Intel 8272 based floppy controller & Missing Address Mark

[Chuck Guzis]

On 11/27/19 8:46 PM, David Kuder via cctalk wrote:
> Hi friends,
> 
> In the continuing saga of building a CP/M system with Pro-Log cards housed
> in a Heathkit dual 8” floppy drive cabinet…

If you're talking about ST2, bit 0 being set as an error, this can occur
for several reasons.

A. You're trying to read an FM-encoded disk in MFM mode, or vice-versa.
B. The address headers on the track aren't what you expect (e.g. first
sector on the track is 63 instead of expected 1.  Whatever formatted the
disk can put just about anything in the CHRN fields--and this is
sometimes used to separate different disk formats.
C. You're not on the right track.
D. If you're using an 8272 and *not* the 8272A, you could be running
into the extra-long "blind spot after index" of the older chip, meaning
that the first sector on the track may not be seen.  Even on the
8272A/NEC 765A, there are some formats written without an IDAM by other
controllers, which causes the 8272A to miss the first sector.

That's right off the top of my head--to see exactly what you're looking
at, try doing a READ ID and see what you get back, if anything.

--Chuck