ModbusRTU

ModbusRTU

Path: 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
• component.h

BEHAVIOUR

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:

#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