On Feb 1, 17:31, Gunther Schadow wrote:
> I read a web article the other day where the guy describes the
> various forms of the Qbus and he also said that you could fry
> certain cards when you stick'em in a wrong version of the Qbus.
> Since I have a uVAX II and a PDP11/03 I would want to know if
> I can mix and much cards with thoese busses or if I would fry
> a K[ZF]QSA board sticking it into the wrong bus.
There are two possibilities here. One is that there are board sets such as
the RLV11 controller which use the CD-interconnect to talk to each other,
and those won't like being in a serpentine backplane, where both A-B and
C-D carry Q-bus signals. They're designed to be used a a straight
backplane, where the Q-bus is only on A-B, and the lower side of each C-D
is connected to the upper side of the next C-D row down. The other
possibility is that some backplanes (notably some Plessey ones) have
battery voltages or occasionally even AC voltages on the "spare" pins and
they may fry boards that expect signals there rather than a power supply.
> Also, why was the need for grant continuity cards an advantage?
Well, a better way of looking at it is that there's a design feature which
is advantageous in providing prioritisation of interrupts according to
position relative to the front (processor end) of the bus, but the penalty
is that you need grant cards as a result. It's the interrupt chain. In
-11 busses it's designed so the chain passes through each device, and
therefore any device can be built with the capability of locking out
devices behind it while it's being serviced. Anything further away will
wait until the locking device releases the interrupt, then it's own request
will automagically be seen by the processor. There's no such scheme in an
Omnibus, as the signal goes "past" each card rather than "through" it.
> The OMNIBUS didn't need it but the UNIBUS (and Q-bus?) do.
> Also, what's the deal about grant continuity cards, they seem to
> just have a few lines shorted. In the UNIBUS box next to my
> VAX 11 it has some intermediary open slots but only one grant
> card plugged in. How could that work?
The grant cards only need to carry the interrupt request and NPR lines;
these are the only ones that have an IN and an OUT on each slot. Other
signals just daisy-chain the slots together like a busbar in a power
distribution panel. The other way is to use a wirwrap on the backplane
between the IN and the OUT. The NPR line is often dealt with in that way,
and G727 grant cards actually only jumper the four interrupt lines (G7272
jumpers NPR as well).
There are a few reasons you may only have grant cards in some slots. It's
quite possible that something has been removed and nobody put a grant card
in even though they should. It may well work like that, if nothing further
back actually needs the signals. Another possibility is that you're
looking at some custom backplane which doesn't have standard SPC slots --
not all Unibus backplanes are wired the same way -- though I can't think of
one that has SPC in some slots and custom wiring in others. No DEC one,
anyway.
> Also, why can you stick
> 1x or 2x cards into the different sections, is there a difference
> where you put them? Why is the feed to the UNIBUS only a 2x card
> and where must you plug that? Is it magic?
The Unibus signals fit onto two rows, A and B. On a normal expansion
backplane, some of those signals are redistributed onto rows CDEF as SPC
signals, so in effect you have two busses side by side -- Unibus (or MUD :
Modified Unibus Device) in slots A and B, and SPC (Small Peripheral
Controller, same signals more or less, but different arrangement) in CDEF.
You can fit a dual-height Unibus card in rows AB, and/or a quad-height SPC
card in CDEF, or a hex card in ABCDEF.
If you really mean single-height (only one slot) or dual-height when you
write 1x or 2x, those are usually cards which are parts of some device
built from simple(ish) logic modules; they're not Unibus or SPC cards, and
generally fit into specific places in a specially-wired backplane where
only A+B of the first and last slot are Unibus. The rest are some custom
point-to-point wiring for a particular device, built from simple modules.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Fri Feb 01 2002 - 19:12:20 GMT