Message-Id: <199812040213.HAA08416@harrier.Uznet.NET>
From: Michael Sokolov <msokolov@harrier.Uznet.NET>
Date: 4 Dec 1998   02:13:33 GMT
To: pups@minnie.cs.adfa.edu.au
Subject: Re: System Industries MSCP disk controller problem
Sender: owner-pups@minnie.cs.adfa.edu.au
Precedence: bulk

   David C. Jenner <djenner@halcyon.com> wrote:
> I have a box with one of these.  I noticed the bussing appeared to
> be non-standard, but exactly what is the problem here?
   
   I don't think it's non-standard. It's probably just a different
standard. Strictly speaking, there is no such thing as a Q-bus backplane.
There are Q/Q, Q/CD, and other types of Q-bus'ish backplanes. Generally,
you call something a "Q-bus backplane" if it has Q-bus in rows A and B.
Rows C and D may be used for lots of different things. In theory you can
have these rows used for something REALLY weird. Allison J. Parent tells me
that once upon a time you could get a special version of BA11 where rows C
and D were customer-wired, i.e., you could have absolutely anything you
want in there.
   
   In practice, however, there are only two types of row C&D wiring. On
some backplanes you have Q-bus in both A&B and C&D, with the grant
continuity going in a serpentine. Some BA11 versions are like this. Such
backplanes are called Q/Q. On other backplanes Q-bus goes straight down the
A&B rows without ever touching rows C and D. In these backplanes rows C&D
are connected in a daisy chain, i.e., the fingers on the solder side of the
board in slot 1 connect to the fingers on the component side of the board
in slot 2, the fingers on the solder side of the board in slot 2 connect to
the fingers on the component side of the board in slot 3, and so on. This
way immediately adjacent boards can use rows C&D as a totally private
interconnect that has zero effect on or relationship with anything else in
the system, just like an over-the-top cable. The best example of this is
RLV11, although the most common example is MicroVAX II and III memory. Such
backplanes are usually called Q/CD, and the examples are some versions of
BA11 and all BA2xx and BA4xx backplanes.
   
   Finally, there are mixed backplanes that have several Q/CD slots
followed by many Q/Q slots. These are specifically designed for MicroVAXen
and other CPUs using rows C&D as a private memory interconnect. These
backplanes are BA23 (3 Q/CD slots and 5 Q/Q slots) and BA123 (4 Q/CD slots
and 8 Q/Q slots).
   
   Getting back to the Sigma backplanes, the inability to put a MicroVAX
CPU in there suggests that they are Q/Q.
   
   Sincerely,
   Michael Sokolov
   Cellular phone: 216-217-2579
   ARPA Internet SMTP mail: msokolov@harrier.Uznet.NET