Massbus - was: Re: VAX 11/750

On Thu, Feb 25, 2021 at 1:43 PM Chris Zach via cctalk <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org>> wrote: Oh this is fun stuff. Is there a specification write-up anywhere on the MASSBUS overall? For example I wonder if the RH70 could do a transfer >128kb at a time. Another is around the RH11: There were two models, the traditional RH11 (which could only do so many words on a DMA transfer) and the RH11-C which could do more words per transaction by basically running the Unibus in "Hog mode". That allowed the 2020 to run RM03's at a full 3600 RPM (and I assume can allow the 2020 to run things like RM80's). Another item I always wondered about was the RH11's support of two unibuses. I think the idea was to do the data transfers on the second bus right to an 11/45's FASTBUS memory without worrying about DMA timeouts while running the control and status registers on the normal UNIBUS (which wouldn't block other devices). I wonder, does the MBA on an 11/70 use similar cards to an RH11? C I have a UNIBUS array of backplanes from an Industrial 11/40 that has a lot of MASBUS controllers in it.? Or, I think that's what these are.? I removed the?backplane chassis from the?rack and replaced it with two RL02 drives. https://www.vintagecomputer.net/digital/PDP11-40_industrial11/DEC_industria… <https://www.vintagecomputer.net/digital/PDP11-40_industrial11/DEC_industrial11_exp_chassis.jpg>