Massbus - was: Re: VAX 11/750

Chris Zach cz at
Thu Feb 25 13:06:36 CST 2021

In answer to my own question: No it does not based on a bit of review. 
Looks like unique cards to the 11/70.


On 2/25/2021 1:43 PM, Chris Zach via cctalk 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
> 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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