3.5" floppy failure mode? Hub not attched to media.

[Fred Cisin]

On Sat, 28 Feb 2015, Eric Smith wrote:
> modified the positioning electronics for microstepping, you *might* be able
> to compensate for increased centering error by attempting to read each
> track at a range of offsets, getting some sectors at one offset and others
> at another, etc. I've done the math only for the case of normal
> positioning, and it looks pretty grim;

Good point.
For 512 byte sectors, the runout error would be minor (a small
fraction of the min/max error o0f the track) within each sector.

> for large sector sizes even the
> microstepping probably wouldn't help with much centering error beyond that
> already caused by the tolerance of the hub clamping.

Still a small fraction of the total error even with 1024 byte sectors.
But, not as easy with something such as Amiga, where the entire track is
read, and then sectors parsed in software.
But, that technique could be considered with a track read (ala WD179x or
flux transition hardware), and then identify "good" and "bad" sections of
the track with multiple reads, and splice together "good" sections to
recreate the track.


> The angle of hub attachment is a far less critical problem. If a controller
> is used that actually cares about the relative postition of the index pulse
> when reading data, a one-shot (or digital equivalent) can trivially be used
> to introduce a delay in the index pulse to compensate for the angular
> position error.

For READ (who would want to WRITE to a damaged disk??), even the NEC765
can READ with the index puls blocked.  'course then you have a few trivial
hassles, such as errors being misreported as 128/"Drive Not Ready"

Would disclosing fluid be adequate for corrcting the rotational error?

It would be nice to have the index pulse.  Using the premise behind the
radial alignment tests of the Dysan Digital Diagnostic disk, it would be
possible to calculate the direction of the radial/runout misalignment,
and be able to keep track of which sectors are out of position.
IFF the sector sequence of the disk is known, and consistent from track to
track, (not Apple][ and Commodore64) then micropositioning based on one
track could be applied to the entire disk.

If the micropositioning could be done in real time, then correction could
be applied for a "clean" read, or, if not, it would still make it possible
to know which sectors will be readable at each microposition and read
those sectors from every track before again modifying the position offset.


I did, on a very few occasions, deliberately take a 5.25" drive out of
radial alignment to read disks created on a misaligned drive.  If I were
still actively doing such, I would rig up a way to calibrate (maybe with a
micrometer head?.  On one group of 8" disks, light finger pressure pushing
the head further towards the spindle made the difference for successful
reads!
Of couse, 48TPI is quite a bit easier than 135, and access to the
positioner is easier on big drives.


So, if radial runout is the problem, read all of the sectors that can be
read on the disk, throw the radial alignment out slightly, read all track,
throw it out the other direction, . . .
And spend a ridiculous amount of time on a single disk.


--
Grumpy Ol' Fred     		cisin at xenosoft.com