On Wed, 25 Feb 2009 12:09:23 +0000, Baron
put finger to keyboard and
composed:

gcd Inscribed thus:

The only thing that is still a little peculiar is the density
select and whether it's an input/output or both.

It sticks in my mind that it was always an output in the sense that it
was used to detect the difference between 720/144 Mb disks and set the
rotational speed, 300/360 rpm I belive.

The IBM PC AT BIOS source listing indicates that IBM used data rates
of 500Kbps, 300Kbps, 250Kbps, and "reserved". In future designs, the
latter became 1Mbps and was assigned to 2.88MB FDDs.

The 3.5" 720KB and 1.44MB diskettes are both spun at 300 RPM, the
difference being that the latter has twice as many sectors per track
(18 vs 9). This results in data rates of 250Kb/s and 500Kb/s. Dual
speed 5.25" drives could spin at either 300RPM or 360RPM depending on
the density. A 1.2MB drive with 15 sectors/track would spin at 360RPM,
whereas a 360KB drive with 9 sectors/track would spin at 300RPM.
Although dual speed 1.2MB drives existed (I have two), IBM accounted
for the case where a 360KB DD 300RPM (250Kbps) diskette is used in a
HD fixed speed 360RPM drive by configuring the controller's digital
data separator with a matching 300Kbps data rate. AFAIK the dual speed
facility was never used on the PC platform. Instead pin 2 was used to
signal reduced write current.

- Franc Zabkar
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