Doing this with RS-232 is just dumb. It's clearly a USB application.
I really believe that there is a relatively large market for such a device
(USB to 5.25 (and possibly 8") interface). I'm thinking tens or hundreds of
thousands of units (not a significant market penetration in a world with
hundreds of millions of PCs in use and an additional 100 million sold
annually, but enough to make design and manufacture easily justifiable).
Because I think that this has much wider application than just "us classic
computer collectors", I think that size, aesthetics and ease of use matter.
Sure, the functionality can be achieved with a whole PC, and an old PC can
be nearly "free". But that's not what the CPA firm that needs to access a
client's old records on 5.25" diskettes wants. They want a nice, small, not
too expensive box that plugs into a USB port and has a CD of drivers and
file transfer software (including file transfer programs for non-Microsoft
OS' ... even, in fact in particular, CP/M).
I personally don't care about a cat-weasel type device that tries to capture
every flux transition on the disc. I don't care about hard sector formats
either (Heathkit, NorthStar) unless I can do them with no great extra
effort. Being able to handle "any" soft-sector format with 128, 256, 512 or
1024 byte sectors, single or double sided, 5.25" (360k or 1.2MB) {and
POSSBILY 8" (ok, for me, preferably)) is good enough and is relatively easy
to do. A USB interface, a microprocessor and a floppy controller. And some
software on the PC side for file transfer. Keep it relatively simple,
relatively cheap, and still meet the needs of the vast majority of people
who might have a use for such a device.
While I might want to use this for CP/M (and the device should allow that,
even if the extent of this is that it allows arbitrary read and write I/O to
specific side/track/sector, and let software on the PC deal with the file
system), the primary use of this is going to be for 5.25" PC diskettes (both
360k and 1.2MB) now that PCs no longer have 5.25" floppies. And, if it
wasn't for the fact that there are USB 3.5" floppy drives, we'd soon need it
for those as well, as floppy drives of all types and sizes are in their "end
days".