11 Sep
2004
11 Sep
'04
9:46 a.m.
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 @ 800 KHz or
so - easily done with a PC parallel port (= 100KHz reading of shift register)
Peter Wallace