Floppy disks again - test-drb@ccmp.vtda.org

3 Oct 1999

On Oct 3, 16:56, Ward Griffiths wrote:
...
  Subject: Re: Floppy disks again
 Bill Yakowenko wrote:

 Hey, with the discussion of diskettes recently, and my own
 search for info on RX50's, I was a little surprised by the
 lack of available info on single-sided single-density 5.25"
 floppy disks (emphasis on SD).  Were they ever actually
 manufactured, or was SS/SD strictly an 8" thing?  I seem to
 remember seeing SS/SD 5.25" way back when.  Can anybody
 give me an authoritative rating of their coercivity?  Were
 they 300 Oerstedts, just like DD?  If so, what is it that
 made them single density?  Something about the size of the
 individual magnetic domains maybe?  Or were exactly the same
 diskettes called "double density" when we figured out how
 to make drives to handle that? 
 Farzino, the Oersted value of SSSD (90k), DSSD (180k), SSDD (180k)
 and DSDD (360k) 5.25" diskettes was the same. 
Those figures for capacity only apply to 40-track disks formatted with 9
sectors per track (IBM PC style).  There are lots of other formats for
40-track disks, and of course there are also 35-track and 80-track drives.

...
  For that matter, it
 was the same with QD (720k) diskettes. 
QD is a misnomer, it really means DSDD 80-track, and the figure of 720K
relates to 9 sectors per track, 512 bytes per sector.

Ignoring unusual techniques, such as GCR recording, as used by Apple and
Commodore, there are two possibilities for density in conventional floppy
disk systems.  Single Density uses FM recording, and Double Density uses
MFM.  They use the same number of magnetic reversals per track, but differ
in the way they include clock pulses, so MFM ends up with twice as many
data bits per track.

Then there are two possibilities fro number of heads, 1 or 2.

Finally, the number and size of sectors can be varied.  IBM happened to use
9 sectors per track (8 on early systems, hence the existence of 320K disks
as well as 360K) with 512 bytes per sector, MFM.  Other manufacturers did
not always use the same layout.  Hence Acorn disks were usually 100K, 200K,
400K, or 800K (800K being 1024 bytes per sector, 5 s.p.t, MFM,
double-sided).

RX50 is MFM single-sided 80-track, 512 bytes per sector, 10 sectors per
track, giving 400K per disk.

...
  Manufacturers "tested" and
 rated the media for different densities, and I know that some disks
 that worked fine as SSSD failed miserable in the Tandy 2000 (720k)
 while others worked just peachy.  I don't know of any failures
 involving using a higher-rated floppy in a lower drive except when
 the media had been previously used in a 720k drive. 
That's not a density issue, merely a question of the width of the track
written by an 80-track drive as opposed to a 40-track drive.

...
  When you get
 to the official HD, 1.2Mb media, the situation changes. 
That's right, the coercivity is different.

-- 

Pete						Peter Turnbull
						Dept. of Computer Science
						University of York