Modbus RTU Communication Project

A complete Modbus RTU communication project implemented from scratch using an Arduino Mega 2560 (Master) and an ESP32 (Slave) over RS485 (MAX485).

This project focuses on understanding the Modbus RTU protocol at the frame level rather than relying on external Modbus libraries. Every Modbus frame, CRC calculation, UART transmission, and response parsing is implemented manually.

---

Project Structure

Modbus_Project/
│
├── README.md
├── .gitignore
│
├── 01_Modbus_RTU/
│   ├── 01_Modbus_RTU.ino
│   ├── uart.cpp
│   ├── uart.h
│   ├── rs485.cpp
│   ├── rs485.h
│   ├── modbus.cpp
│   ├── modbus.h
│   ├── crc16.cpp
│   ├── crc16.h
│   └── README.md
│
├── 01_Modbus_RTU_Slave/
│   ├── 01_Modbus_RTU_Slave.ino
│   ├── uart.cpp
│   ├── uart.h
│   ├── rs485.cpp
│   ├── rs485.h
│   ├── slave.cpp
│   ├── slave.h
│   ├── crc16.cpp
│   ├── crc16.h
│   └── README.md
│
├── captures/
│   ├── rs485_uart_verification.sr
│   ├── rs485_uart_verification.pvs
│   ├── modbus_rtu_frame_verification.sr
│   ├── modbus_rtu_frame_verification.pvs
│   ├── modbusRTU_request_response_verification.sr
│   └── modbusRTU_request_response_verification.pvs
│
└── images/
    ├── modbus_rtu_complete_hardware_setup.png
    ├── modbus_rtu_request_logic_analyzer.png
    ├── modbusRTU_request_response_verification.png
    └── rs485_uart_waveform_decoded.png

---

Hardware Used

• Arduino Mega 2560 (Master)
• ESP32 DevKit V1 (Slave)
• 2 × MAX485 RS485 Modules
• USB Logic Analyzer (24 MHz)
• PulseView (Sigrok)
• Breadboard & Jumper Wires

---

Hardware Setup

The hardware consists of:

• Arduino Mega acting as the Modbus Master
• ESP32 acting as the Modbus Slave
• Two MAX485 transceivers
• USB Logic Analyzer connected for UART frame verification

---

Communication Flow

Arduino Mega
      │
      │ UART1
      ▼
 MAX485 Module
      │
====== RS485 Bus ======
      │
 MAX485 Module
      ▼
ESP32

---

UART Verification

Before implementing Modbus RTU, UART communication through the MAX485 transceivers was verified using a USB Logic Analyzer.

Verified:

• UART timing
• Baud Rate
• Start Bit
• Stop Bit
• Byte Order
• Correct waveform

---

Modbus RTU Request Verification

The master constructs the Modbus RTU request frame manually.

Request sent:

01 03 00 00 00 02 C4 0B

Meaning:

Byte	Description
01	Slave Address
03	Read Holding Registers
00 00	Start Address
00 02	Quantity
C4 0B	CRC16

Logic Analyzer verification:

---

Complete Request / Response Verification

Both request and response were captured and decoded using PulseView.

Master Request

01 03 00 00 00 02 C4 0B

Slave Response

01 03 04 04 D2 16 2E D5 46

Decoded values:

Register 1 = 1234
Register 2 = 5678

---

Features

• Manual Modbus RTU implementation
• Manual CRC16 calculation
• Manual UART frame generation
• Manual frame parsing
• MAX485 direction control
• Request/Response verification
• Logic Analyzer verification
• PulseView captures included

---

Tools Used

• Arduino IDE
• PulseView
• Sigrok
• USB Logic Analyzer
• Git
• GitHub

---

Skills Demonstrated

• Embedded C++
• UART Communication
• RS485 Communication
• Modbus RTU Protocol
• CRC16 Implementation
• Protocol Debugging
• Logic Analyzer Usage
• Embedded System Verification

---

Repository Contents

Folder	Description
01_Modbus_RTU	Arduino Mega Modbus Master
01_Modbus_RTU_Slave	ESP32 Modbus Slave
captures	PulseView capture sessions
images	Project screenshots
README.md	Project overview

---

Future Improvements

• Multiple Slave Devices
• Function Code 06
• Function Code 16
• Exception Responses
• Modbus Poll Compatibility
• Industrial PLC Testing

---

Author

Varun Bhandekar

GitHub:

https://github.com/IMBVK

---

This project was implemented for learning industrial communication protocols by developing the complete Modbus RTU stack from scratch without using third-party Modbus libraries.