11/53 - Qbus backplane question

From: Ethan Dicks <erd_6502_at_yahoo.com>
Date: Fri Jan 4 09:47:26 2002

--- Adrian Graham <Adrian.Graham_at_corporatemicrosystems.com> wrote:
> > -----Original Message-----
> > From: Gooijen H [mailto:GOOI_at_oce.nl]
> > Sent: 04 January 2002 08:42
> > To: 'classiccmp_at_classiccmp.org'
> > Subject: 11/53 - Qbus backplane question
> >
> >
> > I got in the 11/53 running Micro/RSX.
> >
> > The 11/53 has an H9278-A backplane. According to
> > Megan's fieldguide the first 3 slots are Q22/CD
> > and the last 5 slots are Q22.
> > What does that mean?
> If I remember rightly the CD slots have grant continuity, which means you
> can have half height boards in position 1/2 without needing grant
> continuity cards in position 3/4 to carry signals over to the next slot.

Not exactly. Right consequence, wrong cause. Q22 slots are where you
put your CPU, peripherals, etc. If you have Qbus memory, it's where
you put your memory cards. Empty slots between the CPU (first card)
and the last card must have a bus grant in them or the devices behind
the gap won't be detected when they make an interrupt. You can put
cards that do not interrupt all the way at the back with multiple gaps
and no harm done. Think of it like a bucket-brigade when someone steps
out of line - there's no path from the back to the front.

CD slots are special. They are wired up so the bottom face of one card
is connected to the top face of the next card. Prior to the Qbus, when
DEC made a multiple-card peripheral (RK8E, for example), there were H851
connector blocks to provide interconnection. This was not an ideal
solution, especially given the risks that a customer might install a
card upside down if it had 4 backplane fingers on each end and no handle.
The first peripheral I worked with that required a CD backplane was
the RLV11 - two cards, one always above the other (so they were properly
interconnected) and no external interconnect was required (H851, ribbon
cable, etc.)

In more modern boxes, BA-23, for example, they have Q22/CD for the first
three slots either for the CPU (uVAX-I) or the CPU and memory (uVAX-II,
etc.) since late-model Qbus CPUs have PMI memory (Private Memory Inter-
connect) and do not sit on the Qbus. CD slots never require grant cards.

Personally, I have a bunch of 11/23-era (KDF-11) stuff, including a
few BA-11N boxes - 9 Qbus slots next to 9 CD slots. It's handy because
back when I got this stuff, an RLV11 was $100 and an RLV12 was $900
(since it worked in MicroVAXen, etc). By the time the KDA-50 was
created, CD slots were no longer the answer and there's a ribbon cable
to connect the two boards.

Hope this clarifies the issue. CD slots don't require a grant card,
but that's because they are outside the grant chain.


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Received on Fri Jan 04 2002 - 09:47:26 GMT

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