How can I communicate over USB in ROS 2?

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You can make your own node in ROS 2 either in python or CPP,

I have worked with Python, below is the code for that, you can use library serial in python, and you need to convert it to ros2 code based on your requirements.

import serial

ser = serial.Serial("/dev/ttyUSB0", baudrate=9600, parity=serial.PARITY_NONE, bytesize=serial.EIGHTBITS,
                            stopbits=serial.STOPBITS_ONE, timeout=10000)
if ser.is_open == True:
            return True
        else:
            return False

while (ser.is_open == True):
            line = ser.read_until(b'#')

In case of any ques, feel free to drop commet.

If your robot is running off a small microcontroller and you want to communicate over ROS2 via serial you can utilize micro-ROS as found here: https://micro.ros.org/

In ROS1 there is also the rosserial package, which encompasses the popular rosserial_arduino package that can also do the same. They have a tutorials page on the ROS1 wiki here

Again this would mainly apply if your robot is running off a microcontroller, which is the most common use case as to why you would run serial operations. If you are instead using a small computer (not q microcontroller) outfitted with Ethernet or Wi-Fi, this would be a more preferred way to communicate over ROS.