UK PDP11 wanted

Johnny Billquist bqt at
Tue Apr 14 17:44:39 CDT 2015

On 2015-04-14 23:16, Noel Chiappa wrote:
>      > From: Johnny Billquist
>      >> But the QBUS part of _any_ QBUS backplane is not 'directional'
>      >> ...
>      >>
>      > there is still a question on whether the CPU just pass all signals
>      > through that you need.
> Perhaps I'm confused, but the only QBUS signals which are not 'broadcast'
> (i.e. sent down a wired-OR bi-directional bus transmission line to all cards)
> are the grant lines. The CPU doesn't pass them through, it originates them -
> but memory cards ignore those lines anyway?

I think that is correct.

>      > With PMI memory, it obviously works somewhat different, since the CPU
>      > sources signals in just one direction (hence the reason the PMI memory
>      > have to be before the CPU)
> Actually, the PMI signals are all (I'm pretty sure, but have not checked) just
> like most other QBUS signals - 'broadcast' bi-directionally down a
> transmission line. The 'PMI memory before the CPU' thing in 11/83's is simply
> an artifact of the way 'standard' Q/CD backplanes are wired on the CD side
> (used for PMI) - see that second post for details.

But because of the way the CD slots are wired, it cannot be a general 
broadcast thing. Even if the cards just connect two pins together, in 
order to pass the signals along, it means that those signals can be cut.

> In the 11/84 backplane, the PMI memory lines _are_ bussed to all the slots,
> just like the QBUS lines, which is why 11/84 PMI memory cards go after the
> CPU. (So you could in fact have an empty slot between the CPU and the first
> PMI memory card in an 11/84, and it should still work.)

Right. The backplane of the 11/84 is wired differently than a Q-CD 

>      > [the CPU] also interacts/intercepts with all other memory accesses to
>      > the PMI memory.
> My impression is that the CPU does not get involved in DMA access to PMI
> memory.

You might be right. This is one thing I'd like to check though. And I 
think this is my point of worry...

> In an 11/83, I gather the DMA devices just do ordinary QBUS memory cycles to
> it (see the MSV11-J manual, pg. 1-1, middle). In an 11/84, with UNIBUS
> adapter, IIRC the UNIBUS adapter does PMI cycles direct to the memory (but
> don't quote me on that, I haven't checked the 11/84 documentation, although
> the MSV11-J manual indicates it does - see pg. 1-1, bottom).

Yes, the the Unibus adapter speaks PMI all the time.

>      > Which is yet another reason why the PMI memory sits before the CPU. If
>      > you put PMI memory after the CPU, it will instead work as normal Qbus
>      > memory.
> I'm pretty sure that whether PMI memory works as PMI memory or QBUS memory
> depends only on what it hears on the busses, and nothing else. If it sees a
> PMI cycle for it, it does a PMI cycle; if it sees a QBUS cycle for it, it does
> a QBUS cycle.*1 So in an 11/84, where it's on the bus after the CPU, it does
> PMI cycles with the CPU because the CPU does a PMI cycle.

Well, in a way that is obviously correct. The memory works as PMI or 
plan Qbus memory simply based on where it is located in relationship to 
the CPU, so obviously it does Qbus cycles or PMI cycles depending on 
what it hears on the buses.

> (1: Note that the MSV11-JB and -JC variants don't do QBUS properly, and are
> PMI only, and so can't be used in an 11/83. I seem to recall hearing it's
> just QBUS block mode transfers that they don't do properly?)

That is more than I knew.

> Which all does raise an interesting question: if an M8190 is in a system with
> a _mix_ of PMI and regular QBUS memory, do accesses to both memories work
> fine? (E.g. if it's in a Q/CD backplane with a PMI board before it, and a
> regular QBUS memory card after it.)
> Something to try, I guess - I have a working Q22/CD box, and plenty of cards
> - should just plug them in and try it!

As far as I know/understand, this will work just fine if you put the 
Qbus memory after the CPU and PMI memory before. I don't know what 
happens in general if you put Qbus memory before the CPU. That is 
another interesting question.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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