16 Feb
2008
16 Feb
'08
4:31 p.m.
Tony Duell 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.
In my specific case, it would be a very early Sony 400K SS/DD 3.5" drive
as used in the Lisa and the early Mac's. But let's see if I can get at
this stuff some other way first.
Still the above info is very useful to have for future projects, once I
get my hands on something like Philip's box.
Ideally, I'd love to have a setup with 3.5", 5.25", and 8" drives.
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 ;-))
-tony