Massbus - was: Re: VAX 11/750

Chris Zach cz at
Thu Feb 25 13:04:54 CST 2021

That is at least one RH11-AB in there, minus the knucklebusters of course.

On 2/25/2021 1:49 PM, Bill Degnan wrote:
> On Thu, Feb 25, 2021 at 1:43 PM Chris Zach via cctalk 
> <cctalk at <mailto:cctalk at>> 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.
> <>

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