Serial protocol design in Raspberry Pi

Introduction

Serial protocol design helps devices talk to each other using a simple, step-by-step message system. It makes sure data is sent and understood correctly.

When connecting sensors to a Raspberry Pi using wires.

When sending commands from Raspberry Pi to a motor controller.

When reading data from a GPS module over serial connection.

When debugging communication between two devices using serial ports.

When building a custom communication system between Raspberry Pi and another microcontroller.

Syntax

Raspberry Pi

Start byte | Command byte | Data bytes | Checksum byte | End byte

Each part has a clear role: start and end mark the message limits.

Checksum helps catch errors in the message.

Examples

A message starting with 0x02, command 0x10, data bytes 0x05 and 0xFF, checksum 0x14, and ending with 0x03.

Raspberry Pi

0x02 | 0x10 | 0x05 0xFF | 0x14 | 0x03

Another style using 0x7E as start and 0x7F as end, with one data byte.

Raspberry Pi

0x7E | 0x01 | 0x00 | 0x01 | 0x7F

Sample Program

This program creates a simple serial message with a start byte, command, data, checksum, and end byte. It sends the message over the serial port and reads a response.

Raspberry Pi

import serial

# Open serial port
ser = serial.Serial('/dev/ttyS0', 9600, timeout=1)

# Function to calculate simple checksum
# Sum of command and data bytes modulo 256

def calculate_checksum(command, data_bytes):
    return (command + sum(data_bytes)) % 256

# Build a message
start_byte = 0x02
command = 0x10
data = [0x05, 0xFF]
checksum = calculate_checksum(command, data)
end_byte = 0x03

message = bytes([start_byte, command] + data + [checksum, end_byte])

# Send message
ser.write(message)

# Read response (for example)
response = ser.read(10)
print('Sent message:', message)
print('Received:', response)

ser.close()

Important Notes

Always pick unique start and end bytes that won't appear in data.

Checksum helps detect errors but is simple here; more complex methods exist.

Timeouts prevent your program from waiting forever for data.

Summary

Serial protocol design organizes data into clear parts for reliable communication.

Start and end bytes mark message boundaries.

Checksum helps catch mistakes during data transfer.