16 Feb
2008
16 Feb
'08
4:21 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.
That sounds like a fun little project for later on... maybe once I've learned
about FM and MFM encoding, I'll move onto magnetic flux pulses in the analogue
domain...
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.
I'd have thought the head would have a bearing on that too...
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.
The problem being getting enough resolution to figure out how much the speed
varies over a single revolution. A single pulse per revolution just isn't
enough to say that with any degree of accuracy. It's enough to say that
revolution #1 took 0.2% too long and revolution #2 was 0.34% too fast, but not
enough to say where in the cycle that error was.
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 ;-))
More fun things to scope out with my DSO. Guess I know what I'll be spending
my Sunday doing :)
Thanks,
--
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