UK PDP11 wanted

Sure; glad you liked it. Perhaps I'm confused, but the only QBUS signals which are not 'broadcast' (i.e. sent down a wired-OR bi-directional bus transmission line to all cards) are the grant lines. The CPU doesn't pass them through, it originates them - but memory cards ignore those lines anyway? Actually, the PMI signals are all (I'm pretty sure, but have not checked) just like most other QBUS signals - 'broadcast' bi-directionally down a transmission line. The 'PMI memory before the CPU' thing in 11/83's is simply an artifact of the way 'standard' Q/CD backplanes are wired on the CD side (used for PMI) - see that second post for details. In the 11/84 backplane, the PMI memory lines _are_ bussed to all the slots, just like the QBUS lines, which is why 11/84 PMI memory cards go after the CPU. (So you could in fact have an empty slot between the CPU and the first PMI memory card in an 11/84, and it should still work.) My impression is that the CPU does not get involved in DMA access to PMI memory. In an 11/83, I gather the DMA devices just do ordinary QBUS memory cycles to it (see the MSV11-J manual, pg. 1-1, middle). In an 11/84, with UNIBUS adapter, IIRC the UNIBUS adapter does PMI cycles direct to the memory (but don't quote me on that, I haven't checked the 11/84 documentation, although the MSV11-J manual indicates it does - see pg. 1-1, bottom). I'm pretty sure that whether PMI memory works as PMI memory or QBUS memory depends only on what it hears on the busses, and nothing else. If it sees a PMI cycle for it, it does a PMI cycle; if it sees a QBUS cycle for it, it does a QBUS cycle.*1 So in an 11/84, where it's on the bus after the CPU, it does PMI cycles with the CPU because the CPU does a PMI cycle. (1: Note that the MSV11-JB and -JC variants don't do QBUS properly, and are PMI only, and so can't be used in an 11/83. I seem to recall hearing it's just QBUS block mode transfers that they don't do properly?) Which all does raise an interesting question: if an M8190 is in a system with a _mix_ of PMI and regular QBUS memory, do accesses to both memories work fine? (E.g. if it's in a Q/CD backplane with a PMI board before it, and a regular QBUS memory card after it.) Something to try, I guess - I have a working Q22/CD box, and plenty of cards - should just plug them in and try it! Noel