idea for a universal disk interface

[Paul Koning]

> On Apr 17, 2022, at 1:28 PM, shadoooo via cctalk <cctalk at classiccmp.org> wrote:
> 
> hello,
> there's much discussion about the right  method to transfer data in and out.
> Of course there are several methods, the right one must be carefully chosen after some review of all the disk interfaces that must be supported. The idea of having a copy of the whole disk in RAM is OK, assuming that a maximum size of around 512MB is required, as the RAM is also needed for the OS, and for Zynq maximum is 1GB.

For reading a disk, an attractive approach is to do a high speed analog capture of the waveforms.  That way you don't need a priori knowledge of the encoding, and it also allows you to use sophisticated algorithms (DSP, digital filtering, etc.) to recover marginal media.  A number of old tape recovery projects have used this approach.  For disk you have to go faster if you use an existing drive, but the numbers are perfectly manageable with modern hardware.

If you use this technique, you do generate a whole lot more data than the formatted capacity of the drive; 10x to 100x or so.  Throw in another order of magnitude if you step across the surface in small increments to avoid having to identify the track centerline in advance -- again, somewhat like the tape recovery machines that use a 36 track head to read 7 or 9 or 10 track tapes.

Fred mentioned how life gets hard if you don't have a drive.  I'm wondering how difficult it would be to build a useable "spin table", basically an accurate spindle that will accept the pack to be recovered and that will rotate at a modest speed, with a head positioner that can accurately position a read head along the surface.  One head would suffice, RAMAC fashion.  For slow rotation you'd want an MR head, and perhaps supplied air to float the head off the surface.  Perhaps a scheme like this with slow rotation could allow for recovery much of the data on a platter that suffered a head crash, because you could spin it slowly enough that either the head doesn't touch the scratched areas, or touches it slowly enough that no further damage results.

	paul