rosserial and motor controllers

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You can always use standard serial I/O for communicating with serial devices from a ROS node; ROS serial is a mechanism through which you can program any two systems that can communicate via serial to use ROS' API instead of bothering to deal with low level serial port I/O.

Specifically, in your case your motor controller will most probably come with a SDK of sorts that deals with sending motor commands over a serial port. You would simply use that API within your ROS node on your laptop to communicate with the motor controller.

ROS Serial would be useful if you were to write your motor controller from scratch on say an Arduino, and wanted to easily communicate with ROS devices over the serial port using the ROS Serial API.