Re: Latest mac that supported 800k ProDos disks

[dempson@actrix.gen.nz (David Empson)]

Charlie <charlieDOTd@verEYEzon.net> wrote:

> On 9/3/2012 3:53 PM, D Finnigan wrote:
> > J. Vernet wrote:
> >>
> >> As I remember, it's not a drive harware issue but a controler hardware
> >> issue... In an USB external floppy, there is an USB controler that do
> >> not let the drive being used in 800k (variable rotation speed).
> >
> > It's very likely that the drive can't spin at more than one speed, that the
> > motor is fixed.
> >
> > As far as I know, only Apple computers had variable-speed 3.5" floppy
> > drives. These were manufactured by Sony.
> >
> > The Apple drives used GCR at their lowest level, whereas the IBM drives used
> > MFM.
> >
> I believe the 3.5" Apple drive can read/write either GCR or MFM if the
> controller is designed for both.  Consider that a normal 3.5" Apple 
> drive when connected to a PC Transformer can read/write 800K ProDOS 
> disks and 720K MSDOS disks that are compatible with IBM clones.  Note
> also that with the 720K disks the floppy drive speed doesn't vary.

The Apple 3.5 Drive (800K model) has the variable drive rotation speed
as a built-in feature, unlike earlier Macintosh 400K/800K drives, where
the rotation speed was controlled by a timing signal from the Mac.

If the Apple 3.5 Drive is used with a PC Transporter to access 720K
disks, the disk controller on the PC Transporter has to cope with
variable bit cell timing depending on which range of tracks are passing
under the head. I don't recall whether I used that capability much, but
I expect it would be somewhat unreliable for writing, and not too bad
for reading.

-- 
David Empson
dempson@actrix.gen.nz