11/34a issues

Ethan Dicks ethan.dicks at gmail.com
Tue Nov 8 17:33:48 CST 2005

On 11/8/05, Julian Wolfe <fireflyst at earthlink.net> wrote:
> When is it exactly that I need a grant card?  Do I need them whenever
> position D isn't occupied?  Or is there a stipulation to that?

In order for devices to be able to have their interrupt request seen
by the CPU, the "grant chain" must be preserved.  Unlike an Amiga or
PC or other architecture that has IRQ-like lines that a card pulls
true to request an interrupt, on the Unibus and Qbus, each peripheral
is responsible for keeping each BR signal connected (there are several
grant chains on each bus, not just one) so that each card behind it
can be "seen" by the CPU.  To request attention from the CPU, a
particular card breaks that connection.  If, prior to the interrupt
being serviced, another card closer to the CPU also breaks the
connection, then the closer card will be serviced first.  Any card to
the rear will not be seen until it is the front-most card with the
interrupt chain broken.

The consequence of this technique is that if there is no card in the
slot, then a passive device, a "grant card" must be installed, to
preserve the chain past the "empty" slot to any cards that are
installed further away from the CPU.  You can easily see what the
grant card does by inspecting one - it bridges two adjacent pins in
the same slot, and _that_ is what keeps the chain intact.

On the Unibus (not Qbus), to make things a little more complicated, in
a different part of the backplane is the DMA request pin - NPR
(Non-Processor Request).  Because originally, it was expected that a
DMA device would be an entire backplane, the NPR signal is chained
across a DD11DK backplane with wire-wrap wires, requiring a
single-slot DMA device, an RL11, say, to have the NPR jumper removed
from the bottom before it will be able to request the bus.  When
removing such a peripheral, one must either replace the NPR wire
manually on the pin-side of the backplane, _or_ use a more modern
dual-height grant card that grants _both_ the interrupt chains _and_

If you really have an empty 11/34 backplane, save for CPU and mem, I'd
check your NPR wires, too.  If they are missing, you will need a
larger grant card, not the tiny, square, single-height G727 cards. I'd
tell you exactly what pin to check, but I don't have that info right
here... visually inspect a dual-height grant card and see where the
one lone jumper is on the other slot - that will tell you where the
NPR wire goes.  If you don't have any dual-height grant cards, I'd
recommend tracking down one or two - they are as useful while
debugging as an extender card - you can pull a DMA peripheral and not
have to fiddle with the backplane.

Since I used to work for a place that _made_ dual-height grant cards,
we *always* loaded our Unibus backplanes with them - we never had to
worry if slots 4 and 6 had the NPR wire out - we just jumpered all 9
slots.  Naturally, if you lack a pile of dual-height grant cards, that
won't work for you, but it was an easy thing back in the day.


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