>- A few external passives are allowed, but no additional ICs.
>
>Challenge: How few connections between the PICs can you get
>away with, not counting power and ground?  How few pins?

2 pins and 2 resistors. Look at the protocol that TI uses on the TI-83 and 
related calculators. It is a totally self clocked bi-directional protocol 
that can be done with a pure state machine.

See http://www.ticalc.org/pub/text/calcinfo/tixx_guide.zip for info. 

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