[cctalk] Re: Massbus emulation

Does anybody here still use Massbus? Or have a Massbus system you’d like to run, but the idea of dealing with washing-machine disk drives is a bit less appealing these days? In what may classified as a momentary lapse of reason, I have taken up the old Living Computer Museum Massbus emulator project and am in the process of resurrecting / modernizing it. Current progress photo here: http://www.dmv.net/mbe/mbe1.jpg Why, you ask? "Has he gone insane?" are you thinking? I have three PDP-11/70s that I would like to get running again. Sure, I could cheat and hang a UNIBUS SCSI controller with a ZuluSCSI disk simulator, but where is the fun in that? Also, the 11/70 was designed and optimized around Massbus for primary storage, so it seems only fitting to use it that way. The original LCM emulator used a PC with a Mesa 5I22 FPGA card as the Massbus interface. The FPGA implemented the drive-side bus logic, while the PC software emulated the backing disk or tape image. A driver/receiver (“D/R”) board sat in the middle to translate the Massbus differential signals into logic levels suitable for the FPGA. It worked... Since Mesa 5I22 cards are now pretty much unobtainium, I went looking for a cleaner and more modern approach. The result is a redesigned D/R board that accepts a Terasic DE10-Nano directly. The DE10-Nano is a small Linux SBC with a Cyclone V FPGA onboard, so it can host the emulator software itself while the FPGA handles the Massbus interface duties that were previously done by the Xilinx FPGA on the Mesa card. Same general architecture, but much tidier. There is still work to do, but it is coming along nicely. The board in the photo is not yet fully populated, since I am doing incremental testing before committing the rest of the parts. As I have gone down this rabbit hole, naturally a few questions in the “why did they do THAT??” category have come up... paging @Rich Alderson ... If anyone here is still actively using Massbus, has experience with the original LCM project, or just has relevant war stories, comments, warnings, or encouragement, I would be glad to hear them. Cheers! Jim