Re: Apple 3.5" drive questions

["Charlie" <charlied@NOSPAMbboard.com>]

"David Empson" <dempson@actrix.gen.nz> wrote in message
1frbldu.gj851zlqfetcN%dempson@actrix.gen.nz">news:1frbldu.gj851zlqfetcN%dempson@actrix.gen.nz...
> Charlie <charlied@NOSPAMbboard.com> wrote:
>
> > Recently, through the work of Simon Williams, a problem long held to be a
> > hardware incompatibility was solved by software.
> > This made me wonder for the millionth time if software could solve the
> > problem of reading and writing MS-DOS with an 800k Apple 3.5" floppy drive
> > connected to an Apple II.
> >
> > Yes, I know, this is supposed to be impossible.  The hardware is supposed
> > to be incompatible with writing MS-DOS.
>
> Most Apple II 3.5" disk controllers are not capable of reading and
> writing MFM encoded disks.  The major exceptions are the SuperDrive card
> (which only supports it in conjunction with a SuperDrive) and the PC
> Transporter (which can do it with an Apple 3.5 Drive, albeit somewhat
> unreliably).
>
> > What I don't know is why.
>
> Compared to GCR, MFM requires double the resolution for placement of
> bits.  On a double density 3.5" disk, GCR needs 2 microsecond
> resolution, while MFM needs 1 microsecond resolution (flux reversals
> must still be at least 2 microseconds apart, but higher data density can
> be achieved if the bits may be 2, 3 or 4 microseconds apart rather than
> just 2 or 4 microseconds).
>
> The standard 3.5" drive controllers on the Apple II (including the IWM
> chip) only support 2 microsecond resolution for bit cells, so they can't
> access MFM data.
>
> Another problem is that when reading data, the IWM requires bit 7 to be
> set in each disk byte.  This is true for the values used with GCR, but
> MFM allows arbitrary 8 bit data to be read.
>
> I'm sure there are other technical problems.
>
> The SWIM (used on the SuperDrive card, and an early version is also used
> on the PC Transporter) presumably allows for both of these issues when
> operating in MFM mode.
>
> > What I do know is that the drive itself (in an Apple IIgs) can read and
write
> > MS-DOS when connected to PC Transporter.  So what is it about the PC
> > Transporter that is different from the Apple IIgs smartport/IWM?
>
> It has a variant of the SWIM chip, which supports MFM.  The IWM doesn't.
>
> > One guess would be that the Apple firmware doesn't allow for maintaining
the
> > same disk rotational speed.  MS-DOS disks rotate at the same speed no
matter
> > which track the head is on.  Apple disks vary the speed.
>
> The variable speed of rotation is hard wired into the Apple 3.5 Drive.
> When used with a PC Transporter, it is somewhat unreliable compared to a
> 3.5" TransDrive (720K MFM, fixed rotation speed) because the bit cell
> timing varies depending on the track range in which the head is located.
>
> > Would it be possible to write a driver that could hold the speed steady?
>
> No.
>
> (Note that the older Macintosh 400K and 800K drives used a timer output
> from the VIA chip on the motherboard to control the rotation speed, so
> the speed could be controlled through software.  This mechanism was
> built into the drive for the Apple 3.5.)
>

Thank you David.  As always, your explanation is so clear that even I can
understand it.  Now I can go on to worry about other things.

Charlie