Help identifying UNIBUS PDP-11 CPU upgrade

Noel Chiappa jnc at
Wed Jun 28 13:02:09 CDT 2017

    > From: Josh Dersch

    > I pulled the ... bootstrap boards from slot 3 ... The BOOT button
    > causes the RUN light to momentarily flash, but that's about it. 

That's not too surprising. The way booting works with the M9301 boot card (not
sure if you know this already; if you do, apologies, and ignore) is that the
'BOOT' button simulates a power-fail/restart - i.e. it toggles the 'power OK'
line (forget whether it's ACOK or DCOK). When the CPU 'powers up', and tries
to fetch the power fail restart vector from 024/6, the M9301 steps on the bus
address lines and modified the address to 773024/6, which is in the ROM on the
M9301. The new PS/PC retrieved at that point then sends the CPU into the ROM.
So, with no M9301 plugged in, it's not too surprising it doesn't do much.

The DEC QBUS processor cards (not sure about the chips themselves) can be
jumpered on power-up to i) halt, or ii) fetch a PS/PC from 024/6, or iii) jump
to ROM. I've no idea if this board set has something similar (and if so, what
it's set to do).

    > Ran the front panel cable to the connector on the CPU board and powered
    > it up.

Now I'm confused. There is no cable from the front panel to the CPU in a
standard 11/34? (There's one from the front panel to the backplane; another
from the front panel to the M9301; and another from the programmer's front
panel [if present] to the programmer's front panel board, the M7859; but
that's it, that I know of.)

Did you mean the console serial line cable?


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