Re: Latest mac that supported 800k ProDos disks

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

Charlie <charlieDOTd@verEYEzon.net> wrote:

> On 9/3/2012 8:15 PM, David Empson wrote:
> > 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,
> 
> I agree, but apparently it can be disabled by the controller since the
> PC Transporter manages to do it for the 720K MFM disks.

Are you sure it actually stops the variable rotation speed, as opposed
to the controller adjusting the bit cell timing? I no longer have one
one of those drives connected to a PC Transporter and it would be a pain
to test it again.

The timing signal sent by the Mac to older drives is on the same pin as
the Apple II write protect signal (output by the drive), but there is an
"Apple II" mode control pin which might be related to this: if the drive
was set to operate in Mac mode, it might support the timing control
signal, and the PC Transporter's controller could just use fixed timing.
(The write protect signal can still be read via a status command.)

> > 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.
> 
> Mine works flawlessly for reading and writing *if* the disks are 
> formatted on a PC or pre-formatted.  If they are formatted on the PC 
> Transporter they still work flawlessly if they are used only with the PC
> Transporter card.  The only problem I've found is when a disk is 
> formatted on the PC Transporter some PCs (less than half of those I've
> tried) have problems with those disks.

The formatting may be an issue with the "start of track" position. The
Apple 3.5 Drive doesn't provide access to the start of track detector
(hole for 5.25" disks, locating notch for 3.5"). If some PC's BIOSes are
expecting the tracks to start at the same position, they would not be
happy with a staggered start of track position laid down by the Apple
3.5 Drive.

-- 
David Empson
dempson@actrix.gen.nz