RS485

dmx use RS-485 check out the wiring at the playground:

http://www.arduino.cc/playground/DMX/DMXShield

but this set up there is just for a master talking to a slave,
for communcating you could do something like this:

http://tomekness.files.wordpress.com/2008/06/rs-485_communcation.pdf

you can be a little bit smarter and use just one arduino out pin for the RE and DE.

ha det bra
tomek