On Board Unit communication via RS485

I've heard of On Board Units (OBU) can communicate via RS485. My question is, how?

For example, I would like to ask the state of some runtime variables from the OBU (what doors are open?, etc.) Or anot开发者_运维问答her example, I would like to send some data to the OBU (outside temperature).

What protocol must I implement to do these? Are the any sample implementations available somewhere?

Thanks, krisy