pyserial library usage in Raspberry Pi - Mini Project: Build & Apply

Reading Data from a Serial Port using pyserial

📖 Scenario: You have a Raspberry Pi connected to a sensor device that sends data through a serial port. You want to read this data using Python.

🎯 Goal: Build a Python program that opens a serial port, reads data from it, and prints the received messages.

📋 What You'll Learn

Use the pyserial library to open and read from the serial port.

Set the serial port to /dev/ttyUSB0 with baud rate 9600.

Read one line of data from the serial port.

Print the received data as a string.

💡 Why This Matters

🌍 Real World

Reading sensor data or communicating with microcontrollers via serial ports is common in Raspberry Pi projects.

💼 Career

Understanding serial communication is useful for embedded systems, IoT development, and hardware interfacing roles.

1

Setup the serial port connection

Import the serial module and create a serial.Serial object called ser with port '/dev/ttyUSB0' and baud rate 9600.

Raspberry Pi

# Import serial and create serial.Serial object ser
# Your code here

Hint

Use import serial to bring in the library. Then create ser = serial.Serial('/dev/ttyUSB0', 9600) to open the port.

2

Configure a timeout for reading

Set the timeout property of the ser object to 1 second to avoid waiting forever when reading.

Raspberry Pi

import serial
ser = serial.Serial('/dev/ttyUSB0', 9600)
# Set ser.timeout to 1
# Your code here

Hint

Set ser.timeout = 1 to make read operations wait at most 1 second.

3

Read one line of data from the serial port

Use ser.readline() to read one line of bytes from the serial port and decode it to a string called line.

Raspberry Pi

import serial
ser = serial.Serial('/dev/ttyUSB0', 9600)
ser.timeout = 1
# Read one line from ser and decode to string line
# Your code here

Hint

Use ser.readline() to get bytes, then .decode('utf-8') to convert to text, and .strip() to remove newline characters.

4

Print the received data

Print the string variable line to display the data read from the serial port.

Raspberry Pi

import serial
ser = serial.Serial('/dev/ttyUSB0', 9600)
ser.timeout = 1
line = ser.readline().decode('utf-8').strip()
# Print the line variable
# Your code here

Hint

Use print(line) to show the received message.

Practice

1. What is the main purpose of the pyserial library when used with Raspberry Pi?

easy

A. To communicate with devices over serial ports

B. To create graphical user interfaces

C. To manage Wi-Fi connections

D. To control GPIO pins directly

Solution

1. Step 1: Understand pyserial's role

   pyserial is designed to handle serial communication, which means sending and receiving data over serial ports.

2. Step 2: Compare options with pyserial's function

   Options B, C, and D relate to GUI, Wi-Fi, and GPIO control, which are not pyserial's purpose.

3. Final Answer:

   To communicate with devices over serial ports -> Option A

4. Quick Check:

   pyserial = serial communication [OK]

Hint: pyserial = serial port communication library [OK]

Common Mistakes:

• Confusing pyserial with GPIO control libraries
• Thinking pyserial manages network connections
• Assuming pyserial creates user interfaces

2. Which of the following is the correct way to open a serial port using pyserial in Python?

easy

A. serial.Open(port='/dev/ttyUSB0', speed=9600)

B. serial.connect('/dev/ttyUSB0', 9600)

C. serial.Serial(port='/dev/ttyUSB0', baudrate=9600, timeout=1)

D. serial.start(port='/dev/ttyUSB0', baud=9600)

Solution

1. Step 1: Recall pyserial syntax for opening ports

   The correct method is to create a Serial object with parameters: port, baudrate, and optionally timeout.

2. Step 2: Check each option's method and parameters

   Only serial.Serial(port='/dev/ttyUSB0', baudrate=9600, timeout=1) uses serial.Serial() with correct parameter names and values.

3. Final Answer:

   serial.Serial(port='/dev/ttyUSB0', baudrate=9600, timeout=1) -> Option C

4. Quick Check:

   Use serial.Serial() with port and baudrate [OK]

Hint: Use serial.Serial() with port and baudrate [OK]

Common Mistakes:

• Using incorrect method names like Open or connect
• Using wrong parameter names like speed or baud
• Omitting required parameters

3. What will be the output of this code snippet if the serial device sends the bytes b'OK'?

import serial
ser = serial.Serial('/dev/ttyUSB0', 9600, timeout=1)
response = ser.read(2)
print(response)
ser.close()

medium

A. b'OK'

B. 'OK'

C. OK

D. b"\x00\x00"

Solution

1. Step 1: Understand ser.read(2) behavior

   The read(2) method reads 2 bytes from the serial port and returns them as a bytes object.

2. Step 2: Interpret the print output

   Printing a bytes object shows it with a leading b and quotes, so b'OK' will be printed.

3. Final Answer:

   b'OK' -> Option A

4. Quick Check:

   ser.read() returns bytes, printed as b'...' [OK]

Hint: ser.read() returns bytes, print shows b'...' [OK]

Common Mistakes:

• Expecting a string instead of bytes
• Confusing print output with raw string
• Assuming null bytes if device sends data

4. Identify the error in this pyserial code snippet:

import serial
ser = serial.Serial('/dev/ttyUSB0', 9600)
ser.write('Hello')
ser.close()

medium

A. The close method should be called before write

B. The Serial constructor is missing the timeout parameter

C. The port name '/dev/ttyUSB0' is invalid

D. The write method requires bytes, not a string

Solution

1. Step 1: Check the write method argument type

   pyserial's write() expects bytes, but 'Hello' is a string, causing a TypeError.

2. Step 2: Verify other parts of the code

   Timeout is optional, port name can be valid, and close() should be after write().

3. Final Answer:

   The write method requires bytes, not a string -> Option D

4. Quick Check:

   write() needs bytes, encode strings first [OK]

Hint: Always encode strings to bytes before write() [OK]

Common Mistakes:

• Passing string directly to write() without encoding
• Thinking timeout is mandatory
• Closing port before writing data

5. You want to read lines from a serial device that sends text ending with newline characters. Which pyserial method and parameters should you use to read a full line safely with a 2-second timeout?

hard

A. Use serial.Serial('/dev/ttyUSB0', 9600, timeout=None) and ser.readline()

B. Use serial.Serial('/dev/ttyUSB0', 9600, timeout=2) and ser.readline()

C. Use serial.Serial('/dev/ttyUSB0', 9600) and ser.read(100)

D. Use serial.Serial('/dev/ttyUSB0', 9600, timeout=0) and ser.readline()

Solution

1. Step 1: Set timeout for safe reading

   Setting timeout=2 seconds ensures readline() waits up to 2 seconds for a line, avoiding blocking forever.

2. Step 2: Use readline() to read full lines

   readline() reads until newline character, perfect for line-based text from serial devices.

3. Final Answer:

   Use serial.Serial('/dev/ttyUSB0', 9600, timeout=2) and ser.readline() -> Option B

4. Quick Check:

   timeout + readline() = safe line read [OK]

Hint: Use timeout with readline() to avoid blocking [OK]

Common Mistakes:

• Using read() instead of readline() for lines
• Setting timeout=None causes blocking
• Setting timeout=0 causes non-blocking read with no wait