How to use rosserial with Arduino hardware serial3?

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Assuming you have rosserial_arduino installed, navigate to the source code:

roscd rosserial_arduino

edit the file ArduinoHardware.h (inside roslib folder)

Search for iostream = &Serial; and change that to iostream = &Serial3;

There is no need for Serial3.begin() in your arduino code. Refer to any of the Examples from ros_lib inside the Arduino IDE and use that for testing first.

Edit: Note that you need to delete the ros_lib folder inside the Arduino libraries and re-run the command (and possibly re-upload the arduino code afterwards)

rosrun rosserial_arduino make_libraries.py .