11 Feb
2003
11 Feb
'03
8:40 a.m.
Jay wrote:
Not trying to start any holy wars or
anything, but to my "newbie with unibus" mind, the design of having to
change backplane wiring for certain cards seems to be rather... ummmm
silly (euphamism{tm}), at least as compared to other systems of the
period.
Of course, they expected this to be taken care of by field
service, not the end user. But the obvious alternatives were:
1) Require the use of jumper modules in all otherwise empty SPC
slots. This seems reasonable but in the early Unibus days they
only had the short G727 jumpers which didn't pass the NPR signals
because they only went into one slot. The later G7273 modules
do pass NPR, and are more convenient because they are full
length modules with a handle.
2) Design a modified connector that shorts the grant in and out
contacts together when no card is inserted. Presumably they
didn't want to invest in a new connector block design, and
have to worry about stocking another type and making sure that
the right type was used in the right location.
3) Put active electronics on the backplane to detect the absence of
a card and pass the grant. Probably deemed too expensive when
Unibus was designed back in 1969 or so.
Probably there is something I'm missing as to why
this is a good
thing.
Not especially.
On my Unibus machines, I normally cut the grant jumpers on all the
SPC slots, and use G7273 cards in the otherwise empty slots.
Regarding the unibus terminator card...M9302 -
everyone says "the last
slot of the unibus". Does this mean physically the last slot in the
backplane, or "logically the last slot of the unibus" meaning after the
last card?
Physically the last slot.
The manual states that for each open SPC slot, you
have to use a grant
card. But it's a little unclear as to "SPC" slot. In the "main"
backplane, there is a hex SPC slot and a quad? SPC slot after the
memory, the AB spots on the 2nd SPC slot being for the 9202 jumper card.
This gives rise to a few questions: If the last 2 slots in the "main"
backplane are for SPC, are the remaining slots (in the 2nd backplane,
AFTER the 9202 jumper) SPC slots?
That depends on what the 2nd backplane is. If it's just a DD11-CK
or DD11-DK generic SPC backplane, yes, you need grant cards. If it's
a specialized System Unit backplane, such as an RH11, RK11, etc., you'll
need to find out for that specific backplane.
I assume you need a grant card in the
hex SPC slot if nothing is there. But what about the quad SPC slot that
also has the 9202 in it - does that slot need a grant card too?
Yes.
And
about the 1st slot in the 2nd backplane that has the other side of the
9202 in it - that needs a grant card as well? I'm assuming it does...
Again, that depends on the backplane. For a DD11-CK or DD11-DK, all
the slots are SPC, so they all need grant cards.
So in the example where you have no cards at all in
the 2nd backplane
except the initial 9202 and the final 9302 at the other end.. you must
have a grant in every one of those open slots? If so, ummm I need a
bunch of grant cards! Anyone have some to trade?
If you don't have any cards in the second backplane, take out the Unibus
jumper and install the terminator in its place.