UK PDP11 wanted

Noel Chiappa jnc at
Wed Apr 15 09:04:36 CDT 2015

{Replies to all three recent posts bundled together to reduce list traffic;
hope that's OK...}

    > From: Glen Slick

    >> 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.

    > when PMI memory boards are installed above and adjacent to the M8190
    > CPU the PMI signals are bused through one PMI memory board to the one
    > above and adjacent to it

At a low level, yes; but I'm trying to understand PMI at a higher level - one
that applies across _all_ backplanes, including the 11/84 backplane, which is
a purer implementation of 'PMI is just another broadcast bus'.

    > From: Johnny Billquist

    >> 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.

I checked (Micronote #30, 11/84 Tech Manual) and this is basically correct;
all the additional PMI signals are implemented as bi-directional transmission
lines (although some are only used uni-directionally, e.g. PMAPE, PUBSYS,
etc). The UNIBUS adapter does use the BDMG and BIACK lines, which are
uni-directional grant lines, but they are both actually QBUS signals.

    > But because of the way the CD slots are wired, it cannot be a general
    > broadcast thing.

Repeat previous reply to Glen...

    >> My impression is that the CPU does not get involved in DMA access to
    >> PMI memory.

    > This is one thing I'd like to check though. And I think this is my
    > point of worry...

I couldn't find any 11/83 manuals online. There is a User Manual for the
KDJ11-B, and it indicates (Section 1.6.1) that DMA devices can become QBUS
masters and do any normal data transfer over the QBUS, which I take to mean
that the CPU is not involved (other than to do the initial
arbitration). Section 7.3 confirms this.

    >> 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.

    > The memory works as PMI or plan Qbus memory simply based on where it is
    > located in relationship to the CPU

Again, that's looking at one special case (the Q/CD backplane); I'd rather
look at PMI in a more general way.

    > From: Pete Turnbull

    > QBus non-PMI memory does work (as QBus memory) if placed before the CPU.

That part I was pretty sure about (given the way the QBUS works). The part I
didn't know was whether an M8190 would work correctly with a mix of PMI and
non-PMI memory.

I wonder what it does - does it start a PMI cycle, and then convert to a QBUS
cycle when the PMI doesn't happen? Or does it somehow scan for both types
during startup, and have some sort of configuration table that it relies on
thereafter? I didn't see anything in either the KDJ11-B or KDJ11-E User
Manual, and there is no KDJ11-x Tech Manual that I could find.


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