoration process, but the videos and accompanying articles as well.
Computer history has always been a favorite topic of mine. I was only 7 when
my father gave me my first computer (an IBM 5150), so there is a lot that I
simply wasn't around to experience. These videos and articles give me a taste
of that. Thanks.

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Animats
This is looking good. Disk returns data, CRT lights up, CPU almost working.
It's good to hear that data is coming in from the disk. Looks like they won't
have to build something to emulate the disk drive.

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ChuckMcM
I really enjoy these. Having brought a number of MicroVAXen back to operating
condition it is really fun to have such a complex system suddenly back in
operation. Understanding a problem at the chip level is so much more
satisfying than saying "well I swapped this board and it works now."

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agumonkey
Wow, seriously impressed by the regular progress on each step. Great
"documentary".

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throwanem
Maybe a dumb question, but why dead-bug the existing 7414 instead of dropping
in a new one?

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kens
We didn't have a new 7414 handy, so we hacked the old one. We'll replace the
chip with a new one next session.

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fit2rule
That was a particularly nice little angle, I felt .. when I showed my kids
this video, I had to explain how the chip contained multiple gates, and it was
likely that one of them was borked, so you hacked over some wires to use one
of the other gates - and my kids tuned into this right away .. "you mean that
not all the features of these chips are used in the original?" .. typical
engineering lesson, in a few seconds of video. :)

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digi_owl
That's why Woz's boards were so crazy, he found ways to make use of every last
gate and such on each chip he put in.

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justin66
Fascinating thing I saw recently in social media, though I don't have a URL:
Woz woke up one night in 2014 with the thought of a way to knock a chip off
the Apple II's design. I haven't looked at the details but of course this was
very endearing to me, the pure love for that design that Woz has always felt.

[https://groups.google.com/forum/#!topic/comp.sys.apple2/iiLg...](https://groups.google.com/forum/#!topic/comp.sys.apple2/iiLgD-7INtE%5B1-25%5D)

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fit2rule
Thanks for that, great read. . .

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donpdonp
Thank you for this series! Its a real education and fun walk back in time to
see how the Alto was made and how it operates.

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AceJohnny2
Since it wasn't mentioned in this update, I assume the memory parity problem
from last week remains?

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kens
Near the end I mention that the parity errors from last week have disappeared.
There's no obvious reason for them to go away, but memory appears to be
working okay now.

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AceJohnny2
Oh, I missed that sentence, thanks! Since it was such a big part of last
week's debug, I thought there'd be more followup coverage.

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ifdefdebug
Problems which go away alone tend to come back alone, so maybe a followup
comes next week :)

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sghiassy
I watch these videos all the time on YouTube. Thanks for documenting the
work!!

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source99
Why not proactively check all the chips to make sure all gates are working on
each chip, rather than try to debug when something breaks?

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kens
Checking all the chips would be very, very time consuming. Not to mention we'd
need to build some sort of test setup for each chip. It's more likely we'd
cause a problem (e.g. bend a pin) than fix a problem.

~~~
source99
But wouldn't it be fun to build a raspberry pi based chip tester!

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avian
We used to have an old EPROM (the UV-erasable kind) programmer in the lab. I
remember it also had a mode for testing 74xx series logic. I don't think it
was ever used in that capacity though.

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makomk
I actually watched a YouTube video the other day where someone used a similar
programmer to test all the logic chips in his ZX-80:
[https://youtu.be/qqUxXrh_xgM](https://youtu.be/qqUxXrh_xgM) (Spoiler warning:
it didn't find the failure.)

