Re: Disk II Stepper Phase Motor?

[ag471@yfn.ysu.edu (Eric S. Ford)]

In a previous article, vantune@fraser.sfu.ca (Joseph Van Tunen) says:

>How are the four stepper motor phase soft switches (5.25" disk drives)
>used to move the disk arm inward and outward over the tracks
>($C0E0..$C0E7)? How is the arm moved 1, 1/2, (and 1/4) track at a
>time?  (Copy II Plus has options for doing 1/4 and 1/2 tracks). What
>timing is required? Not all drives can step a 1/4 of a track, right?
>But the timing should remain the same no matter what kind of drive, I
>guess...  I have the Beneath Apple DOS book (c1981) which doesn't

The book you need is "Understanding the Apple IIe" by Jim Sather.  BTW,
I have a copy for sale.

>explain it all. It does mention where in the DOS 3.3 code (RWTS) the
>seek routine is but the Disk II has 70 phases, 1 track every second
>phase, which allows only half tracks. What about 1/4 tracks (is there
>such a thing)?  Are there drives with more than 80 phases? I could

Yes, there is such a thing.  The arm is moved by the following sequence:
Turn on the next stepper phase, turn off the current phase, wait for the
motor to respond (at this point the arm has moved 1/2 track), then repeat.
To arrive at 1/4 track, the following sequence is used:  Turn on the next
phase, turn off the current phase, wait (a shorter period) then turn the 
"current" phase back on trapping the arm halfway between the phases.

>disassemble the Copy II Plus code and find how it seeks tracks, but
>I'ld rather not. Does ProDOS have commands for seeking tracks (1/4 and
>1/2 also)?

No.  Prodos deals in blocks, not tracks.  The device driver might accommodate
calls to accomplish this but I do not believe it has been implemented in
the Apple Inc release.  You might find it in "protected" versions of Prodos
but I haven't heard of any.


-- 
Eric S. Ford    ag471@yfn.ysu.edu
		ac569@dayton.wright.edu
		E.Ford6@genie.geis.com