RE: CBM 8280

From: William Levak (
Date: 1999-09-03 22:46:35

On Fri, 3 Sep 1999 wrote:

> Hallo William,
> Thanks for the explanation but to be honest I still did not understand it 
> completely.
> > Judging from the number of the 8280 drive, it should be analogous to the
> > 8250 drive, that is Double sided Quad Density (DSQD).  Almost nobody uses
> > QD disks in an 8250.  DSDD disks work just fine.
> Anyway the above took my interest. I've been collecting 2S/DD 96TPI en 2S/4D 
> 96TPI floppys for the last year because I just did not trust the above 
> statement.
> My question: is there a little more simple way why I can use DDas well, why 
> they sometimes can be used in PC 1.2 MB-drives (and sometimes not) and why HD 
> can not be used in the 8250?
You are dealing with two different things here.
  1)  Magnetic incompatibility in the disks
  2)  The disks themselves have changed over time.

SD, DD, and QD that are made today are magnetically compatible.  That was
not the case in the past.  If you have an old SD disk, it may not work in
a DD or QD drive, while a new one should work in any of those drives.

In the old disk the magnetic particles are too big.  If you write on an
old SD disk in a DD or QD drive,  the signal will spread over adjacent
tracks.  The new disks all use particles that are small enough to do QD.
The difference is in the testing.  a DD disk was only tested at DD, so it
might not work at QD, but it probably will.

HD disks use a magnetic particle that requires a higher electic field to
write them.  In addition, the write head is narrower.  If you use a SD,
DD, or QD disk in a HD drive, the higher electric field will cause the
signal to be spread over adjacent tracks.  If you format a DD disk to a DD
capacity in a HD drive, then you will be able to read and write it because
of the greater spacing between tracks.  You will, however, only be able to
use it in a HD drive because the main signal is too narraw to be read
reliably by a DD drive.

ED disks are even less compatible.  Not only are the magnetic particles
different, but the configuration of the read/write head is different.

Commodore's GCR encoding was used because the timing circuits on drives at
that time were not as reliable as they are today.  Signals are written to
the disk using a NRZ (Non Return to Zero) encoding scheme.  If the signal
changes, that is a one bit.  If it doesn't change, that's a zero bit.
With that scheme, you have to know exactly when to look for a signal
change.  If you have a long string of zeros (no signal change), a slight
error in timing may cause it to be interpreted as too many or to few
zeros.  The GCR scheme converts 4 bit (half byte) data into 5 bit data.
The valid 5 bit data representations do not include any that can cause
more tha 2 zeroes in a row.  This more reliable method allows disks to be
used that may not work in any other machine.

Another problem with writing to magnetic media is hysteresis.  If you
create a magnetic field on a disk, it takes slightly more energy to
reverse the field than it took to create it.  The drive head always uses
the same field strength, so you always end up with a residual signal under
the main signal that you last wrote.  If you change the format on a disk,
you can end up with residual signals between tracks, or in sector gaps
that can interfere with reading the disk.  This can be a serious problem
for secret information.  Military security protocols specify that a disk
containing secret information must be overwriten a certain number of times
with random data in order to obliterate the residual signal.  Or, you can
bulk erase it.

This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail

Archive generated by hypermail 2.1.1.