General Question about UNIBUS backplanes

william degnan billdegnan at
Sun May 22 11:02:41 CDT 2016

Background - I have a PDP 11/40 that came to me with a CPU backplane and a
general purpose 4 slot peripheral backplane.  The CPU backplane came to me
connected to the 4 slot via a std 11/40 M981.  In the 4-slot I have I have
a working 64K solid-state RAM card, M7856 serial card, M9312
bootstrap/console card, and an RL02 drive controller.    I can access RAM
from the console, run the console program (monitor program) off the M9312
through the serial card.  I can't boot yet off the RL02 and I am thinking
this is due to a termination issue, not sure yet.  I decided I need more
backplane to go any farther.

So I decided to insert two 32K core memory backplanes in between the
original two backplanes that came with the system.  My plan is to replace
the solid-state RAM card with core, and this will free up a slot on the
4-slot backplane for a terminator.  Am I correct in my logic?

I have connected the "new" backplanes using M920's.  Imagine the original
system with a core backplane wedged in the middle.  Power seems fine, DC LO
AC LO fine.

Right now there is nothing in the core backplanes, First I want to verify
that I have them arrayed correctly and that I can still communicate with
the (now) 4th backplane in the back.  I can run "chase the lights" from the
console but the serial card seems to have stopped working.  It appears that
the console program is being loaded and runs from the M9312 but nothing is
appearing on the terminal.  I cannot send an "A" to the terminal, the most
basic test I can think of.

Could be the the M7856 card decided to die on me just now, or something
else is wrong.  Does anyone have a PDP 11 /05/10/35/40 and have
experimented about what you can and can't do with backplanes?

I know that there are a lot of variables here, so I'd like to start with
the basics

Basic Question #1
Do I need grant cards in *every* empty slot of the core backplane,
including the end slots with the M920's?
Basic Question #2
Can you only use the core backplanes for core memory (assume yes), or are
they capable of holding other types of cards temporarily? (yes/no/maybe)

I am researching the system config on my own, but I thought if anyone had
any advice I'd love to hear it, make troubleshooting a little easier.

I have read up on the subject:

I see this is not a simple little thing.  Working to find more GC cards now.



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