Re: Apple 5.25 floppy drive

["Michael J. Mahon" <mjmahon@aol.com>]

mwillegal wrote:
> There are actually 69 steps from inside to outside of a disk with
> normal DOS disks set up with tracks on the even steps and odd steps
> left empty.
>
> The actual number of steps of of the stepper motor is 96*2 when re-
> calibrating.   This number is set by the the LDA #$60 instruction at
> location $BDD2 of the RWTS DOS driver.
>
> Basically the RWTS is told that we are on track 96 and should seek to
> track 0.  Then the actual track we want is stepped to from there.
>
> Depending on where the head is at the beginning of a re-calibrate, we
> will bounce off of the stop more or less times.
>
> This re-calibration seems like drastic overkill, though I have never
> tried changing the LDA #$60 to something in the low 40s to see what
> would happen.

It would seem that the author of this code misunderstood the seek
interface and pre-multiplied the track count by 2.

But at boot time, the head is positioned to track 0 by the controller
ROM code at $Cs3B to $Cs50, which seeks outward 80 "phases" or 40 
tracks, without the normal acceleration/deceleration time delay ramps.

Depending on the initial positioning, this will step the head out 38-40
tracks (since only an "adjacent" phase is effective in moving the head,
the first 0-4 phase energizings could result in no net head motion).

So David's pre-positioning the head at track 39 or 40 prevented
the cam from "hitting the stop", assuming that the phase sequence
starts with the phase for track 40.

> On Jan 4, 11:26 pm, David Wilson <mcs6...@gmail.com> wrote:
> > On Jan 5, 8:48 am, Eric Rucker <bhtoo...@gmail.com> wrote:
> >
> > > He is. IIRC, when the drive controller is initialized, it basically
> > > sends commands to the drive to step down a track 40 times, because it
> > > has no idea what track it's on, meaning... if you're on track 35 (the
> > > highest track on a normally formatted floppy, although you can
> > > sometimes get more than 35 tracks on a floppy,) it will click 4 times.
> > > If you're already on track 0, it'll click 40 times.

> > This used to annoy me so much that I wrote a little program to seek
> > the head to track 40 (or perhaps 39) and I would run this before I
> > powered the computer off. That way it would seek silently in before
> > booting when I powered it up the next time.
> >
> > Slightly later I obtained a newer 1/2 height direct drive FDD unit
> > which was based on a standard drive with a small 20 pin to 34 pin
> > adapter circuit board. This circuit used the track zero sensor on the
> > drive to disable outward seeks so it booted quietly no matter where
> > the head was left.

-michael

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