RFC: Floppy reader/writer project (recovering UniPlus Unix for the Lisa)
ray at arachelian.com
Fri Feb 22 08:14:46 CST 2008
So I do have some news on the earlier subject. I've received the disks
for UniPlus yesterday and was able to image them.
I'm wondering if temperature had anything to do with it. They sat
outside my house (in the box) for a few hours and were obviously cold
when I read them. I disregarded the usual good idea of acclimation and
immediately attempted to image them, mostly because I keep an air
conditioner on at all times in my computer room (which of course in this
weather acts as a fan). So if anything, the difference between the
machine and the disks was less than 20F.
All of the disks read just fine, with the exception of a single disk,
which initially failed to read after about the 1st 10 tracks, but later,
after I finished imaging the rest of them, I retried it, and that disk
worked. I suspect that either the lower temperature helped, or perhaps
that one disk that initially failed had some dust which was loosened and
allowed to it read the second time around.
I'm loathe to mess with these disks anymore as I need to return them to
their owner, and don't want to expose them to any more risk.
This isn't a very large test set since I didn't attempt reading them at
various temperatures and with different drives/machines. Maybe I'll try
just one of the disks to see if it reads at warmer temperatures, but
it's an interesting hypothesis anyway. Maybe if I find a set of
slightly bad non-critical disks in my collection, I can try this again
with those at varying temperatures.
So I'm wondering if temperature has any affect on media readability.
To wildly jump to a conclusion here, would colder temperatures stiffen
the media, making it easier to read? Certainly they would cause the
metal in the center of the cookie to contract, but would it have the
same effect on the plastic or whatever's under the magnetic coating?
(NOTE: I'm not suggesting that if you have fragile unreadable media that
you should try this, I'm only asking if this is a possibility. Don't
risk it based on this single one-off sample! There's always the danger
that the plastic does stiffen and in doing so may cause the magnetic
coating to flake off?)
(Of course a much simpler explanation is that the disks were originally
read with a marginal drive, possibly misaligned or dirty.)
There were a few other factors that I'm wondering about. The media was
written by a 400K GCR floppy drive in the Lisa. These of course have a
larger r/w head. The drives vary the motor speed depending on the track
number - there are 4 speeds for these drives.
Later versions of the Apple drives known as SuperDrives (not to be
confused with the modern DVD "SuperDrives") have smaller heads in order
to support HD media (which they do so only in MFM), but they still
support physical reading and writing of GCR disks for DS/DD and SS/DD.
The SuperDrives however do not vary the motor speed, but rather have a
fixed rate spindle motor and vary the shift register read/write rate.
I would theorize that the smaller r/w heads might make it harder for
data recovery iff the tracks are slightly misaligned or the data is
partially damaged, the smaller r/w head wouldn't see the whole width of
the written track, and any damage to the area that it would see would
prevent it from reading the drive, while an original 400K or even an
800K drive would see the whole width of the track, and average out the
missing data with the existing data, possibly making it more readable.
Any merit to this idea?
Perhaps for the original subject for the floppy reader/writer board,
having actual DS/DD drives as well as a DS/HD drives is more ideal?
i.e. have both a 720K and a 1.44M 3.5" drive as well as both 5.25" 360K
drives and 1.2M drives and switch according to whatever was used to
write to the drive?
In my case, I was able to read all of the disks with a SuperDrive, and a
later model of a SuperDrive at that, so this didn't apply at all. (The
TAM is built from notebook components, and was built later than most
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