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

vrs vrs at msn.com
Mon Aug 14 01:23:07 CDT 2006


From: "Ethan Dicks" <ethan.dicks at gmail.com>
> On 8/14/06, vrs <vrs at msn.com> wrote:
> > I'd start out by building replacements for the Rxxx and Sxxx modules,
all
> > scaled down.  (Once a few of those work, you could start in on a
backplane
> > 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
little
> > 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
right
> > angle connector on it, and insert the miniature card being tested.
Replace
> > an original module in some old gear with the result, to see if you've
got
> > things right.   You could also use the technology to resurrect old gear
with
> > 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
might
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.
It
> > 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
work-alike.

> 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
quibbling
about the number of designs on them.  So I'd recommend panelizing about
8"x10"
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.

Agreed.

    Vince




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