Serial

If your Python program needs to communicate with an Arduino or other micro-controllers, you would often need to use the serial protocol. The pySerial module can help with that.

PySerial does not come default with Python, so you'll need to install it.

python -m pip install pyserial

or

python3 -m pip install pyserial

Writing Data

The sample code below prompts the user for an input, and sends it to the serial device. It is meant for a Linux or Mac computer that has a serial device at /dev/ttyUSB0. If your serial device is not at this location or if you're using Windows, the the serial device name will need to be changed.

import serial

ser = serial.Serial('/dev/ttyUSB0', baudrate=9600)
while True:
    msg = input('Type your msg here: ')
    ser.write(msg.encode() + b'\r\n')

import serial : Imports the serial module. Note that while the package is named pySerial, the module is named serial.

ser = serial.Serial('/dev/ttyUSB0', baudrate=9600) : Create a new serial object using device /dev/ttyUSB0 (...see the section on serial port to learn how to determine the correct device name). Baudrate is set to 9600 in this example; you need to make sure this matches the baudrate for the connected device (eg. Arduino). Some devices may require additional settings; read the PySerial Documentations to learn how to set them.

msg = input('Type your msg here: ') : This reads a string from the user.

ser.write(msg.encode() + b'\r\n') : This sends the user's message to the serial device. Since serial can only send bytes object, we'll need to encode the string msg into a bytes object using encode(). See the next line for explanation of the b'\r\n'.

b'\r\n' : This are the Carriage Return (\r) and Line Feed (\n) characters. The b at the front indicates that this is a bytes object and not a string. These characters are added to the end of your message to mark the end of the line. Depending on your device, it may expect just a carriage return, just a line feed, or both.

Reading Data

This sample code reads from the serial device, one line at a time, and prints it to screen.

Before reading this code, be sure to read the section on Writing Data first, as some of the code are identical and won't be explained again here.

import serial

ser = serial.Serial('/dev/ttyUSB0', baudrate=9600)
while True:
    data = ser.read_until() # Blocks!
    print(data.decode(), end='')

data = ser.read_until() : Read until a (newline) character is found, then return everything. Basically, this allows you to read one line at a time from the serial port. This is a blocking function, which means that the next line of code will not run until serial receives the newline character.

print(data.decode(), end='') : This prints the data that was received. Note that the data is a bytes object, and not a string. To print it properly, we'll need to decode it into a string first. The end='' indicates to the print function that it should not add a newline to the end when printing (...as the serial data already contains a newline).

Serial Port

To use the serial module, you'll need to know which serial port your device is connected to. There are a few ways to do this...

Linux only

Plug in your serial device (...if it's already plugged in, unplug it then plug it in again).
In a terminal, run sudo dmesg. Key in your password if requested.
Look in the last few lines for a message containing...
- ttyACM0: Your device is /dev/ttyACM0
- ttyUSB2: Your device is /dev/ttyUSB2
- Note that the actual message is longer than the above, and the exact message may vary.

Linux, Mac, Windows

Unplug your serial device.
Start up the Arduino IDE software.
Click on "Tools -> Port", and note down the devices that are listed there.
Plug in your serial device.
In the Arduino software, click on "Tools -> Port" again and note down which is the new device that appeared. That is your serial device.