Strange third party board in PDP-11/45

Mattis Lind mattislind at
Wed Jul 25 03:02:37 CDT 2018

>     > The sandwiched dual boards are sitting in 27 / 26 AB. The board in
> 27AB
>     > was empty (quick glance), while the board in 26AB has a few TTL chips
>     > on it. Slot 26AB is the Unibus A slot, Slot 27 AB should be a
>     > terminator on Unibus B.
> I'm more interested in _what_ the two boards are doing! :-)
I will take a picture of the boards in more detail so we can figure out
what they are doing later on.

> It seems they must be jumpering UNIBUS A and UNIBUS B together. (Which I
> didn't expect, but maybe... will have to ponder.)
> As to what _else_ it is doing, and why it has the cable to the main
> card... I
> think that it must intercept MSYN from the processor and only let it pass
> if
> there's no hit in the cache.
> (To explain why it would need to do that... normally with the MS11,
> there's a
> static partitioning between FastBus memory and UNIBUS A memory. So when the
> CPU goes to do a memory cycle, it can put the address out on both the
> and FastBus, with the certainty that it will only get a reply on one. But
> with
> the cache, if there's a hit, it would in theory get a reply on both, which
> might confuse it. Or if it takes the cache copy, and terminates the UNIBUS
> cycle, that might confuse the memory.)
> Or maybe I'm confused, because now that I think about it, UNIBUS A goes
> straight from the CPU to the UNIBUS A out slot, so the Able board couldn't
> intercept MSYN? I guess I need to understand the fine details of the
> and B stuff, maybe it will make sense at that point.
> Oh, wait a moment: slot 26 is UNIBUS A out, slot 27 is 'UNIBUS B in', and
> slot
> 28 is UNIBUS B 'termination'. (27 is 'in' because when the M9200 is
> installed
> in 26/27 to join the two UNIBI together, obviously one has to connect an
> 'out'
> to an 'in'... and then 28 is not 'UNIBUS B termination', it's 'UNIBUS out'
> to
> the rest of the system.
> OK, so that works - MSYN coming out of slot 26 is intercepted by the dual
> double-card, and is only allowed to pass on cache miss. Yeah, that sounds
> like
> it should work.

I am not sure I follow you entirely. My understanding is that slot 1AB and
slot 26 AB is tied to each other. So if there would be no expansion unibus
there should be a M930 in each of these slots. The same goes for slots 27AB
and 28AB. It corresponds with what I see on page 111 in
if I am not missing something.

I cannot see how a device in slot 26AB or 27AB would be able to intercept
MSYN here. What it could do though is to force some signals active (low).

>     > The hex ABLE/ ACT board sits in slot 21 which is the memory
> controller
>     > board for the MS11.
> One of two; the other is slot 16.
>     > From: Paul Birkel
>     > I wonder whether this CACHE/45 can coexist with MS11 memory on the
>     > Fastbus itself
> According to that marketing thing you found, "User may optimize hit ratio
> by
> upper/lower limit switch settings", so one would have to configure the
> Cache/45 to not cache the block that the 'other' MS11 controller thinks it
> owns... otherwise both might respond to requests for addresses in that
> range.... :-)
>         Noel

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