SMT PDP-8 (was Re: group buy for homebrew CPUs?)
vrs at msn.com
Sun Aug 13 23:45:10 CDT 2006
From: "Ethan Dicks" <ethan.dicks at gmail.com>
> > That has been discussed before. It would be fun to do something like the
> > PDP8-S on a single PCB with only resistors, capacitors, SOT-23
> > dual diodes (And yes LEDs in all the FF outputs would be nice).
> I'd build one, especially if it was Negibus-compatible. I could
> attach a real DF-32 to it. ;-)
That would be quite a conversation piece :-)!
Naturally, it should be Negibus compatible!
> Has anyone estimated the physical size? A real PDP-8/S has about 1000
> transistors and a lot more passives than that, not counting external
> things like a TTY interface (besides serializing the ALU, that's one
> of the ways they shoehorned Straight-8 technology into such a tiny
I'd guess something like 1/4 scale would be acheivable. If you make it
too small, you'll have trouble with the user interface (as you mentioned
about the 1/12th scale IMSAI).
> One could always start small and build something simpler from the same
> technology - a simple clock or something - sort of a test-drive to see
> what the scope of a full processor would entail.
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
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. (Were there double-sided Rxxx or Sxxx modules? I don't
think so, because the connector blocks used to be one sided.) Then you
could make the modules the right shape, and route the "handles" right out
of the PCB material :-). The result would be about 1"x2.25".
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 only a little better than half-scale, though. One quarter scale
need finer pitched connectors, which would probably be way more expensive.
Another goofy idea I had (for inexpensive interconnect) was to try to
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.
More information about the cctalk