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

[Ray Arachelian]

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
>
>
>