Massbus - was: Re: VAX 11/750
cz at alembic.crystel.com
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?
> 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
>> > 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
>> 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
>> -11/70 memory bus (as well as the UNIBUS, which is only used for
>> control, not
>> Similar questions about the speed point; passing data through an RH11
>> increase the speed of the UNIBUS? Yes, the RH70 is faster, but that's
>> of its connection to the -11/70 memory bus.
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