Re: Some Apple // hardware questions!

[mjmahon@aol.com (MJMahon)]

Michael Kelsey <kelsey@mail.wsu.edu> wrote:

<snippage>

>of other disk bytes.  I am aware of the Disk II's inability to realiably
>read zero bytes, and with wear and tear I'm sure the accuracy of my
drives
>[which currently read zero bytes quite well] will fade.  Despite the

As David Empson pointed out, the Disk II is unable to read reliably more
than two consecutive zero bits in a "raw" byte.  In fact, all disk drives
are
subject to a similar restriction, because of the need to recover the clock
from the serial bit stream.  A low-density PC disk actually writes _all_
bytes in 4+4 code--that is, with explicit clock pulses (1's) between the
data bits.

So don't worry about your Disk II's "forgetting" how to read zero bits--
the GCR coding scheme used on the Disk II eliminates any need to
read more than two consecutive 0's!  I haven't experienced a single
case of "floppy data degradation" in over 15 years of use--and I have
a lot of 10-15-year-old floppies.  Although I have heard anecdotal
evidence of "wearout", experience suggests that many of these disks
could have been read by a properly adjusted, clean Disk II drive.

The Apple disk recording format is very tolerant of flakey media, and
its error checking is relatively robust.  You should not be very worried
about "silent" loss of data.  Just keep them stored vertically, in a cool,
dry place that is relatively free of magnetic fields (especially AC
fields)
and your 5.25" disks will be fine until the oxide coating flakes off. 
(Then
you can back them up in a hurry!  ;-)

-michael

 Email:  mjmahon@aol.com
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