
Restoring Y Combinator's Xerox Alto, day 3: Inside the disk drive - kens
http://www.righto.com/2016/07/restoring-y-combinators-xerox-alto-day_11.html
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ChuckMcM
Ah the RK05 (that was the DEC version of the Diablo drive). I used to have a
replacement set of R/W heads for one but gave them to a friend who was
restoring a couple of drives for a PDP-11 project. Fun times.

I've wondered what it would take to build a drive and its media from scratch.
DEC used to do it all the time of course but given today's tools and tech.
Could you build a 5MB removable platter drive? What sort of seek rate could
you get on it? Clearly one of those projects that people that build their own
vacuum tubes would get in to.

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rzzzt
For the platter, you'd need a substrate with the ferromagnetic material
applied – I'm not sure what process is involved here, electroplating, coating?
Some equipment might be borrowed from the tape manufacturing process if that
is not considered cheating.

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ChuckMcM
I'd go with a spin coating, easiest to create. However coming up with a
material that could be easily spin coated that would be a bit harder. There is
a lot of math involved [1] :-). There was an excellent "Secret Life of
Machines"[2] video about tape which made magnetic tape by sprinkling ferrous
oxide (rust) over "sticky tape" (which in the US is often called 'Scotch Tape'
as that is the dominant brand).

[1]
[http://ecee.colorado.edu/~mcleod/teaching/EandM3400/Lab%20Bo...](http://ecee.colorado.edu/~mcleod/teaching/EandM3400/Lab%20Book/Chp_13.pdf)

[2]
[http://www.secretlifeofmachines.com/secret_life_of_the_video...](http://www.secretlifeofmachines.com/secret_life_of_the_video.shtml)

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ricksplat
Scotch tape is something slightly different to what I'd consider "sticky
tape", which is what I'd also know as _Sellotape_. We have Scotch tape here as
well (W. European seaboard ;) but it's a neater more refined (cloudy) tape
specifically used for paper, whereas Sellotape is a more general purpose clear
tape.

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Animats
Ah, the joys of 1970s electronics, or "it's full of parts!" Electronics was
finally good enough that you could design and build complex systems and have
them work. But it took a huge number of 7400 series TTL ICs to do anything.

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dboreham
That disk emulator fpga would be helpful for people looking to resurrect other
old computers (looking at a couple of closets full of pdp-11/70 boards in my
office...) so hopefully the design will be published/open sourced. RK-05s I'm
sure are not identical but probably sufficiently similar for the design to be
a good starting point.

~~~
kens
There's a MFM disk emulator project here, but I don't know if it's compatible
with your drives:
[http://www.pdp8.net/mfm/mfm.shtml](http://www.pdp8.net/mfm/mfm.shtml)

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chiph
I'd like to see a photo of the linear motor that drives the heads. They come
in massive and MASSIVE sizes...

~~~
kens
Unfortunately the drive motor is underneath the head assembly, so you can't
see it without more disassembly of the drive. The servo motor moves the heads
via a rack and pinion gear.

There's a (bad) photo of the motor on page 52 of the manual:
[http://bitsavers.trailing-
edge.com/pdf/diablo/81503-03_Serie...](http://bitsavers.trailing-
edge.com/pdf/diablo/81503-03_Series_30_Disk_Drive_Maintenance_Jul78.pdf)

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asciimo
While watching this video, the significance of the advent of SSD really
strikes home.

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jloughry
Steve Wozniak's disk controller for the Apple II used the same kind of
approach: a few simple chips on the disk controller card with all the timing
and marshalling done by code running on the CPU. Doing it that way is
practical if you have confidence that the CPU speed won't change.

~~~
kens
I think Wozniak would have fit in well on the Alto team; he had the same
approach of minimizing hardware as much as possible.

One unusual thing about the Alto is the code to run the disk (and other
devices) was in microcode, not machine code. (Are there other computers that
used microcode for programming, not just to implement the instruction set?)

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flukus
Wasn't the Alto expensive because they didn't minimize the hardware enough?

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pinewurst
It was expensive because minimized discrete hardware (not soon enough for any
kind of real integration) was still expensive. Even the successor 2901-based
D-machines were expensive in absolute terms. By the time Alto-level
functionality was relatively cheap to implement, the software requirements had
grown well beyond. Hence PARC moving eventually to the likes of ECL for
research platform development.

~~~
kens
To expand on what pinewurst said, the Alto was built minicomputer-style out of
TTL chips, so even with minimized hardware the CPU took three large boards of
chips. Within a few years, microprocessors took over and Moore's law crushed
computers built from discrete chips.

Here are some bonus pictures of boards from the Xerox Dorado, which used ECL
chips for higher speed. First is the ALU board; the large chips are ECL
versions of the 74181 ALU. Second is a detail of chips from the 10K family, as
well as unusual square memory chips.

[https://lh3.googleusercontent.com/\--KYoSBUuOhg/V4Rxs7tLB-I/A...](https://lh3.googleusercontent.com/--KYoSBUuOhg/V4Rxs7tLB-I/AAAAAAAA4vM/_HY8Xvkg0DgRjl36M4ZyjY5PYXrvlWh2gCCo/s800/alu.jpg)
[https://lh3.googleusercontent.com/-MWYhn7enXdA/V4RxsQ8YHRI/A...](https://lh3.googleusercontent.com/-MWYhn7enXdA/V4RxsQ8YHRI/AAAAAAAA4vI/5hAeS8i7ZX8M3DNANpXNrcXStB3_0qZmACCo/s800/square.jpg)

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13of40
I found a drive just like that in a dumpster in Eugene Oregon in 1997. No
other parts, and I couldn't get it to do anything interesting, so it
eventually went back in the trash.

