Re: Apple 3.5" drive questions

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

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.)

-- 
David Empson
dempson@actrix.gen.nz