Massbus - was: Re: VAX 11/750

Chris Zach cz at
Thu Feb 25 12:43:17 CST 2021

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?


On 2/25/2021 1:30 PM, Noel Chiappa via cctalk wrote:
>      > From: Paul Koning
>      > There's a good reason why the big disks on many DEC machines were Massbus
>      > devices until MSCP arrived.  It's quite clear on Unibus PDP-11s, which
>      > needed Massbus both for speed and for a cleaner answer to more-than-18
>      > bit addressing.
> I follow the first sentence, but I'm confused by the second, especially "a
> cleaner answer to more-than-18 bit addressing". The UNIBUS MASSBUS
> controller/adapter, the RH11, only has 18-bit addressing on the main memory
> side. It does have more than 18-bit addressing on the device side, but so does
> the RP11 (sort of). Are you thinking of the RH70? That does have access to
> more than 2^18 bytes of main memory, but that's because it connects to the
> -11/70 memory bus (as well as the UNIBUS, which is only used for control, not
> data).
> Similar questions about the speed point; passing data through an RH11 doesn't
> increase the speed of the UNIBUS? Yes, the RH70 is faster, but that's because
> of its connection to the -11/70 memory bus.
>     Noel

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