[Coco] Multipak redesign/replacement

[Andrew]

Gene,

When I was originally thinking about the idea I proposed regarding a 
"backplane" - I wasn't thinking of it having a case, and it being only a 
kit. For a low run (50 units) - something like that I think could be 
done for the prices you suggested ($60 kit, $120 assembled - and the end 
user would need to supply a case). I seem to recall something like this 
being sold in Rainbow (basically an open-frame MPI, with a cable to the 
CoCo side)...

Hmm - I guess my memory was faulty on the details - but it was called 
the "A-BUS" by ALPHA Products (242-W West Avenue, Darien, CT 06820) - 
from the advertisement on page 21 of the December 1987 Rainbow (and I am 
pretty sure they were advertising before this issue and somewhat afterward).

Each "motherboard" could hold 5 cards, up to five adaptors could be 
connected; they seem to have had their own line of cards for various 
purposes (all look like they were for industrial/robotics control 
purposes). Various adaptors (to connect to the motherboards) were 
available for a variety of different systems, of which the CoCo was one. 
They also mention that the adaptor could work in an MPI - so in theory 
with a CoCo (if I am reading right) - 100 slots were potentially 
possible (power req's aside)...

Anyhow - take a look at that ad if you care; that was what I was 
imagining, except with the cards (and "motherboard") turned 90 degrees 
so that they were all the same "height".

I'm ignorant about the design of the MPI, and I have not seen the 
schematic; I know it is more than just some buffered I/O data/address 
lines from the port ganged together (for one, there's the switch up 
front, plus it needed it's own power supply) - but I have to admit that 
I didn't think it was much more complex (maybe a few extra chips for 
address selection - plus other passives for filtering and such).

I do agree that the initial system for design and debug purposes should 
be kept as simple as possible, to learn from and see what works, etc. I 
also wonder if maybe (with the right design - again, being ignorant of 
the design and schematic of the MPI, this may be impossible) the number 
of slots could be configurable; that is, start with 2 or 3 slots, and 
then if you need more, extra slots could be added. That way, the final 
kit could be customizable to the needs of the end-user, and in the 
future (providing the design files were kept available to the 
community), if more slots were needed and you or whomever couldn't make 
them, they might not be to difficult to DIY. It might also lower the 
cost of the project (?).

Finally - and again being ignorant of the schematic and PCB for an MPI - 
does milling make the most sense (to you)? From what I gather, it seems 
you have the equipment and skill to do it - so that's a "sunk cost" - 
but would other methods make the turn-around faster for revisions (toner 
transfer, UV transfer, or even sending off to an el-cheapo chinese 
boardhouse)?

Maybe not for the very first one (which may be all you are after 
initially) - but for revisions afterward...? I'm not questioning your 
skills or experience here; I'm just trying to gain some understanding 
myself about the process, as I said that I don't have any experience in 
design and development of a PCB. It also may be the size of the PCB 
involved (which I wonder if a smaller but modular design like I 
suggested above might not help in the initial design and debug stage as 
well).

Well - that's enough for now...oh, and I would definitely be willing to 
buy one of these, even if it came to more than $120.00 for a kit. Also, 
I have a few different cartridges that could be tried out (not just rom 
paks - though I have plenty of those - but also others; RS-232, floppy, 
speech-sound, Orchestra-90, even a DS-68B video digitizer somewhere).

-- 
Andrew L. Ayers
Glendale, Arizona
http://www.phoenixgarage.org/