How to do an I/O bus?

Other Hardware LEDs and Multiplexing
1

How hard would it be to configure the I/O pins to act as a data/addressing bus to allow multiple shields to be stacked? Similar to ISA architecture; would allow several shields to be plugged together in parallel, and the arduino could address/talk to whichever shield was needed.

(Is this already being done and I'm missing something?)

Thanks!

2

It would be quite easy, but all the shields in question would have to have appropriate hardware and at present none do.

Rob

3

OneWire, I2C and SPI are already busses... (OK SPI needs an enable signal per slave, but the 3 other signals are bussed).