From Computer History Wiki
The H9276 backplane is a 9-slot Q/CD backplane, which could hold quad-sized cards. It was a Q22 backplane (the only significant difference from predecessor H9273 backplane, which was Q18); it was the standard backplane in the BA11-S mounting box.
Since it was a Q/CD backplane, dual-width QBUS cards could only be placed in the left-hand slots, and quad QBUS-only cards had to not use any pins on the right-hand C/D connectors. (The only DEC card with a problem with this seems to have been the MMV11-A.)
- PDP-11/23B Mounting Box User's Guide (EK-23BMB-UG-001) - the H9276 is covered on pp. 12-13
- BA11-S Field Maintenance Print Set (MP-01233) - the H9276-B Field Maintenance Print Set (MP01243)is on pp. 51-59