RFC: Floppy reader/writer project (recovering UniPlus Unix for

Tony Duell ard at p850ug1.demon.co.uk
Sat Feb 16 16:53:14 CST 2008

> Ray Arachelian wrote:
> > Philip Pemberton wrote:
> >> 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.)
> 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 :)

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 
> > 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)

Most floppy drives have a tachogenerator on the motor anyway. On the old 
full-height belt driven units, the spindle motor, a permanent magnet DC 
motor -- has 4 wires. 2 go to the motor, the other 2 to an AC-output 
tachogenerator inside the motor. I can look up the colour coding, IIRC 
most mangufacturers used the same motor.

More modern drives have a tacko track on the PCB under the spindle motor 
rotor. If you remove that, you'll see a 'square wave shape' track round 
the outside of the motor coils. That's it. It might be labeleld 'FG' when 
it gets to a useful testpoint (== Frequency Generator, do not confuse 
this with a Frame Ground point ;-))

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