[Coco] Prototype cartridge board?

[Andrew]

 > Don Barber <don at dgb3.net>:
 >
 > As such, i'm wondering if there is any source for a prototyping board
 > for the coco 40 pin cartridge port? What do you hardware hackers use
 > when playing with a design for a new cartridge-based peripheral for
 > the coco?

Don, I don't know what other people use off-hand; but not too long back 
(ok, maybe 6 months ago? Can't remember) I tried my hand at designing an 
edge connector (and ideally expand it out to a full prototyping board) 
for the CoCo, using KiCad.

I had never used the software before, though I had played around with 
both EagleCAD and a few other tools of similar nature. KiCad turned out 
to be fairly easy and intuitive to work with; I was quite impressed.

I took a file I found for the PC ISA edge connector; since the CoCo has 
the same 0.1" center per-pin layout and sizing (back in the day, people 
used to use a cut-down ISA board for prototyping - while there were 
boards specifically for the CoCo for prototyping, they weren't as cheap 
as what was available for the PC).

Taking that file, deleting some parts, copy-pasting the "fingers", 
bringing out some traces to pads...I ended up with something that looked 
reasonable (in the 3D view) but who knows whether it would've worked.

That aside - there is this board design; I've never had one made or 
anything to try it out, but it looks reasonable and workable. Everything 
is there for you to send it off to a cheap board company and have some made.

https://github.com/JayesonLS/TandyCircuitsAndLogic/tree/master/CoCoProtoBoard

If you go this route, and need someone to take some of the extras off 
your hands (for a price, of course!) - let me know. It's one of the 
downsides of this kind of thing. You always end up with extras you may 
or may not use.

But I wouldn't try it using any homebrew methods (that is, something 
like toner transfer or such); even if you end up with something decent 
(and can align both sides perfectly) - you still would have to drill all 
the pads - ugh, no thank you!

Another option - and again, I haven't tried it yet (though I did 
purchase a few of the boards):

If you go on Amazon or AliExpress, there is a particular double-sided 
prototyping PCB (0.1" centers on the individual pads) which has 
"fingers" along the edge that are size about the same as the individual 
hole pads, but slightly shifted to the hole pads; the length of each 
"finger" is just about the same as a standard ISA edge-connector "finger".

I bought some of these boards, with the intention of trying to see if I 
could make one work for prototyping - but that project is still on a 
back burner, so to speak. My idea was to use some wire-wrapping wire to 
solder to the edge fingers, and bring them to a set of 20x2 pads on the 
board, then solder a 20x2 header on and go from there. It's only 
"downside" seems to be that you have to cut away some of the fingers (it 
has 27 fingers per side) - but for the price, that didn't seem that 
terrible!

https://www.amazon.com/gp/product/B081PHJPR7

This is just an example - there might be other sellers on Amazon, or you 
can find this same board sold by various vendors on AliExpress. Just 
don't go with any other size or layout. I have only found that this 
particular size PCB (80 x 120mm, double sided, etc) has the fingers the 
right length and size. Other boards either have shorter fingers, spacing 
wrong, etc - which won't work.

Oh - and on github while you're at it, you can look up other CoCo 
hardware projects, some of which use the expansion slot; maybe one of 
those board designs can be used as a starting point, if you want to 
learn whatever software EDA CAD they used, or your own, etc...

I hope this helps!

Andrew L. Ayers
Glendale, Arizona
phoenixgarage.org
github.com/andrew-ayers