29 Oct
2008
29 Oct
'08
5:52 p.m.
-- ethan.dicks at usap.gov wrote:
On Wed, Oct 29, 2008 at 10:56:13PM +0000, jeff.kaneko
at juno.com wrote:
A-HA! I think you nailed it! That explains why you could use
an older drive that could be used with an XT controller,
on an AT comntroller, but *NOT* vice-versa! I was totally
unaware that the fourth head-select line on the bigger drives
had been previously used for RWC.
Geez, the 419 was uncommon, even in it's own
time.
Indeed. I don't ever recall running across one.
There's some difference (which I have been
unable to quantify)
between different models of MFM drives (of equal or similar
geometry) that allows some drives to work in some applications,
but not others.
If there's anyone out there intimately familiar with the
ST-506/412 interface, now would be a good time to offer
some suggestions as to why this is so.
If you had to play with MFM and RLL drives back in the day,
there were *four* drive geometry parameters you had to match
up with what your controller/firmware was expecting:
Cylinders
Heads
Write Precomp
Reduced Write Current
RWC was important with the ST-506 standard, but, IIRC, it fell
away with the ST-412 standard, and the pin that the controller
used to tell the drive to reduce the write current was recycled
as another head-select line, allowing an ST-412-compatible drive
to have 9-16 heads. More modern drives kept track of which track
they were on and handled RWC internally, sometimes based on the
Write Precomp value, I think.
I don't know anything about the HP9133D, but I did fiddle with
plenty of MFM and RLL drives on ISA controllers and non-PC
controllers (DEC RQDXn, Commodore D9060/D9090, Dialog DQ614, etc.,
etc.) There was no hard and fast rule that X drive could always
be replaced by Y drive of a larger size, but there were many
specific cases where it worked - one I've done is replacing a
Tandon TM602S with a Seagate ST225 in a Commodore D9060. They
have the same number of heads (4), and although the ST225 is 20MB
and the TM602S is 5MB, what you end up with is a 75%-unused
ST225, and there's no conflict between the Write Precomp and the
RWC parameters.
OKay, looks like the 9133 wants a drive that supports RWC (or,
at least, does not have a fourth head select bit!). Looks like
the write precomp has to match also. Wow, I must have led a
sheltered life: This is the first instance I've seen where the
RWC and precomp actually have a discernable effect.
I'll have to consult my drive guide to confirm this . . . .
Thanks Ethan!
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