[Coco] hardware port mapping question

[Mike Pepe]

Roger Taylor wrote:
> This relates to a very interesting project I want to attempt if it's 
> possible.
> 
> What I'd like to do is connect 4 CoCo's to each other via ribbon cables 
> between all of the expansion ports.  On each CoCo I'd like a dip switch 
> (or maybe just hard-wired) that sets the CoCo # to 0-3 and let each CoCo 
> see each of the others 8-bit data port in a block of ports starting from 
> some free address.
> 
> An example:
> 
> 65384 CoCo A
> 65385 CoCo B
> 65386 CoCo C
> 65387 CoCo D
> 
> I realize 65384 is reserved for the RS-232 pack, but I'm just using the 
> address as an example.
> 
> Each CoCo would map those addresses and see 8 data lines from the other 
> CoCos.  Can this project be put together with just ribbon cable and a 
> few soldered-in components for each CoCo board?
> 

Essentially what you're making here could be a 4x4 crossbar switch:

  ABCD
A++++
B++++
C++++
D++++

It could be done with a bunch of latches and tri-state buffers. 
Arbitration could be an issue, but it sounds like what you're looking 
for is for each CoCo to have one "out" port that can be read by the 
others. If that's the case, it becomes more possible. If you're looking 
to write values into a common register, it becomes much more complex.

-Mike