🚀 go-pugleaf

This message is directed to K.C.Lee, but it's interesting enough I thought
I'd better post it globally.

The Mac has five track zones:

track range / sectors per track / zone
----- -----   ------- --- -----   ----
0-15          12                   5
16-31         11                   4
32-47         10                   3
48-63          9                   2
64-79          8                   1

Zones one and two can be read by the Amiga disk controller without any
special effort.

Note that the zone designation is not neccessarily used by Apple; I just
made it up for convenience.

The Mac format is extremely similiar to that of the Apple II, so 
Beneath Apple DOS or Beneath Apple ProDOS are very useful.

I'm surprised and pleased that you thought of controlling drive speed with
software. It's really a great concept (although whether it is feasible
remains to be proven). I worked on this last summer, so I can get you
off to a good start.

The first problem that arises is the fact that the drive motor signal
can't normally be toggled without deselecting the drive. This causes
data to be lost. If there weren't a fix for this, I wouldn't be typing
this text now. It turns out that you can disable this feature, without
creating any potential problems, with a simple jumper. I installed the
jumper in my A1000 - what machine do you have? (so I can provide details
pertinant to your situation).

There are a couple other very interesting tidbits you should know. One,
the motor line is connected to PB7!!!! I put the exlamation marks there
because this line is VERY special to a CIA. It can be toggled or pulsed
according to the countdown of one of the timers! That *suggests* the
possibility of simply loading up the timer and reading from the Mac disk.
(in reality, the read routine will undoubtedly be much more complicated).

There's another interesting thing I found out. I found that I could read
an entire sector of a Mac disk (zone 5) by simply letting the motor
decelerate from 300 RPM to 0 RPM! Again, that *suggests* the possibility
of reading a Mac disk rather easily. (at least in a one-shot conversion
situation).

I should mention that this is not the only way to slow down a drive motor
on a standard drive. The other method is to vary the input voltage. This
is exactly what the Cyclone copy hardware does (or so I am told). They
use it to help them copy long track and nibble counting protection schemes,
but it could also be used to read (and of course write) Mac disks!

Of course, there is yet another method. One could design a floppy drive
controller just like the Commodore 1541 drive, which can read and write
at various rates. The so-called "data seperater" of the 1541 is completely
digital and quite simple. A 16Mhz clock is an input to a programmable
counter. Every time the counter counts down, a zero is read. A transition
at any time is read as a one and resets the counter. Because the on-board
CPU (a 6502) can write the countdown value to the counter chip, many data
rates are easily supported.

I have absolutely no idea how the Amiga disk controller works internally.
If anyone has the schematics to Paula lying around, please provide some
information (yes I am serious!).

Of course, the OTHER method is to simply use a Mac 800K drive, which
AUTOMATICALLY adjusts its speed depending on what track it is on (quite
unlike the 400K drives, which required a special control pulse from the
host).

If anyone needs more methods to read a Mac disk, just let me 
know (grin!)...

-- Dan Babcock

1106 Jefferson Road
Pittsburgh, PA 15235
(412)-373-1753