How connect two distinct hardware(robots/sensors)

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For the Rapiro and other similar arduino compatible robots, the best way to do and learn about sending messages is check out the rosserial libraries available. For example in my project, i am able to connect Arduino and raspberry pi by using the package http://wiki.ros.org/rosserial_arduino . That package has all the essential communication knowledge and tutorials available for testing to send messages form your PC directly to your arduino and so much more. Regarding multiple connections of robots distributed across a wide platform. The best available link is http://wiki.ros.org/Robots/TurtleBot/... . As it shows you how to set-up a host computer and master in your setup to communicate with the ROS. It's also essential to note that there are other serial libraries bundled with ROS like for python and so on.