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