Re: Write Signal on Disk II Question

ferdimh@xxxxxx wrote:
Right, but each "1" shows up as a 1us pulse on the read data line.


On my computer, I need ~650ns to read from the parallel port.
This means that I can't read the signal from an original Disk II
reliably if it don't process it.

That is correct. It may be hard for an Apple II-familiar person
to realize how "remote" PC I/O ports are from the processor. There
are several layers of bus conversion involved, each with a step-down
in clock speed and timing resolution. On top of that, of course, is
the timing indeterminacy inherent in modern cached microprocessors.

Several years ago I did some experiments in sampling the read
signal using a PC parallel port, and I found that I could not
sample the raw signal fast enough to reliably capture all pulses.

My conclusion was that an external shift register would be needed,
but its clocking should be based on clock recovery from the data
stream, which is the entire purpose of the Woz state machine.

That realization was enough to cause me to drop the project. It was
clear that an external microcontroller or a Disk ][ Controller would
be needed. ;-)

But while I was experimenting with modern 3.5 '' drives on the Apple
(I just finished it) it found out that the pulse can be much longer
(3µs) without problems. So I would use a monoflop to make the pulse
~2µs long so that I can read it without problems.

Subject, of course, to the remaining timing indeterminacy of an
interrupt-rich environment and a cached processor...

But: According to "Understanding the Apple ][" the problem in
detecting more than 2 adjacent zero bits is not in the controller, but
in the drive. If that is true (I could test it - my 3.5'' drives CAN
read more zeroes). We're not improving anything if we're thinking
about other controllers.

Correct--after two bit periods without a transition, the MC3470 (?) on
the analog card continues "turning up the gain" to find a transition,
and, of course, it succeeds when it interprets noise as signal.

In principle, this period could be extended to reliably handle more than
two bit periods without a transition, but that would mean lengthening
the AGC time constant which would make the drive less reliable in the
presence of short media "weak spots". The AGC time constant is an
engineering tradeoff that compromises reliable detection of strings
of zeroes against tolerance for media fluctuations. I presume that
the time constant is chosen for good overall reliability (on the media
of the time).

The good news is that even copy protected disks seldom relied on reading
more than two consecutive zeroes. The exception was nibbles written as
*all* zero, so that dynamically changing noise could be detected when
they were read back. This served to flag a region of the disk as
unrecorded, which most copy programs did not reproduce.

And if a copy protected disk works using the Disk ][ controller card
it CAN be read. If it can't be copied this can't really be the
controller's problem.

More accurately, the data on it can be recovered. It is possible to
read data from a disk whose complete transition timing cannot be
reliably determined. The trick of some copy protection schemes was
to have a way of determining that a disk read was "consistent with
a certain transition string" even though the exact transition string
could not be determined using the Disk ][ Controller.

The utility of this approach was that a copy program would write an
"apparent" transition string that was different from the actual
string, and the difference was detectable by the copy protection code.

-michael

NadaPong: Network game demo for Apple II computers!
Home page: http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it's seriously underused."
.

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