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

vrs vrs at
Mon Aug 14 01:23:07 CDT 2006

From: "Ethan Dicks" <ethan.dicks at>
> On 8/14/06, vrs <vrs at> 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.)
> Would it be worth the expense of a backplane?  I would have thought
> that the way to do it would be to lay it out on both sides of single,
> largish PCB with off-board connections for I/O and front panel.

That would be a lot cheaper, but would sacrifice some of the "vintage"
aesthetics :-).

> > I was thinking of a 9x2 header/socket arrangement for the interconnect
> > and soldering the header to pads on each side of the board to make
> > 1" wide modules.
> While the originals R and S modules were 18-pin, I'd think that 20-pin
> headers, etc, would be easier to find in quantity (not that they
> couldn't be trimmed down, of course).

Sure.  One could use keyed 20 pin receptacles, and 18 actual pins.

> > For testing, you just need a standard size module with a female 9x2
> > angle connector on it, and insert the miniature card being tested.
> > an original module in some old gear with the result, to see if you've
> > things right.   You could also use the technology to resurrect old gear
> > missing modules :-).
> That's certainly true, but in some cases, they hand-picked modules to
> get the timing right.

Sure, and depending on how closely one followed the old schematics, that
still be needed :-).

> > Another goofy idea I had (for inexpensive interconnect) was to try to
> > squeeze
> > the SMT modules down into the footprint of, say, an 18 or 20 pin DIP.
> > wouldn't look much like a replica anymore, though.
> I would think the point would be a work-alike replica more than a
> board-for-board form-factor-in-miniature replica, but whatever works.

One would certainly have to pay extra for the look-alike over the

> I would think that the individual PCB
> costs, even for something about 2 sq in., and the interconnect
> hardware would start to make this a rather expensive project.

I know a couple of places that will do panels inexpensively without
about the number of designs on them.  So I'd recommend panelizing about
of the things and produce them 30 at a time or so :-).

> It
> would, of course, be easier to _test_ dozens of individual modules
> than a monolithic CPU PCB (since there wouldn't be any interaction
> issues), but quite a bit pricier.  OTOH, even with a $1000 budget,
> it's cheaper than the going rate for a PDP-8/S these days.



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