Massbus - was: Re: VAX 11/750

Bill Degnan billdegnan at
Thu Feb 25 12:49:04 CST 2021

On Thu, Feb 25, 2021 at 1:43 PM Chris Zach via cctalk <cctalk at>

> 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.

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