I'm doing some experiments with an Arduino Mega and the OpenROV software. What pins do I need to I need to hook up on the Beaglebone and Arduino (Mega) in order to get them to communicate via USART just like the OpenROV cape would?
do you have a cape? or just the BB and the Mega?
the BB has 3.3V signaling so you will need a 3.3 to 5v level converter between the BB and the mega. this is available on the OpenRov cape. you can also get this cheap on ebay, sparkfun or similar places.
you can see the level converter and connections on page 2 and 4 in this document, https://github.com/OpenROV/openrov-electronics/blob/master/BeagleBone%20OpenROV%20Cape%20Rev%20A1.PDF here you will also find how the pins are connected to the arduino uno atmega328.