how to use rosserial? How to use rosserial_python.

My task is writing a ROS node(python) to send command and receive status code from my mcu(had been programmed). through serial port(/dev/ttyUSB0, 9600)

I noticed that rosserial package is only working for "a connected rosserial-enabled device". Is that true?

How can I finish my task using the package"rosserial"? if can not, what is the most efficient way to achive that?

Thank you very much!!!!!