Interesting. So it's not bklock-mode on the QBUS which is screwed up, but normal QBUS transfers. That jibes with the comment abour "gate array incompatibilities" (which I take to mean "errors" :-). RH70's are totally different from RH11's - a hex card, and a couple of quads - and the interface to the /70's memory system is totally different from the RH11's (which goes to the UNIBUS): https://gunkies.org/wiki/RH70_MASSBUS_controller It has interfaces to both the cache, and the memory bus (although the diagram in the 11/70 CPU handbook shows it as only connected to the cache). I didn't follow the "the old FASTBUS on the 11/45 cpu core (which is what an 11/70 really is, with cache)"; the 70's cache is what's connected to what used to be the FASTBUS, the memory bus connects to the cache, IIRC. Noel