<both ABCD. In a BA23 box, the top 3 slots are ABCD. I presume <that if you put the CPU in the top slot and the RLV11 in the 2nd and <third slot, it might work. Has anyone ever done that successfully? NO, Dont! There is a different backplane that would fit and work but the standard (BA23) one is not usable with RLV11. Most BA11 (S or N) boxes have the right one though. <Also, if you are running RT-11, you can still use more than 256 KBytes <in the system, just don't try to do it with the RLV11. PIP can <copy anything from the RL02s to the RD53 and then let you use <the rest of the memory over 256 KBytes. RT11 will run in more than 256k (mine has 2mb). The RLV11 however limits you to 256k, though you could write your one handler... nah. <The RQDX2 allows RD51, RD52, RD53. It is a quad board (a -YB). <Most will not accept an RD52 from an RQDX1 without re-format. RQDX1 and 2 are the same board with different firmware. The later RQDX2 has knowledge of RD50->53 where the RQDX1 only knows the RD50->52. The later RQDX3 (dual width rhater than quad) required the reformatted drives as it's slightly differnt format. Allison