Modbus Communication

Classes
struct	SensorData
	Sensor configuration parameters for Modbus discovery. More...

Macros
#define	SLAVE_ID   1
	Modbus slave device address.
#define	NUM_REGISTERS   18
	Total number of Modbus registers available for RMS data.

Functions
HardwareSerial	ModbusSerial (1)
	Serial port instance for Modbus RS485 communication.
ModbusServerRTU	MBserver (2000)
	Modbus RTU server with 2000ms timeout.
void	dataUpdateTask (void *pvParameters)
	Modbus register update task with per-channel conversion factors.
ModbusMessage	readHoldingRegistersWorker (ModbusMessage request)
	Worker to handle Modbus holding register read requests.

Variables
uint16_t	holdingRegisters [NUM_REGISTERS]
	Modbus registers for RMS data.
SemaphoreHandle_t	dataMutex
	Mutex for Modbus register protection.
TaskHandle_t	dataUpdateTaskHandle
	Handle for Modbus data update task.
SensorData	sensor = {1, 3, 10, NUM_REGISTERS, PROCESS_INTERVAL_MS, 1, 1, 0}
	Sensor configuration for discovery responses.
const int	MODBUS_UPDATE_INTERVAL_MS = 300
	Modbus register update interval in milliseconds.

Detailed Description

Overview

This module implements the Modbus RTU server (ESP32) and the complete flow of:

• Response to sensor discovery requests from RS485 masters.
• Holding register management for RMS data and configuration parameters.
• Periodic data updates from the RMS processing subsystem.
• Error handling and address validation in the Modbus protocol.

Structure and Components

• Data and configuration
  • SensorData: sensor parameter definition for discovery.
  • holdingRegisters[]: Modbus register array with RMS data.
  • sensor: global instance with device configuration.
• Communication and protocol
  • ModbusServerRTU: RTU server with configurable timeout.
  • HardwareSerial: RS485 serial port for physical communication.
  • readHoldingRegistersWorker(): main worker for function 03.
• Main tasks
  • dataUpdateTask: periodic register update from RMS history.
  • Callbacks: registered worker for READ_HOLD_REGISTER.

Main API

• Configuration: SLAVE_ID, NUM_REGISTERS, MODBUS_UPDATE_INTERVAL_MS.
• Data: SensorData, holdingRegisters[], sensor.
• Communication: ModbusSerial, MBserver, dataMutex.
• Functions: dataUpdateTask(), readHoldingRegistersWorker().

Usage

Flow summary:

1) Modbus initialization

• RS485 UART configuration (19200 bps, 8N1) on pins RX=20, TX=21.
• ModbusServerRTU initialization with SLAVE_ID=1 and timeout=2000ms.
• Worker registration for function 03 (holding register read).
• Creation of dataMutex for register protection.

2) Sensor discovery

• readHoldingRegistersWorker() responds to requests for addresses 0-7.
• Returns SensorData structure with: sensorID=1, numberOfChannels=3, startAddress=10.
• Includes parameters: maxRegisters=18, samplingInterval=1000ms, dataType=1, scale=1.
• Enables automatic identification by Modbus masters.

3) Data update

• dataUpdateTask runs every 300ms on core 0 with priority 1.
• Extracts RMS history using get_rms_history() for each channel.
• Applies CONVERSION_FACTOR=0.618 to convert to physical units.
• Updates holdingRegisters[] thread-safely with dataMutex.

4) Data read response

• readHoldingRegistersWorker() handles requests for addresses 10+.
• Validates address range and number of requested registers.
• Protects access to holdingRegisters[] with dataMutex (100ms timeout).
• Returns RMS data organized by channel or ILLEGAL_DATA_ADDRESS error.

5) Modbus error handling

• Validation of out-of-range addresses (>= NUM_REGISTERS).
• SERVER_DEVICE_BUSY response if dataMutex cannot be obtained.
• Debug logs for received ServerID and FunctionCode.

Warnings and best practices:

• Ensure consistent layout of 8 parameter registers for discovery.
• Respect data offset in Modbus RTU registers according to specification.
• Protect access to holdingRegisters[] with dataMutex in all operations.
• Verify NUM_REGISTERS limits to prevent buffer overflow.
• Maintain consistency between numberOfChannels and data organization in registers.
• Configure appropriate server timeout according to expected RS485 bus latency.

Function Documentation

dataUpdateTask()

void dataUpdateTask

(

void *

pvParameters

)

Modbus register update task with per-channel conversion factors.

Periodically updates Modbus registers with latest RMS values from all channels. Dynamically organizes data according to configured number of channels. Applies channel-specific conversion factors from CONVERSION_FACTORS array.

Parameters

pvParameters

Task parameters (unused).

Application of channel-specific conversion factor.

Uses CONVERSION_FACTORS[ch] instead of global factor to enable individual channel calibration.

readHoldingRegistersWorker()

ModbusMessage readHoldingRegistersWorker

(

ModbusMessage

request

)

Worker to handle Modbus holding register read requests.

Responds to two types of queries:

• Registers 0-7: Sensor configuration parameters
• Registers 10+: Historical RMS data Implements address boundary validation to prevent invalid access.

  Parameters

  request

  Received Modbus request message.

  Returns

  ModbusMessage Formatted Modbus response.