---
title: "ModbusRTU"
description: "ESP-32 industrial Modbus-485 implementation for bidirectional communication"
keywords: ["modbus", "rtu", "485", "esp32", "industrial", "communication", "protocol"]
---

## ModbusRTU

**Path**: [`src/modbusRTU.h`](../../src/modbusRTU.h)

**Revision History**: Initial documentation

ModbusRTU is a comprehensive implementation of the Modbus RTU protocol over RS-485 for ESP32 devices. It provides a robust, industrial-grade solution for bidirectional communication between master and slave devices, supporting both master functionality for querying other devices and slave functionality for responding to external queries.

## REQUIREMENTS

- **Hardware**:
  - RS-485 transceiver (e.g., MAX485)
  - RX pin (configured in `pins_arduino.h`)
  - TX pin (configured in `pins_arduino.h`)
  - DE/RE pin for RS-485 direction control

- **Software**:
  - Platform.io with ESP32 support
  - C17 compiler support

## FEATURES

- Supports both Modbus master and slave functionalities
- Implements standard Modbus function codes (3, 4, 6, 16)
- Automatic CRC calculation and validation
- Configurable timeouts and retry mechanisms
- Interrupt-driven communication with hardware buffer
- Thread-safe operation
- Extensible register mapping system
- Support for various data types (uint16_t, float, etc.)

## DEPENDENCIES

- [`ArduinoLog`](https://github.com/thijse/Arduino-Log)
- [`component.h`](../../src/component.h)

```mermaid
graph TD
    ModbusRTU --> Component
    ModbusRTU --> ArduinoLog
```

## BEHAVIOUR

```mermaid
stateDiagram-v2
    [*] --> Idle
    Idle --> Transmitting: Send Request/Response
    Transmitting --> Waiting: Master Mode
    Transmitting --> Idle: Slave Mode
    Waiting --> Processing: Receive Response
    Waiting --> Timeout: No Response
    Processing --> Idle: Success/Error
    Timeout --> Retry: Retries Left
    Timeout --> Idle: Max Retries
    Retry --> Transmitting
```

## TODOS

### PERFORMANCE

- Consider implementing a more efficient buffer management system to reduce memory usage
- Optimize CRC calculation for speed using lookup tables
- Evaluate interrupt priorities to ensure timely processing of incoming data

### SECURITY

- Implement message authentication to prevent unauthorized commands
- Add support for encrypted Modbus communication where security is critical
- Consider implementing access control lists for sensitive register operations

### COMPLIANCE

- Complete full compliance with Modbus RTU specification
- Add support for additional function codes as needed for specific applications
- Ensure timing requirements meet the Modbus specification under all operating conditions

### RECOMMENDATIONS

- Use shielded twisted pair cables for RS-485 communication to maximize reliability
- Implement proper line termination (120Ω) at both ends of the RS-485 bus
- Consider using galvanic isolation for the RS-485 transceiver in noisy environments
- Regularly test communication with various slave devices to ensure compatibility

## EXAMPLE

This example shows how to initialize and mount the ModbusRTU component in master mode:

```cpp
#ifdef PIN_RS485_DE
  modbus = new ModbusRTU(
      this,                  // owner
      SERIAL_RS485,          // serial port (defined in pins_arduino.h)
      PIN_RS485_DE,          // direction control pin
      MODBUS_BAUD_RATE,      // baud rate (typically 9600, 19200, or 115200)
      SERIAL_8N1,            // data format (8 bits, no parity, 1 stop bit)
      1                      // device ID for slave mode
  );
  
  if (modbus)
  {
    components.push_back(modbus);
    Log.infoln(F("ModbusRTU initialized. DE/RE Pin: %d, Baud: %d, ID: %d"),
              PIN_RS485_DE, MODBUS_BAUD_RATE, 1);
  }
  else
  {
    Log.errorln(F("ModbusRTU initialization failed."));
  }
#endif
```

### References