General Question about UNIBUS backplanes

Mattis Lind mattislind at
Tue May 24 09:12:40 CDT 2016

2016-05-24 15:37 GMT+02:00 william degnan <billdegnan at>:

> >
> >
> >
> >
> > But the M9312 has bus terminating resistors (unless you actually
> desoldered
> > those!). Then you have three terminators in your system! One in the M891
> at
> > the CPU, one in the M9312 in slot 3 of last DD11, and then a M930 in
> slot 4
> > of DD11.
> >
> > If you want the M9312 you should put it with W1-W5 installed in the slot
> 4
> > where you have the M930. You should not have both a M930 and M9312 (when
> > you have the M891).
> >
> > /Mattis
> >
> After I sent the email to this group, I checked the M9312 and found it was
> terminated.  I was told it was not, I should have checked for myself but I
> grew suspicious that it should be by default.  Now, the M9312 is in the
> last slot, and I removed the M930.  There is nothing except a grant card in
> the 3rd slot.

Good. But what is the status of W1-W5 of the M9312? Installed or not?

The usage of AU1 and AV1 differs in Unibus slot versus a Modified Unibus
(MUD) slot. The MUD slot uses those two pins for +20V Core power. You don't
want to have the jumpers installed when you put the M9312 in a MUD slot (if
there is +20V in your system). Then smoke will escape I guess.

But If you use it as a terminator in a unibus slot then then you want them
to terminate the BGx and NPG lines thus you need to install the jumpers.

Now I don't really know if your backplane has MUD wiring or not. I think
MUD wiring is a later DEC invention when there were selfcontained unibus
core memory modules. Not entire backplanes.

But be carful with the jumpers. On the M9302 for example there are big
print that tell you something will go bad if you install it in a MUD slot...

> I fixed the core problem I described in my email.  I also am able to store
> into core and run "chase the lights",  I have generally speaking a working
> 16K system.

Sounds really good!

> Thanks again.
> Next - Get a serial card running and resume efforts to boot RT11 from an
> RL02 drive.

What never was clear to me was if you actually could access the serial card
or not from the console. I.e. did it respond when addressing it? If it did
respond I would check that there were correct power supplied to the 1488


> Bill
> --
> @ BillDeg:
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