Plug-n-Play for vintage machines?

[Jim Brain]

I'm considering designing some cartridges for the CBM line of computers, 
but I'd prefer to skip masses of jumper blocks and move to a soft-config 
option.

I know, some of you love jumpers, but for carts, it's more important to 
offer flexible options so that the SW can reconfigure carts on load.

I have no issues with adding a small uC to a cart to do the heavy 
lifting, but I'm struggling with a way to communicate with the uC from 
the CBM machine.

In the past, people have added config registers to a IO space, but that 
introduces its own issues (how to hide the registers, what about 
conflicts, etc.)

Ideally, I'd like to use an approach that:

    * works via the existing address/chip select/data lines of the cart
      ports
    * will handle multiple equipped carts on the same port (port expanders)
    * uses as few lines as possible to communicate with the host machine
    * will not affect non-equipped cartridges
    * not require major amount of horsepower from the uC side.

I thought using and SPI/I2C-like approach using an address line or two 
and a data line to communicate with the cartridges.

But, that approach requires something to "unlock" the config system and 
lock it again.

Any alternative ideas?  Is there any prior work in this area I should 
re-use?

-- 
Jim Brain, Brain Innovations                                      (X)
brain at jbrain.com 
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times! 
Home: http://www.jbrain.com