David Brownlee <abs@netbsd.org> wrote:
> 	Many thanks - one thing, I believe 'MFM' is just the disk encoding
> 	while the interface was 'ST-506'.
Yes. Actually, it's usually called ST-506/412. However, this is not a
specification, but a de facto standard. The very first drives of this type were
Seagate ST-506 and ST-412, and all other drives simply clone their interfaces.
There was a slight interface improvement between ST-506 and ST-412 (namely,
buffered seeks), and therefore people sometimes call ST-506 and ST-412 different
interfaces. All controllers for this type of drives support buffered seeks, but
work fine with drives that don't support them, and therefore people usually
write ST-506/412.
With VAXen, however, I write MFM instead of ST-506/412 because the MFM interface
of RQDXn, KA410's built-in MFM controller, and the optional MFM controller for
KA42 is more generic than ST-506/412. First, these controllers support floppy
disks, and the support for the two types of disks (floppy and Winchester) is
really integrated, not like on combo Winchester/floppy disk controllers for IBM
PC AT which merely put two controllers on the same card. Even the
Winchester-only part is more generic than ST-506/412. Remember, ST-506/412 is
not a formal standard, it's only one type of interface for what is called
colloquially an "MFM disk", the one that Seagate happened to pick for its
St-506 and ST-412 drives. These drives are internal and have internal
connectors, and therefore these internal connectors are usually considered to
be part of the de facto ST-506/412 spec. External drives can't use the same
connectors, obviously, and therefore in the strictest sense they are not
ST-506/412. Of course, an external drive is usually an internal drive in a box,
and the external interface is converted to ST-506/412 inside that box by an
appropriate cable, but this doesn't matter for the purposes of the external
interface. The latter has exactly the same signals as ST-506/412, but uses
cables and connectors, and therefore is not strict ST-506/412. Since most VAX
MFM controllers support some external interface, I say that they are more
generic than ST-506/412.

Sincerely,
Michael Sokolov
Phone: 440-449-0299
ARPA Internet SMTP mail: sokolov@alpha.ces.cwru.edu