It's not exactly clear to me what you're asking, Chuck. If you're asking a general backplane expansion question, the answer is probably contained in Micronote 29, *Q-bus Expansion Concepts*, available over the web from http://metalab.unc.edu/pub/academic/computer-science/history/pdp-11/microno… Only the AB slots are continued between backplanes, not the CD slots, in any conventional configuration. Note that there are both 18-bit and 22-bit bus cable-cards; the M9400/01 series are 18-bits, and the M9404/05 series are 22-bits. For more information on 22-bit vs 18-bit options, see Micronote 5, *Q22 Compatible Options*. According to Micronote 5, not only does the RLV11 pay attention only to the low 18 bits, it also uses backplanes pins BC1 and BD1 for purposes other than BDAL18 and BDAL19, so other things are likely to break if its plugged into a 22-bit backplane. -- Tim Shoppa Email: shoppa(a)trailing-edge.com Trailing Edge Technology WWW: http://www.trailing-edge.com/ 7328 Bradley Blvd Voice: 301-767-5917 Bethesda, MD, USA 20817 Fax: 301-767-5927