AVR-Chat

I suggest using RS485.

All the devices sit in "parallel" on a single bus. You DO
NOT need additional enable lines. Assign each device an
address and make the address the initial part of the
message. 

The biggest "trick" with '485 is what to do when no device
has electrical control over the line. The usual solution is
to apply a little bit of bias at one point (often the
master) so that the line is always in a known state.

National Semi has one or more VERY GOOD app notes on use of
422/485. Many sources of transceivers (TI, National, Maxim,
Linear Tech, etc).

Jim
---------------------------------------------------------------
The Think Different Store
http://www.thinkdifferentstore.com/
For All Your Mac Gear
---------------------------------------------------------------