On Jan 19, 20:47, lothar felten wrote: If I were to be really picky, I'd say you meant microPDP-11/23. 11/23-plus would mean a BA11-S enclosure with a different type of backplane :-) Not the 54- number, that's only the PCB part. The backplane itself has a model number; for a BA23 it should be H9728-A. In an H9728-A the top three slots are Q22/CD-interconnect, and the rest are Q22/Q22 serpentine (the 11/23-plus backplanes are different). When I replied to your earlier email I assumed you just gave the order of the boards, not the layout. The layout should be (reverse the positions of CPU and memory if you wish): ---------KDJ11-B-CPU--------- ---------MSV11-memory-------- ------------RLV12------------ ----RQDX3---- ----TQK50---- empty ----DEQNA---- I'm guessing you have a single memory board, probably an MSV11-Q (M7551), and an RLV12 (M8061, one quad board) rather than an RLV11 (two quad boards) -- if not, that makes a difference to the layout. I'm also guessing at a DEQNA (M7504) rather than any other Ethernet controller, but it makes no difference to the placement, so long as it's a dual-height board. If you added another dual-height board it would go under the RQDX3 (M7555), the next would go under that, and the next under the DELQA, etc. The arrangement of the slots after the first three is called "serpentine" or occasionally "zig-zag". All the KDJ11-B processors, whether 11/73 or 11/83, use the same printed circuit board and module number. There were some differences about whether an FPU could be fitted (due to an error on the original boards); those that would not take an FPU were only sold as KDJ11-BC and all had 15MHz clocks. Others with 15MHz clocks were sold as KDJ11-BB (upgradeable but FPU not fitted). There are also some with 18MHz clocks, these were sold as KDJ11-BE, -BF, or higher. Normally an 11/83 has a KDJ11-BE or higher suffix. Early 11/73 are 15MHz. Just to add to the confusion, the -Bx suffix actually refers to the EPROMs on the board, not the clock speed or the FPU. The *only* difference between a normal KDJ11-BE or -BF or -BH is the firmware in the EPROMs. However, the biggest difference between 11/83 and 11/73 is whether the memory is used as QBus memory, or PMI memory, which is faster. All of the KDJ11-B boards can use PMI memory. Beware, not all quad memory boards are PMI-capable, but all the 1MB and bigger ones that I can think of are. Yes. They will work with QBus memory instead (and if you put a PMI board after the processor instead of before it, it will run as normal QBus memory) but then what you have is effectively an 11/73, not an 11/83. No, there's no memory on the CPU board, but the memory you have is running as QBus memory. Literally "hanging"? Not fixed to the front of the BA23? Is this actually a floor-standing (or possibly rack-mounting) BA23 with space for a TK50 and a drive unit, or a rackmount BA11-S or BA11-N chassis with no space for drives? Neither have I. DEC used pushbuttons for the disk controls on microPDP-11 panels. Each section is separate, though; it sounds like someone has replaced the pushbuttons or used third-party sub-panels. The round hole (if this is an original DEC panel) is for the badge that says whether it's a microPDP-11/23, microPDP-11/73, microPDP-11/83, microPDP-11/53, etc. The rectangular hole is for the power switch in a BA23 or BA123 cabinet. -- Pete Peter Turnbull Network Manager University of York