idea for a universal disk interface

[Paul Koning]

> On Apr 18, 2022, at 6:44 PM, Mike Katz via cctalk <cctalk at classiccmp.org> wrote:
> 
> Which is more generic.
> 
> ESDI, SMD or SCSI.
> 
> In my opinion, SCSI is as close are you are going to get to a universal interface.
> 
> As for reading raw data from a drive.  The newer the drive, the higher the bit density and lower the strength of the magnetic fields and hence the lower the flying height.  You have to deal with linear (or horizontal) recording, perpendicular recording, Heat assisted magnetic recording, microwave assisted magnetic recording.  The latest technologies are approaching 1TB (yes that's TB) per square inch.
> 
> If you spin the platters too slowly you will not be able to distinguish individual magnetic fluctuations from noise.  What do you propose as your maximum data density (in BPI) and what is the minimum speed you will need to accurately decode it.

I know about some of the modern drive magic, but I wasn't talking about those at all.  My comments about recovering raw signals from disk surfaces is for much earlier disks, especially removable packs.  In more recent disks you always have the drive if you have the disk since they are the same thing.  (I'm ignoring "data recovery" services here that deal with mechanically failed drives; that's a specialized business and as you said it's increasingly difficult with modern drives.)  

If you consider 1960s through 1980s you're likely to run into disk packs for which the drives may be hard to find.  The mechanical tolerances of those devices require care but are not crazy difficult, as my RC11/RS64 example last week was meant to illustrate.  The bit densities are not all that high, nor the track densities.  Consider for example that the track positions on a 1311 drive are set by mechanical detents, and are low enough that no servo system is used at all.

My mechanical skills and tools are perhaps a bit better than some of the readers here, undoubtedly worse than others.  I could sketch a "spin table" that could handle, say, an RK05 pack.  Without a milling machine I can't build it, but that could be fixed, and I could refine my skills to make it work.  Do I plan to?  No, but if an interesting enough pack showed up I could imagine doing it.  The RA60 pack I have would be a bit more of a stretch -- more platters and higher densities, not to mention lack of documentation -- but it's still on the edge of possibilities.

So I think there are two different possible projects here.  One involves a generic controller/emulator for common disk to controller interfaces, like ESDI or SMD.  (Or SCSI but those are off the shelf items, right?)  I'd imagine there would be plenty of takers for that, just as there are for the MFM and floppy emulators.  The other, more difficult and less needed, is the pack reader that I was discussing.  There is a little overlap between the two but not much.

	paul