Serial protocol design in Raspberry Pi - Mini Project: Build & Apply

Serial Protocol Design on Raspberry Pi

📖 Scenario: You are working on a Raspberry Pi project that communicates with a sensor using a serial connection. The sensor sends data as a string of comma-separated values representing temperature, humidity, and pressure.Your task is to design a simple serial protocol in Python to read this data, parse it, and display the values clearly.

🎯 Goal: Build a Python program on Raspberry Pi that reads a serial data string from a sensor, parses the temperature, humidity, and pressure values, and prints them in a user-friendly format.

📋 What You'll Learn

Create a variable to simulate the serial data string exactly as '25.3,40.2,1013.1'

Create a variable called delimiter set to ','

Use the split() method with delimiter to separate the data into a list

Print the temperature, humidity, and pressure values with labels

💡 Why This Matters

🌍 Real World

Serial communication is common in electronics projects where Raspberry Pi talks to sensors or other devices. Parsing data correctly is essential to use sensor readings.

💼 Career

Understanding serial protocols and data parsing is important for embedded systems developers, IoT engineers, and anyone working with hardware interfaces.

1

DATA SETUP: Create the serial data string

Create a variable called serial_data and set it to the string '25.3,40.2,1013.1' to simulate the sensor data.

Raspberry Pi

# Create the serial_data string with exact value
# Your code here

Hint

Use quotes to create a string exactly as shown.

2

CONFIGURATION: Define the delimiter

Create a variable called delimiter and set it to the string ',' to separate the sensor values.

Raspberry Pi

serial_data = '25.3,40.2,1013.1'
# Create the delimiter variable with ','
# Your code here

Hint

The delimiter is a comma inside quotes.

3

CORE LOGIC: Parse the serial data string

Use the split() method on serial_data with delimiter to create a list called values containing the separate sensor readings.

Raspberry Pi

serial_data = '25.3,40.2,1013.1'
delimiter = ','
# Split serial_data by delimiter into values list
# Your code here

Hint

Use split() with the delimiter variable.

4

OUTPUT: Display the parsed sensor values

Print the temperature, humidity, and pressure values from the values list with labels exactly as:
Temperature: 25.3 C
Humidity: 40.2 %
Pressure: 1013.1 hPa

Raspberry Pi

serial_data = '25.3,40.2,1013.1'
delimiter = ','
values = serial_data.split(delimiter)
# Print the sensor values with labels
# Your code here

Hint

Use f-strings and index the values list to print each value with its unit.

Practice

1. What is the main purpose of using start and end bytes in a serial protocol?

easy

A. To increase the speed of data transfer

B. To mark the beginning and end of a message

C. To encrypt the message data

D. To compress the message content

Solution

1. Step 1: Understand message framing

   Start and end bytes are special markers that show where a message begins and ends in a data stream.

2. Step 2: Identify their role in communication

   They help the receiver know exactly which bytes belong to one message, avoiding confusion with other data.

3. Final Answer:

   To mark the beginning and end of a message -> Option B

4. Quick Check:

   Start/end bytes = message boundaries [OK]

Hint: Start/end bytes frame messages clearly [OK]

Common Mistakes:

• Thinking start/end bytes encrypt data
• Confusing start/end bytes with speed control
• Assuming they compress data

2. Which of the following is the correct way to represent a start byte in a serial protocol in Python code?

easy

A. start_byte = 02

B. start_byte = '0x02'

C. start_byte = b'\x02'

D. start_byte = "\x02"

Solution

1. Step 1: Understand byte representation in Python

   Bytes are represented with a b prefix and escape sequences like \x02 inside single or double quotes.

2. Step 2: Check each option

   start_byte = b'\x02' uses b'\x02' which is a bytes object with hex value 0x02. start_byte = '0x02' is a string, not bytes. start_byte = 02 is an integer. start_byte = "\x02" is a string, not bytes.

3. Final Answer:

   start_byte = b'\x02' -> Option C

4. Quick Check:

   Bytes need b prefix and hex escape [OK]

Hint: Bytes start with b and use \x for hex [OK]

Common Mistakes:

• Using strings instead of bytes
• Missing the b prefix
• Using decimal numbers instead of bytes

3. Given this Python snippet for a simple serial message:

message = b'\x02HELLO\x03'
start_byte = b'\x02'
end_byte = b'\x03'
if message.startswith(start_byte) and message.endswith(end_byte):
    content = message[1:-1]
else:
    content = b''
print(content)

What will be printed?

medium

A. b'HELLO'

B. b'\x02HELLO\x03'

C. b''

D. HELLO

Solution

1. Step 1: Check message start and end bytes

   The message starts with b'\x02' and ends with b'\x03', so the condition is True.

2. Step 2: Extract content between start and end bytes

   content = message[1:-1] removes the first and last byte, leaving b'HELLO'.

3. Final Answer:

   b'HELLO' -> Option A

4. Quick Check:

   Strip start/end bytes = b'HELLO' [OK]

Hint: Slice message to remove start/end bytes [OK]

Common Mistakes:

• Printing the whole message without slicing
• Forgetting bytes notation in output
• Confusing string and bytes output

4. This code is intended to check if a serial message has a valid checksum byte at the end:

def check_checksum(msg):
    data = msg[:-1]
    checksum = msg[-1]
    calculated = sum(data) & 0xFF
    return checksum == calculated

message = b'\x02ABC\x06'
print(check_checksum(message))

What is the error in this code?

medium

A. It includes the start byte in checksum calculation

B. It does not convert checksum to integer

C. It uses bitwise AND instead of modulo

D. It compares checksum with data instead of sum

Solution

1. Step 1: Analyze checksum calculation

   The code sums all bytes except the last one, but this includes the start byte (b'\x02'), which usually should be excluded.

2. Step 2: Identify correct checksum range

   Typically, checksum is calculated only on the data bytes, excluding start and end bytes. Here, start byte is wrongly included.

3. Final Answer:

   It includes the start byte in checksum calculation -> Option A

4. Quick Check:

   Exclude start byte from checksum [OK]

Hint: Exclude framing bytes from checksum sum [OK]

Common Mistakes:

• Including start/end bytes in checksum
• Confusing bitwise AND with modulo
• Not converting bytes to int before sum

5. You want to design a serial protocol message with this format: start byte (0x7E), payload (ASCII text), checksum (sum of payload bytes mod 256), and end byte (0x7F). Which Python code correctly builds this message for payload = b'HELLO'?

hard

A. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = sum(payload) & 0xFF
message = start + payload + checksum + end

B. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = sum(payload) % 256
message = start + payload + checksum + end

C. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = bytes([sum(payload)])
message = start + payload + checksum + end

D. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = bytes([sum(payload) % 256])
message = start + payload + checksum + end

Solution

1. Step 1: Calculate checksum correctly

   Checksum is sum of payload bytes modulo 256, so sum(payload) % 256 is correct.

2. Step 2: Convert checksum to bytes and build message

   Checksum must be a single byte, so bytes([value]) converts it properly. Then concatenate start, payload, checksum, and end bytes.

3. Final Answer:

   payload = b'HELLO' start = b'\x7E' end = b'\x7F' checksum = bytes([sum(payload) % 256]) message = start + payload + checksum + end -> Option D

4. Quick Check:

   Checksum as bytes and modulo 256 [OK]

Hint: Use bytes([sum(payload) % 256]) for checksum [OK]

Common Mistakes:

• Not converting checksum to bytes
• Using bitwise AND instead of modulo
• Concatenating int with bytes directly