Shassis Controller Communication Protocol

1. Overview

This protocol is designed for communication between a ROS2 system running on platforms such as RPI and a chassis controller operating on hardware like BeagleBone Blue or STM32. It supports both TCP and UART, allowing for a seamless future transition between the two communication methods.

2. Protocol Architecture

The protocol consists of two layers:

1. Frame Layer (Layer 1) – Ensures data integrity, required only for UART.
2. Protocol Layer (Layer 2) – Defines the core communication commands.

2.1. Frame Layer (Layer 1) – UART Only

(Not used in TCP mode)
The frame layer is only applicable when using UART, as UART does not guarantee message integrity or completeness.

Frame Format (UART Only)

Offset	Size (bytes)	Field	Description
0	1	frame_length	Total packet length (including CRC16). Ensures packet boundary detection over UART
1	N	payload	Protocol Layer (Layer 2) data
N+1	2	crc16	CRC16 checksum. Provides error detection for unreliable UART communication

2.2 Protocol Layer (Layer 2) – Used in TCP & UART

This is the core protocol used in both TCP and UART modes.

Protocol Format

Offset	Size (bytes)	Field	Description
0	1	command_id	Unique command identifier
1	N	payload	Command-specific data

3. Command Set

Command ID	Command Name	Description
0x01	get_api_version	Retrieves the API version of the system
0x02	set_motor_speed	Sets the speed of a motor
0x03	set_all_motors_speed	Sets the speed for all four motors in a single command
0x04	get_encoder	Retrieves the encoder value for a specific motor
0x05	get_all_encoders	Retrieves the encoder values for all motors

get_api_version (0x01)

Retrieves the firmware/API version.

Request

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x01
1	1	ROS2 Driver Version	1-255

Response

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x01
1	1	API Version	1-255

set_motor_speed (0x02)

Sets the speed of a specific motor.

Request

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x02
1	1	motor_id	Motor ID (0-3)
2	4	speed	Speed in RPM multiplied by 100 (signed value, negative value means reverse direction)

Response

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x02

set_all_motors_speed (0x03)

Sets the speed for all four motors in a single command.

Request

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x03
1	4	speed_motor_1	Speed in RPM multiplied by 100 (signed value, negative value means reverse direction)
5	4	speed_motor_2	Speed in RPM multiplied by 100 (signed value, negative value means reverse direction)
9	4	speed_motor_3	Speed in RPM multiplied by 100 (signed value, negative value means reverse direction)
13	4	speed_motor_4	Speed in RPM multiplied by 100 (signed value, negative value means reverse direction)

Response

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x03

get_encoder (0x04)

Retrieves the encoder value for a specific motor.

Request

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x04
1	1	motor_id	Motor ID (0-3)

Response

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x04
1	4	encoder_value	Encoder value (signed value, negative value means reverse direction)

get_all_encoders (0x05)

Retrieves the encoder values for all motors.

Request

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x05

Response

Offset	Size (bytes)	Field Description	Values
0	1	command_id	0x05
1	4	encoder_value_motor_1	Encoder value (signed value, negative value means reverse direction)
5	4	encoder_value_motor_2	Encoder value (signed value, negative value means reverse direction)
9	4	encoder_value_motor_3	Encoder value (signed value, negative value means reverse direction)
13	4	encoder_value_motor_4	Encoder value (signed value, negative value means reverse direction)