PDP-11/34A bring up

I think you're right there :-) Almost certainly both of those would be DMA devices ('NPR' in DEC's terminologiy, meaning 'Non Processor Request') . As a result, the NPG (non provessor grant) jumper will have been removed from the backplane wiring on those 2 slots. I would agree with all that. You've done the first test that I always suggest -- Pull the M9302 (and replace it with an M930 if you have one). )OK, as to what';s going on. This is is basically what should heppen when a Bus Request (BR) or Non Procesor Requeest occurs : 1) Periphral asserts the appropraite BR on NPR line 2) Procesosr (actually the arbiter if you're pedantic) asserts the associated Grant line, procided (in the case of an BR) the priority level is high enough. Note that the grant lines are not bussed along the Unibus slots. Instead, the grant lines from th processor/arbiter) go to the grant inputs on the first Unibus slot. The grant outputs of that slot go to the grant inuts of the next slot, and so on. 3) A device which is not issuing a request at that level will simply pass the grant on, and make it's grant outputs the same state as the grant inputs. That's what a grant contuity card does (in a trivial way by simply connecting the inputs ot the outputs) 4) The device that did issue the reuest does not pass the grant on (it keeps its grant outputs deasserted) so that no device s further from the processor can repsond ot the grant. It then asserts SACK (Select ACKnowlddge) on the Unibus to say that it's accepted the grant. I think at this point the arbiter seasserts the grant lines, the peripohertla has control of the Unibus. Now, normally, a grant should never get as far as the terminator at the far end of the Unibus. Because the grant was issued in response to a requexst from a device, and that device should intercept the grant and not pass it on. However, IIRC there can be a condition where things don't quite synchornise properly. Under those conditions, hte grant is not intercepted, as a result, the SACK line is never asserted and I think the machine hacks. So the M9302 terminator has a little bit of logic to assert SACK if it sees a grant. But that birings in aother problem. The grant lines, unlike the other Unibus signals, are active high. So if the grant chain is broken at some point, th grant inputs of all devices after th break will seem to be asserted. Those grants will get to the terminator, which asserts SACK, and the system hcngs for another reason. That's waht you're seeing. It's a little wire-wrapped jumper between pins CA1 and CB1 of the Unibus slot. That is, the rearmsot 2 contacts on the right hand side (compoennt side of the PCB) on the 3rd edge conenctor from the rear of the machine. Most of the grant contiuity cards used on 11/34s are the little almost square ones which go in connector D. Those connect up the 4 bus grant lines, but not the NPG line. There is a dual-height continuity card which goies into C nad D and completes all grants, but they're not as common. A trick I've usef for testing is to cut a little strip of single-sided copper clad boardthat will go ito the C connector and connect pins CA 1 and CB1 only. When I fit NPG jumpers by wire-srapping them on the backplane, I use red or orange wire-wrp wire ot make them easier to spot in the future. And yes the RL11 is a DMA device, you must remove the NPG jumper from the slot where it's fitted -tony