3B1<->3B2

[Michael "Ford" Ditto]

In article <1447 at neoucom.UUCP> wtm at neoucom.UUCP (Bill Mayhew) writes:
>A 720K quad density drive is much like a 360K drive, except that
>there are 80 tracks of 1/2 the normal width.  The oxide used is
>approximately (if not exactly) the same as that used in 360K media.

These are fairly common, and I beleive that the TEAC model FD55D is
the 80-cylinder version of the very common FD55B (which is what's
in the Unix PC, if I remember correctly).  I doubt that the
manufacturer would call these "quad density" drives, since they are
used at double density, the same as 40 track drives.  I have always
heard them called simply "80 track drives".

>To the best of my knowledge there aren't any readily available HD
>1.2 meg drives that can read the oddball 720K format.

The 720K format isn't oddball at all; it's exactly the same as the
IBM PC 360K format except on an 80 track drive.  It's also the
format used by IBM-PC compatibles with 3.5" drives.  1.2M drives
should read/write that format perfectly (same mode as when being
40-track semi-compatible, except don't skip odd tracks).

I used floppies for data transfer between Unix PCs and some custom
68020-based systems running Regulus with 1.2Meg drives.  The floppy
device driver on the D512 had an "ibm" mode which skipped odd tracks
and used "normal" double density MFM (nine 512-byte sectors), and
this format got along just fine with the Unix PC.

I was able to both read and write normal floppies on either machine,
regardless of which machine formatted them.  Bill's warning about
"thin" tracks not being reliably readable on "thick" drives is very
true as far as the specs are concerned; I did it knowning that it was
"at my own risk" and didn't have any problems.  For my long term
backups, though, I always formatted and wrote using the Unix PC, since
that should always be reliably readable on either type of drive.

I will post my "fdread"/"fdwrite" program which allows the Unix PC
to read/write "raw" floppies, i.e., starting from track zero.  This
latest version allows specifying various floppy parameters, such as
number of sectors and sector size.  I tried reading some 256-byte
sector disks but ran into some trouble; I can give more info if
someone needs it.

If the 3B2 can read/write "generic" 48tpi floppies (without requiring
a partition table or similar nonsense) it should be possible to do
transfers with the 3B1 and a program such as "fdread".
-- 
					-=] Ford [=-

"The number of Unix installations	(In Real Life:  Mike Ditto)
has grown to 10, with more expected."	ford at kenobi.cts.com
- The Unix Programmer's Manual,		...!sdcsvax!crash!elgar!ford
  2nd Edition, June, 1972.		ditto at cbmvax.commodore.com