Rotating memory data recovery

From: Peter C. Wallace <>
Date: Sat Sep 11 16:46:23 2004

On Sat, 11 Sep 2004, Tom Jennings wrote:

> OK, I'm very close to needing to suck the data off the main memory of
> this LGP-21 before I reform caps and all that electronicky stuff.
> It would be nice to have a 16-bit MHz A/D system, but I don't. Any
> *practical* suggestions for someone with an extremely limited budget? I
> also don't have time to build one, though it's within my skills.
> (The LGP-21 I have has a rotating magnetic main memory; bit-serial 80KHz
> clock, 64 physical tracks (128 logical), 4096 31 (32) bit words, NRZ or
> NRZI. I figure A/D sampling of the raw head signal at 8 to 10 times the
> original clock rate will allow offline data recovery, should I smash
> data on later CPU powerup.)
> The platter is driven by an AC motor. It powers up completely
> independently of the rest of the computer, so I can easily do all this
> before I begin the restoration. I planned on rigging up a fast opamp
> buffer/amp, clip-leading it onto each head in turn (the heads are low-Z;
> I do know about ground, common-mode, etc), and taking 2, 4 or a dozen
> snapshots of each track. I'll make the track snapshots available to
> anyone who wants them.
> If there's anything there, it would be really nice to see
> 41-year-old-data.
> Practical suggestions appreciated...

I would think that 16 bits is way overkill (the original read channel gets by
with one bit) 8 bits ought to be good enough if you use most of the bits.
8bit 1 MHz A-Ds are cheap and simple to use.

Also since you have multiple passes available you could just use a comparator
and adjust the threshold (even manually) every rotation (like a sampling
scope) This way you only need to record (multiple) 1 bit streams _at_ 800 KHz or
so - easily done with a PC parallel port (= 100KHz reading of shift register)

Peter Wallace
Received on Sat Sep 11 2004 - 16:46:23 BST

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