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

From: Richard Erlacher <edick_at_idcomm.com>
Date: Thu Apr 8 00:05:10 1999

Well, perhaps you're right about the discussions you recall, Eric. I do
recall that there was an effort afoot to use 80-track drives to read some
diskettes written with the off-track method used by (Bill Budge?) games,
etc, in order to defeat their copy protection. I do recall the comment
being made, however, that a while person might defeat that particular copy
protection, since the mechanism under discussion was capable of putting a
track literally anywhere on the diskette, ( which certainly couldn't have
been done with a stepper ) the protection scheme wouldn't hold up. In any
case, it's good someone else was paying attention at the time, as it didn't
really matter to me, though I found it interesting. It was, after all, over
20 years ago.

Dick
-----Original Message-----
From: Eric Smith <eric_at_brouhaha.com>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Wednesday, April 07, 1999 8:37 PM
Subject: stepping machanism of Apple Disk ][ drive (was Re: Heatkit 5 1/4
floppies)


>"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 Thu Apr 08 1999 - 00:05:10 BST