Arduino + Dual H Bridge Motor Driver

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for example - my ORCP protocol for 4WD robot

Not sure if this will help you, I'm running pretty much the same setup, using a pair of motors with an Adafruit dual H-bridge motor controller. While I have not yet built that part of the code, I have been able to get a sonar to talk to my Arduino Mega (duh, that's the easy part) and then was able to get the Arduino to publish the sensor data to a ROS topic. I have gone through the tutorials here and had no trouble writing a functioning publisher and subscriber. Based on that, I'm planning on just running the Arduino as a node that will receive motor commands from ROS, and send sensor and odometry data to ROS. Pretty straight forward in theory, the execution at the ROS level has been a little tricky thus far, but at the Arduino no problem at all as long as you are using the proper data types (otherwise the buffer gets filled and rosserial shuts down the connection)