16 Feb
2008
16 Feb
'08
2:53 p.m.
Ray Arachelian wrote:
Most, if not all, floppy drives have a pair of testpoints in the read
chain that are used to display the catseye pattern when doing a head
alignment. They're the output of the read amplifier before the filter
(normally) and differentiator stage.
I suspect those testpoints are just what you need. Hook them up to a fast
differential amplifier and thence to the ADC.
The problem might be finding them. If you can find a service manual for
the drive, it'll tell you wher they are. Alternatively look for a couple
of labeleld TP's oround an LC filter network. If you can find such a
pair, they are most likely to be what you want.
Or if you have the 'Microtest' floppy alignmet unit from the 1990s, see
if that lists the drive. 2 of the probes used with that go to those
testpoints.
Philip Pemberton wrote:
It may be a useful thing to build, but it probably isn't practical to build
it. If I had a drive that I had a schematic for, I might be a little less
pessimistic :) What you'd have to do is find the amplified
signal, then bring it into
the range an A/D controller would accept, and digitise it. You'll
probably want at least 20 megasamples/second to get a good amount of
resolution out of the data, and for a revolution time of a second,
you're talking 20 megabytes of data per track (assuming you use an
8-bit converter).
Sounds like it might be a very useful thing to build. (I don't even
have my hands on the disks at this point, he's resorting to using
CopyIIMac right now, which hopefully will be able to read a bit past the
errors.) > I suppose you could also slow down the motor
speed, as to increase
Be careful doing that. There's almost certainly a frequency-dependant
filter network in the read chain which might not like a very low data rate.
resolution,
but all of these would be pretty invasive drive mods.
(you'd still need some way to precisely measure the distance traveled by
the motor.)
Putting a tach on the motor would be fairly uninvasive. Avago (nee Agilent)