Differences between PDP-11/83-84 M8190-AB/AC and AD/AE CPU cards

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun Feb 8 08:25:38 CST 2015

    > From: Noel Chiappa

    > (I suspect, but have yet to verify, that this is in part because bus
    > grant lines are run directly from that slot to the UNIBUS adapter slot.)

So it turns out I was half-wrong, but half-right. I was paging through a copy
of EK-PDP84-TM-PR4 (PDP-11/84 Technical Manual) which I recently acquired: in
section 2.1.14 "Backplane (H9277-A)", pg. 2-6, the following sentence appears:

  Bus signals BDMGI (pin AR2) and BIAKI (pin AM2) for slots 2 & 3 are jumpered
  on the front of the backplane.

(This is not in any other version of EK-PDP84-TM, of which I have several.)
So I got out my spare 11/84 backplane, and sure enough there are two jumpers,
W1 and W2. The traces connected to them are, luckily, on the surface, so it's
possible to see where they go: one end runs to a trace connected from slot 1
to slot 2, and the other to a trace connected from slot 3 to slot 4.

EK-PDP84-TM-PR2 actually contains (pp. D-4 to D-8) prints for the backplane.
Unfortunately, they are very low-res, and only partially readable, but one
can see, on the bottom of the first print (D-4), the two jumpers. The
associated traces do indeed connect to AM2 (BIAKI), AN2 (BIAKO), AR2 (BDMGI)
and AS2 (BDMGO).

In other words, when the jumpers are in, the CPU's BIAKO/BDMGO pins are
connected directly to the UNIBUS adapter's BIAKI/BDMGI pins; when they are
out, those signals are routed through the two 'memory' slots, in the normal
QBUS manner.

What this strongly suggests to me is that those two slots _can_ function as
real QBUS slots. Otherwise, why arrange so that the grant lines can run
through them? I haven't examined all the other pins, to make sure they
contain the full set of QBUS signals - in part because I don't want to pore
over those poor images!

(Speaking of which, does anyone have a copy of either 11/84 Field
Maintainance Print Set - MP-01955 or MP-02536? Neither one seems to be online
- at least, as best I can tell, from some Googling, although others may have
better Google-fu than me - and it would be fabulous to have access to them.)

But I do strongly suspect they can function that way; at some point, when I
have an /84 running, I'll actually try them out. As to why DEC put this
capability in, and then didn't document it or use it - who knows? Maybe they
though it would introduce extra complixity in the user instructions, or

Anyway, I think we are close to cracking this particular puzzle...


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