SMT PDP-8 (was Re: group buy for homebrew CPUs?)

vrs vrs at
Mon Aug 14 01:35:05 CDT 2006

From: "David Griffith" <dgriffi at>
> On Sun, 13 Aug 2006, vrs wrote:
> > I'd start out by building replacements for the Rxxx and Sxxx modules,
> > scaled down.  (Once a few of those work, you could start in on a
> > for them.)
> I'm guestimating that once these miniaturized Rxxx and Sxxx modules are
> designed, manufacturing them in the quantities required for even a single
> Straight-8 would be very cheap.  Now about the backplane: Is it possible
> to make it with a PCB, or is wire-wrap absolutely required?

I have done layout for a board that implements the wire list for the TC08.
(That's a lot simpler than a straight-8, but might compare with the 8/S.)
It required 4 layers to route -- wasn't even close in 2 layers.  The wire
wrap allowed an essentially unlimited number of wire crossings :-).

That was also with DEC connector block spacing.  If you make the connections
closer together, you worsen the congestion.

So the backplane (which also wants to be physically large) ends up being a
really expensive (and nearly impossible to fix) PCB.  Might be better to use
wire-wrap (or maybe space the connectors way apart and try for 2 layers) for
the prototype :-).

> > That's only a little better than half-scale, though.  One quarter scale
> > would need finer pitched connectors, which would probably be way more
> > expensive.
> It would cost more, but I don't think it would be by very much.  How about
> having three rows of .1" connectors?

That would get messy to connect to the board edge, and I'm not sure you'd
find the receptacles affordable?


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