11/53 - Qbus backplane question

Gooijen H wrote: The slots each allow for four "fingers" on each card - a quad card uses all four and a dual card uses only two (there are some single cards too, but not many). The holes on the backplane slot for the fingers are labelled A,B,C,D from left to right from the side the cards plug into. Q22/CD slots have A/B wired as Q22 bus, C&D are just wired straight through. C/D are not used for Q-bus comms and are just for card-card communication. Q22/Q22 slots have Qbus on A&B and Qbus on C&D. It sounds like in your machine, the path followed by the Qbus is: slot-1: A/B slot-2: A/B slot-3: A/B slot-4: A/B slot-4: C/D slot-5: C/D slot-5: A/B slot-6: A/B etc. i.e. Qbus goes down A/B in slots 1,2,3,4 and then hops over to C/D on slot 4, drops vertically down to slot-5 C/D, moves back to slot-5 A/B ... repeat until you run out of slots. The manuals refer to this as a serpentine pattern. There is a diagram in some of the MicroVAX manuals on http://208.190.133.201/decimages/moremanuals.htm (I cannot remember exactly which ones but if you pick up the 630Z Owner's and Technical manuals, I'm 90% certain one of them has the diagram ... note that BA23 has 3 Q22/CD slots and BA123 has four Q22/CD slots ... the principle is the same though). The Q22/CD distinction matters in two cases that spring to mind. The first is that some Qbus signals need to be passed on to the next card correctly (interrupt and grant signals). An empty slot breaks the chain here. So if your first three slots have processor + memory and you put a TQK50 controller in slot 4 (A/B) and an RQDX3 in slot 5 (A/B), the chain is broken because slot 4 C/D and slot 5 C/D are empty - you need grant cards (or other Qbus cards) in there to let the RQDX3 work. Just to make this more fun, the RQDX3 will show up on the bus (it's CSRs are visible) it just won't work :-) The other reason it matters is that quad wide cards need to do the right thing depending on the kind of slot they are in. Most seem to arrive defaulted for a Q22/Q22 slot (which means they can go straight into a BA23/BA123 chassis with no additional jumpering etc). Putting one of these in a Q22/CD slot is usually no problem except that the card is passing on the C/D signals to the next card. This may or may not matter. For those cases where it does matter, cards provide a means of preventing this from happening. On the KDA50 there is a zero-ohm link (looks liek a resistor) that you remove. Of course, if you move a modified KDA50 from a Q22/CD slot to a Q22/Q22 slot you are in for some fun :-) Later series chassis (certainly the BA200 series, probably the BA400 series) were wired Q22/CD throughout. Many years agon, on a PDP-11/23, we had an external box (about the same size as the 11/23 cab - including RL02 and RX01) which was just a frame buffer (i.e. video card). Your peripheral may well be similar (but this is just a guess based on the fact that it seems to have video connectors!) I guess yes. Obviously you loose a really cool peripheral! You can put a DELQA there. You can put a DEQNA there (which is what I guess you meant). But a DECNA is (IIRC) the really *rare* ethernet card for the Pro 300 series of machines. It won't fit and you'd destroy a cool card if you put that there :-) It doesn't *have* to be. The reasons for putting cards in a particular order is described in one (or more?) of the Micronotes that you can find on the web (e.g. http://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/hardware /micronotes/numerical/ ) Basically, the RQDX3 will hog the bus given half a chance, but has enough buffering not to mind yielding the bus to other peripherals (within reason). The nearer (electrically) you are to the CPU, the higher your priority. So the RQDX3 (and KDA50) traditionally go furthest That would be normal. It will probably work even if you don't move the RQDX3 though. It's worth mentioning that the RQDX*2*/RQDX*1* are brain-dead and do not pass on one of the signals (GRANT I think). This means that they *must* go at the end of the bus (nothing after them will ever be able to interrupt!) Antonio