On 2012-12-30 19:00, Christian Gauger-Cosgrove<captainkirk359 at gmail.com> wrote: Correct, PMI memory on PDP-11 machines is just communicating on the CD slots. No ribbon cables. However, I'd like to point out that the 11/83 also can use PMI memory. The thing to understand is that for the memory to be connected on the PMI, it needs to set *before* the CPU on the Qbus, and be PMI capable. The same memory placed *after* the CPU means it acts as normal Qbus memory. Some difference in performance. When sitting in the 11/84, the memory sits after the CPU, but all the first four slots are PMI slots in that box, always. Actually, PMI is also not only on the CD slots. A few signals in the AB slots are working differently than in a Qbus as well. And to point out one last, obvious thing, the PMI on the VAX is a bus, even through it's on a ribbon cable. Why would that make it not a bus? Johnny