More on building PDP-11s

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Dec 17 17:28:59 2003

[...]

> backplane. Be careful when buying modules that go in System Units; if
> you don't have the backplane, they're of no use. Examples of System

If you're a sufficiently dedicated (read : Mad) hacker you can always
wire-wrap your own special backplanes for boards that need them. It's
quite a bit of work (I did one over a long weekend...) but it is possible.

> Units are the RK11-D DECdisk control, RH11 Massbus adapter, and
> MM11-L core memory.

> hard to get out of a full backplane. The M9302 is a tall card, and
> IIRC does a bit more than the M930. On more recent systems, the
> M9301 and M9312 are common, and combine the terminator with bootstrap
> PROMs.

Strictly yuo need a terminator at _each_ end of the Unibus. In the case
of the 11/45, you put an M930 card in the top front slot of the CPU

It provides an SACK signal if a grant gets to the terminator (this will
mean very little unless you've seriously hacked about with the Unibus).
This is a Good Thing under normal circumstances, but a Bad Thing if the
grant chain is open for any reason (missing grant continuity card or
jumper) -- it locks up the entire machine as the arbiter tries to remove
the grant (which it can't do, since it's not coming from the arbiter due
to the open grant chain) and thus release the SACK. It's handy to have an
M930 around for testing -- stick that in place of the M9302 and see if
the machine will at least do something. If so, you can then start looking
for grant chain problems.
 
backplane (CPU end terminator) and one in the Unibus Out slot of the last
backplane on the bus.

Often (certianly in machines like the 11/34) you put the
terminator-with-boot-ROMs in the CPU-end termiator position (often in a
dedicated slot in the CPU backplane). You then put an M9302 (or an M930,
I suppose) in the Unibus Out of the last backplane.

> There are also "Extended Unibus" slots in the PDP-11/24 and PDP-11/44,
> which have extra memory address lines. Once again, be very careful

THey're basically MUS slots with the extr address lines IIRC.

> how you use those slots.
>
> Since all of the PDP-11 systems with "real" front panels used Unibus,
> you can generally use almost any Unibus or SPC peripherals on any of
> the CPUs. Memory is a little bit trickier; the high-end CPUs like the
> 11/45 and 11/70 require special memory. The 11/20 uses Unibus memory,

Not so in the case of the 11/45. You can use normal Unibus memory cards
on that machine, although you need to put them in an MUD slot in an
expansion backplane, not in the memory slots on the CPU backplane. You
can also use the core-based system units.

An 11/45 configured this way will not give the same performance as one
with the speical high-speed memory, but it certainly will run, and run
correctly.

-tony
Received on Wed Dec 17 2003 - 17:28:59 GMT