Huw Davies <H.Davies(a)latrobe.edu.au> wrote: On *some* backplanes, there is no difference. A dual-high board could be plugged into the CD side just was successfully as in the AB side. In other backplanes, the AB side is the Qbus side, and the CD side is set to connect adjacent boards so that they can communicate (such as the VSV11, the RLV11, and the 11/83 and its PMI memory). In yet other backplanes, you find that it is Qbus in the AB slots and CD bus in the CD slots for part of the backplane (BA23 = three slots, BA123 = 4 slots) and then it switches to Qbus in both the AB and the CD sides. Bottom line -- make sure you know what backplane you are plugging boards into before you do so. Also, make sure that quad-high options which can plug into both Q/CD and Q/Q have the CD jumpers on the board set correctly. Megan Gentry Former RT-11 Developer +--------------------------------+-------------------------------------+ | Megan Gentry, EMT/B, PP-ASEL | Internet (work): gentry zk3.dec.com | | Unix Support Engineering Group | (home): mbg world.std.com | | Compaq Computer Corporation | addresses need '@' in place of ' ' | | 110 Spitbrook Rd. ZK03-2/T43 | URL: http://world.std.com/~mbg/ | | Nashua, NH 03062 | "pdp-11 programmer - some assembler | | (603) 884 1055 | required." - mbg | +--------------------------------+-------------------------------------+