Date : Thu, 24 Aug 2006 00:39:25
From : Pete Turnbull <pete@...>
Subject: Re: writing and reading on 3.5 floppy
On Aug 23 2006, 12:37, David Hunt wrote:
> The reason 3.5" drives don't work on the ModelB is the 8271 expects
to get a
> sector start notification from the drive.
You mean an index pulse, presumably.
> 3.5" drives don't have an index hole sensor like 5.25" drives.
I believe that's essentially incorrect; they do have an index sensor.
The INDEX signal is a standard part of the 3.5" interface, just as it
is on 5.25" drives. All my 3.5" drives generate an INDEX pulse, and
it's hard to see how formatting would work without it. Also, the way
missing sectors are detected in most systems, including DFS and ADFS as
I recall, is by counting revolutions and giving up if the desired
header is not read after a certain number of revolutions, which is done
by counting index pulses. On PC systems, there's also an INDEX MARK,
rather like a sector header, written at the start of each track during
formatting, and that's written by the floppy controller when triggered
by the INDEX pulse from the drive.
The disks don't exactly have an index hole, but they always fit onto
the drive spindle the same way, and the spindle, or something attached
to the motor, generates the pulse, rather than an optical sensor
looking for a hole in the disk.
Trivia: if you've ever formatted a 3.5" DOS floppy on an Archimedes,
and had work on the Arc but fail to be recognised by a PC, it's because
that INDEX MARK is optional in the specification for 3.5" formats, and
is neither used nor written by the Arc's 1770 controller. As a result,
the first sector header is too close to the INDEX pulse for some PC
floppy controllers, and they don't see it.
What is true, is that some modern 3.5" drives mask out the INDEX pulse
(and the READ DATA signal) during seeks, which might cause timeout
problems.
--
Pete Peter Turnbull
Network Manager
University of York