Re: Disk II Reference

[Holger Picker <pickerh@uni-muenster.de>]

On Mon, 17 Feb 2003, Bryan Parkoff wrote:

> Date: Mon, 17 Feb 2003 18:35:17 -0600
> From: Bryan Parkoff <BParkoff@satx.rr.com>
> Newsgroups: comp.sys.apple2
> Subject: Disk II Reference
>
>     Do you know where can I obtain Disk II Reference that it has all
> technical information?
Sorry, I don't. (Hopefully someone else.) I've got a book about DOS 3.3 that
covers in great detail how DOS works and also explains some elements of the
hardware, but as it is written in German it probably won't be of any use to
you. (I cannot scan it and send it to you.) There are, however, good books
in English (which I haven't got). Someone mentioned the following: "Beneath
Apple DOS' from Worth & Lechner, 'The Apple II History' by Steven Weyhrich
(available on anon ftp) and the manuals for the Apple ][ and the Disk ][."
This is a quote from a text file I've found on the internet that also
explains how the bytes are stored on the disk. Information about the stepper
motor, however, is hardly found since there isn't really much to know about.
See the following quote from the same text file:
<QUOTE FROM TEXT FILE>
It was in 1977, then Woz evaluated several floppy and harddisk
controllers and found a brilliant solution with minimal hardware.
Soft sectoring (with self sync bytes) should overcome the hard
sectoring and a clever software encoding of the data should make
a clock unnecessary. The head's stepper motor was simply connected
to four flip-flops, which were toggled in a cyclic manner by
software (with timing loops) to move the arm.
All in all, he made it working with 10 IC's and some resistor/
capacitor/transistor sugar.

[..]

4. The stepper motor

The four phases of the stepper motor are connected to
flipflops, which can be controlled by software. They are
switched on and off in a cyclic fashion to move the head
in and out. To overcome inertia, this has to be done at
specific times. Timing loops (with table values) do this
critical job.
</QUOTE FROM TEXT FILE>

> I am interested to study Disk II Family including
> IWM.  First, I would like to study the original Disk II and then IWM at
> later time.  I only get very little information that I can't understand how
> the hardware works.
>     For example, Disk II has over 70-80 tracks.  It means to use even track
> (0,2,4,6,8,...) that equals to decoded track (0,1,2,3,4,5...).  I don't know
> what odd tracks contain data.  I guess that both even-odd tracks are
> combined together like 4-4 encoding.  For example, 0-1 track equals to 0
> track, 2-3 track equals to 1 track, 4-5 track equals to 2 track, etc.
My apologies if I did not express myself clearly enough.
The disk drive has about 70 half-tracks of which only 35 are used. The term
"track" normally refers to the logical track the user can see, which is a
number between 0 and 34, not the "real" half-track found with the drive's
stepper motor, which is a number between 0 and 68. The logical track gives the
corresponding half-track by multplying the value with 2, for example
track 4 * 2 = half-track 8. The odd half-tracks (1, 3, 5...) were not used
for storing data. There was simply nothing on it, because writing data to them
would have destroyed the data on the even tracks (0, 2, 4...). IIRC there was
at least one non-standard disk drive that was precise enough to use also the
odd half-tracks. This then was able to store twice the amount of data
(i.e. 280 KB) on a disk, but you had to modify DOS to make use of it, and
these 280 KB disks could not be read on a standard disk drive.
Kind regards

  Holger