stepping machanism of Apple Disk ][ drive (was Re: Heatkit 5 1/4 floppies) - test-drb@ccmp.vtda.org

8 Apr 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 &lt;eric(a)brouhaha.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Wednesday, April 07, 1999 8:37 PM
Subject: stepping machanism of Apple Disk ][ drive (was Re: Heatkit 5 1/4
floppies)

...
 "Richard Erlacher" &lt;edick(a)idcomm.com&gt;
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 