UK PDP11 wanted

Johnny Billquist bqt at
Sat Apr 11 10:42:54 CDT 2015

On 2015-04-11 17:25, Noel Chiappa wrote:
>      > From: Johnny Billquist
>      >> Remember the jumpers I found on the 11/84 backplane? They can turn the
>      >> PMI section (AFAICT) into a real QBUS.
>      > I don't remember any jumpers.

Cool. I can't remember reading that post before, but those jumpers are 

>      >> For QBUS _memory_, it should 'just work', in the same way that you can
>      >> plug an M8190 into a Q/CD backplane with some QBUS memory, and it will 'just
>      >> work'. (I.e. the M8190 will automatically do QBUS memory cycles if
>      >> that's the kind of memory that's out there.)
>      > I'll believe that when you plug the Qbus memory *before* the CPU in a
>      > Q/CD backplane.
> But the QBUS part of _any_ QBUS backplane is not 'directional' in the same
> way the CD part of the backplane in an ordinary Q/CD backplane is. (Note that
> the 11/84 backplane's CD section is _not_ 'directional' the way a normal Q/CD
> backplane's CD section is. See:
> for more.) So for plain QBUS memory, it doesn't matter where it is in the
> QBUS backplane, in relation to the CPU.

Good point. But there is still a question on whether the CPU just pass 
all signals through that you need. In a normal Qbus system, the CPU is 
*always* at the end, as it terminates the bus. 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), and as far as I understand, also interacts/intercepts with all 
other memory accesses to the PMI memory. 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.

So I'm still not convinced until I actually see that done. :-)


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

More information about the cctalk mailing list