stepping machanism of Apple Disk ][ drive (was Re: Heatkit 5 1/4 floppies)

From: Eric Smith <eric_at_brouhaha.com>
Date: Wed Apr 7 21:31:25 1999

"Richard Erlacher" <edick_at_idcomm.com> wrote:
> (2) - I remember lengthy discussions among those members of the Denver Area
> 6502-Users' Group who were presumably qualified to discuss the intricacies
> of the internals of APPLE's disk I/O routines at a level I neither knew nor
> cared about, beyond the superficial details I gleaned from the several and
> varied sessions discussing that set of details. Now, I attended these
> weekely and typically 4-hour long meetings for several years, and KNOW the
> guys who were hashing out the details of the hardware and software in
> question knew what they were talking about, so I accept that as fact.

Then, as you'll recall, I was one of the people who spoke at those meetings
regarding the operation of the Disk ][. Armed with information provided by
Wayne Wall, Larry Fish, and Peter Boyle, I delved into the disk system in order
to figure out how to defeat various copy protection schemes, including the
various half-track and quarter-track positioning methods. Larry was involved
in Apparat's efforts to interface standard Teac FD55 series disk drives to
the Apple ][ controller.

> These fellows
> spent a couple of sessions talking about and demonstrating the screwy means
> by which certain game vendors in the Apple market were "protecting" their
> wares by altering the timing of the positioning routine, thereby making it
> possible to write tracks "off the track" by changing the time delay between
> a known cylinder position and the point at which the specific track was to
> be written. This made it impossible for someone using the stock timing of
> the positioning mechanism to read the diskettes so written.

No, the postioning that was discussed was the half-track positioning I
described earlier. If you look at the Apple RWTS routines (for either 13-
or 16-sector diskettes), you'll find that the low level postioning routine
actually takes an argument that is two times the track number.

The only thing about it that was non-obvious was the timing of the
acceleration/decelleration profile used to speed up the seek process.
However, this did not affect the final head position.

> The scheme with the tristate multiplexers came later, I believe, than the
> one I remember.

I'm not sure what tristate multiplexers you're referring to. The controller
for the Disk ][ never changed in any non-trivial way. Some later cards for
use with the Unidisk and Duodisk used a 19-pin D-subminiature connector in
place of the pair of 20-pin right angle headers, but the electronics was the
same. Starting with the Apple ][c they used the IWM chip, which was a
slightly fancier single-chip version of the original controller, but the
positioner control method didn't change.

> Apple had several patents, all of which are there to be
> examined if one wishes. I believe this software-timed positioning scheme
> was among them.

They had exactly one patent from that era which covers the disk controller.
It describes (among other things) how they use a stepper motor for positioning.

> I doubt, however, that Wayne Wall would have allowed the waste of
> several sessions of the meetings he so firmly controlled back in those days,
> if the assumptions presented as fact in those discussions had not been
> verified.

I'm sure he wouldn't have. Which is why there was not any discussion of
using DC motors for head positioning.

> The helical cam I remember didn't have a groove, but rather, a ridge or

Regarding mechanical details of the Disk ][ drive I'll readily concede that
you are likely correct, as I never bothered to study the mechanism, only
the electronics and code.

Cheers,
Eric
Received on Wed Apr 07 1999 - 21:31:25 BST