Qbus Backplanes

Yes, a normal BA11-N is Q-CD. So is a BA11-S. If you can read the H92xx number off the backplane (it's often visible without too much effort, for example it's on the top of a BA11-M/N/S backplane) you can look that up to see if it's straight (Q-CD), serpentine (Q-Q) or mixed. The following is not exhaustive, but includes the common ones: PDP-11/03 BA11-M H9270 18-bit 4 x Q-Q PDP-11/23 BA11-N H9273 18-bit 9 x Q-CD H9275 22-bit 9 x Q-Q PDP-11/23+ BA11-S H9276 22-bit 9 x Q-CD microPDP-11 BA23 H9278 22-bit 3 x Q-CD, 5 x Q-Q H9281 18-bit various sizes, only two slots (AB) microPDP-11 BA123 54-17507 or 70-22019 22-bit 4 x Q-CD, 8 x Q-Q, 1 x non-bus DDV11 18-bit 9 x Q-Q, slots EF are separate Q-Q backplanes are usually referred to as serpentine (or sometimes as zig-zag), Q-CD as straight. Serpentine slot order goes like this: slot 1 ----> slot 2 | V slot 4 <---- slot 3 | V slot 5 ----> slot 6 etc Some of the backplanes have optional jumpers to enable/disable LTC interrupt, or for termination. Many carry the SRUN_L signal only on row 1. Beware of such differences when changing backplanes or linking two together. The DDV11 is unusual in being hex-height instead of quad-height; the extra slots are nor bussed, except for +5,V, 0V, and -12V, but may carry odd voltages, depending on which OEM or experimenter last used it! There's a rumour that the first 3/4 slots in a BA23/123 are special and you can't use them for anything except memory or processor; that's rubbish, they're normal Q22-CD slots and you can put anything that's Q-CD compatible in them, so long as it's after the processor. What *is* special is the range of PMI memory boards; if you put them in *front* of a KDJ11-B they use the CD interconnect for the Private Memory Interface but if you put them *after* the CPU they use the Q-Bus like all other memory boards. -- Pete Peter Turnbull Network Manager University of York