You want to look at Micronote #5, "Q22 Compatible Options", for the straight scoop. If you want to look at it over the web, start at the Micronote index at http://metalab.unc.edu/pub/academic/computer-science/history/pdp-11/hardwar… In there you'll see the H9278 classified as Micro/PDP-11 H9278 4 X 3 Q22/CD and 4 X 5 Q22/Q22 Backplane In other words, this is your bog-standard BA23 backplane. The first three slots are CD, and the rest are serpentine. -- 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