General Question about UNIBUS backplanes

Paul Anderson useddec at
Sun May 22 14:02:29 CDT 2016

Hi William,

If I follow right, you have gotten some great advice,,but I'm not sure
about a few things.

Is your 4 slot SPC a DD11-A, DD11-B, or DD11-CK/CF?  The standard one used
in most 1140s is the DD11-B, and a lot had DD11-Cs added later. The DD11-B
will not work with the RL11  and probably not with the MOS board.

The last slot of the the Unibus should have a terminator in A/B.

I don't know how well you know the 11/40, but most cpu options require
jumper changes on various cpu boards. These can can weird problems also.
You can take out the options, M7236, 37, 38, 38, and M787, reset the
jumpers, then put them back in when everything else works.

Shortening the bus is also helpful when troubleshooting.

PDP 11/40s are great! Good luck and have fun!


On Sun, May 22, 2016 at 1:19 PM, william degnan <billdegnan at>

> On May 22, 2016 1:22 PM, "Mark Darvill" <mark.darvill at> wrote:
> >
> > The core memory/CPU backplane is a special case and doesn’t need grant
> cards.
> >
> > If you have linked up to the expansion Unibus then you need to check you
> have the NPG jumper in row C, Pins A1 and B1 in each slot. If these do not
> have wire wrap then you will either need to wire wrap them or insert a G727
> into the backplane slot. Without these the units will hang on attempting to
> use a card that requires DMA access.
> >
> > Hope this helps.
> >
> > Mark
> >
> Thanks Mark..yes I have one slot that needs the npg, it's in place.
> I have not had a chance to test with grant cards removed hope that does the
> trick

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