For those who never had to suffer through it,
in case there isn't sufficient confusion, . . .
<pedantic>
The 1.2M drives ran at 360 RPM.
To handle 360K, which would be every other track and ran at 300RPM, some
could switch speed to 300 RPM, while others relied on the FDC to
compensate for the speed.
The FDC had to be able to handle 500K bps data transfer rate for 1.2M
(2.8M required 1000K bps)
It also had to do 250K bps for use with 360K drive.
To be able to handle a 360K disk in a 1.2M drive, most, but not all, also
had a 300K bps data transfer rate for a 360K disk spinning at 360RPM.
To handle a 720K diskette, you need the 96tpi, either by using a 1.2M
drive, or the more obscure 720K 5.25" drive.
But, in addition to the 96tpi, you also needed to read and write at "360K"
bit rates. That could be done either at 300RPM at 250K bps, OR at 360RPM
at 300K bps. If you have a "single speed" (360RPM) drive, and an FDC with
250K/500K bps, but without 300K bps support, then you can't do it.
The simplest is to use a "720K" drive, such as Teac 55F, Shugart 465,
Tandom TM100-4 (NOT M!). You can lie to the SETUP and tell the AT CMOS
that it is a "360K". Or on later machines, tell the AT that it is a 3.5"
"720K".
And even simpler, use an XT, instead of an AT, where the FDC is ONLY 250K
bps.
Diskettes must be 300 Oersted (same as "360K").
HD (600 Oersted) diskettes are absolutely unusable.
"180K" SSDD, "360K" DSDD, and "720K" DSQD diskettes are the same
formulation. The difference is whether they have been tested/certified
for using both sides, and/or all tracks.
At one point in the early days, when diskettes were hideously expensive,
there might or might not have been manufacturers who sold diskettes that
passed testing on both sides as "double sided", and sold ones with only
one good side as "single sided". But, as production ramped up, the
price came down and failure rate declined, and it wasn't worth heroic
efforts for a reputable manufacturer to try to save a diskette that had a
known bad side.
Nevertheless, DS testing is more through than SS testing, and 96tpi
testing would be more thorough than 48tpi testing. With almost total
falloff of sales of "quad" density, few manufacturers bothered to continue
SS or 96tpi testing. So, a "DSQD" diskette is likely to be the same, but
OLDER than a "360K" diskette.
Again, DO NOT USE "HD" diskettes for anything other than "1.2M" formats.
</pedantic>
YMMV
--
Grumpy Ol' Fred [email protected]
