MFM and ESDI SCSI bridge boards (was Re: SCSI emulator) - test-drb@ccmp.vtda.org

5 Nov 2010

On 11/5/10, Jules Richardson <jules.richardson99 at gmail.com> wrote:
...
  Dave McGuire wrote:
  On 11/5/10 2:00 PM, Jules Richardson wrote:
  It's a bit more murky than that (albeit a
related issue) because a lot
 of the bridge boards expect a vendor-specific command to tell them the
 drive geometry of the attached drive... 
   MFM- or ESDI-to-SCSI bridge boards (Adaptec ACB-4000, Emulex MD21 if
 memory serves) are weird to begin with... 
 I think the Adaptec was one of the most sane boards out there 
I do have some experience with a couple of them, primarily the ACB-4000

...
  IIRC it stores
 the drive geometry on block zero of the drive at format time, then at
 power-on fetches that block back and initialises itself with the loaded geometry. 
That sounds somewhat familiar to me, but I wouldn't have been able to
have said that's what it did without looking it up first.

...
  I can't remember if it just hides the first block
from the user, or hides the
 whole track. 
I really can't remember that detail - been way too long, but if I had
to guess, I'd guess "block" since I have a fuzzy memory about some
detail about moving, say, an ST225 on and off an ACB-4000 and a
different kind of controller (an XT MFM card most likely, possibly a
WD WX-1 or Everex clone).

...
  I think the board responds to Inquiry, too (although
it just returns "ACB
 4000" rather than anything to do with the geometry of the attached drives) 
That also sounds familiar (having played at the low-level back in my
early Amiga days).

...
  I've not played with the Emulex boards - I think
the only ones I have are
 tape bridges rather than disk. 
I have a couple of the Emulex bridging boards from the Sun-3/early
Sun-4 days, but I never had to set them up - they were black (beige,
really ;-) boxes to me.  I just stuck them on a Sun box and don't
remember any special fiddling.

...
  The Xebec and Omti boards seemed reliable, but their
SCSI implementations
 were
 a little lacking (one of them I think had the option of a user-supplied ROM
 with the expected disk geometry encoded into it, but it still wasn't "SCSI
 enough" to work with a modern system) 
I think I tried to fiddle with an Omti board once but didn't achieve
enough success to use it.  One reason I tried was it was a combo
hard-drive/floppy drive model.  It would have been handy, but alas no.
 It's likely I was running into one of those "almost-SCSI" problems
and lacked sufficient documentation.

Since I moved from PETs and C-64s and small DEC machines first to
Amigas and Macs, then much later to PCs, I was very happy when
embedded SCSI drives displaced all the Adaptec and Emulex and Xebec
and Omti boards - much easier to set up and move from environment to
environment.

-ethan