Modbus Connector

Overview

The Modbus connector enables communication with PLCs and industrial devices using the Modbus protocol, supporting both serial (RTU) and TCP/IP connections.

Connector Types:

• ModbusReader - Read data from Modbus devices
• ModbusWriter - Write data to Modbus devices

Features

• ✅ Multiple transport protocols (TCP, RTU, UDP, TLS)
• ✅ All register types (Holding, Input, Coil, Discrete Input)
• ✅ Multiple data types (Int16/32/64, UInt16/32/64, Float32/64, ASCII)
• ✅ Configurable byte and word order
• ✅ Unit ID support for multi-device networks

Transport Protocols

Meddle supports the following Modbus transports:

• TCP - Modbus TCP over Ethernet
• TCPWithTLS - Modbus TCP with TLS encryption
• UDP - Modbus over UDP
• RTU - Modbus RTU over serial
• RTUOverTCP - Modbus RTU encapsulated in TCP
• RTUOverUDP - Modbus RTU encapsulated in UDP

Basic Configuration

Modbus TCP Reader

{
  "type": "ModbusReader",
  "config": {
    "endpoint": "192.168.1.100:502",
    "transport": "TCP",
    "pollingRate": 1000,
    "defaultUnitId": 1,
    "defaultBitOrder": "BigEndian",
    "defaultWordOrder": "HighFirst"
  },
  "variables": [
    {
      "key": "temperature",
      "address": 0,
      "registerType": "HoldingRegister",
      "dataType": "Float64"
    },
    {
      "key": "pressure",
      "address": 4,
      "registerType": "HoldingRegister",
      "dataType": "Int32"
    }
  ]
}

Modbus RTU Reader

{
  "type": "ModbusReader",
  "config": {
    "endpoint": "/dev/ttyUSB0",
    "transport": "RTU",
    "pollingRate": 1000,
    "defaultUnitId": 1,
    "baudRate": 9600,
    "dataBits": 8,
    "parity": "None",
    "stopBits": 1
  },
  "variables": [
    {
      "key": "sensor_value",
      "address": 0,
      "registerType": "InputRegister",
      "dataType": "UInt16"
    }
  ]
}

Modbus Writer

{
  "type": "ModbusWriter",
  "config": {
    "endpoint": "192.168.1.100:502",
    "transport": "TCP",
    "defaultUnitId": 1
  },
  "variables": [
    {
      "key": "setpoint",
      "address": 100,
      "registerType": "HoldingRegister",
      "dataType": "Float32"
    },
    {
      "key": "enable",
      "address": 0,
      "registerType": "Coil"
    }
  ]
}

Register Types

Holding Registers (Read/Write)

General-purpose registers that can be read and written.

{
  "key": "temperature",
  "address": 0,
  "registerType": "HoldingRegister",
  "dataType": "Float64"
}

Function Codes: Read (03), Write Single (06), Write Multiple (16)

Input Registers (Read-Only)

Read-only registers typically used for sensor data.

{
  "key": "sensor_reading",
  "address": 0,
  "registerType": "InputRegister",
  "dataType": "Int16"
}

Function Code: Read (04)

Coils (Read/Write Boolean)

Single-bit registers for digital outputs.

{
  "key": "motor_enable",
  "address": 0,
  "registerType": "Coil"
}

Function Codes: Read (01), Write Single (05), Write Multiple (15)

Discrete Inputs (Read-Only Boolean)

Read-only single-bit registers for digital inputs.

{
  "key": "limit_switch",
  "address": 0,
  "registerType": "DiscreteInput"
}

Function Code: Read (02)

Data Types

Integer Types

16-bit

// Signed 16-bit integer (-32768 to 32767)
{"dataType": "Int16"}  // 1 register

// Unsigned 16-bit integer (0 to 65535)
{"dataType": "UInt16"}  // 1 register

32-bit

// Signed 32-bit integer
{"dataType": "Int32"}  // 2 registers

// Unsigned 32-bit integer
{"dataType": "UInt32"}  // 2 registers

64-bit

// Signed 64-bit integer
{"dataType": "Int64"}  // 4 registers

// Unsigned 64-bit integer
{"dataType": "UInt64"}  // 4 registers

Floating Point Types

// 32-bit float (4 bytes)
{"dataType": "Float32"}  // 2 registers

// 64-bit float (8 bytes)
{"dataType": "Float64"}  // 4 registers

String Types

// ASCII character (1 byte per register)
{"dataType": "AsciiChar"}  // 1 register

Byte and Word Order

Modbus devices may use different byte and word ordering. Configure these to match your device:

Bit Order (Byte Order)

{
  "defaultBitOrder": "BigEndian"  // or "LittleEndian"
}

• BigEndian: Most significant byte first (default, most common)
• LittleEndian: Least significant byte first

Word Order

For multi-register values (32-bit, 64-bit):

{
  "defaultWordOrder": "HighFirst"  // or "LowFirst"
}

• HighFirst: High word first (default)
• LowFirst: Low word first

Per-Variable Override

{
  "key": "special_value",
  "address": 10,
  "registerType": "HoldingRegister",
  "dataType": "Float32",
  "bitOrder": "LittleEndian",
  "wordOrder": "LowFirst"
}

RTU Serial Configuration

For Modbus RTU over serial connections:

{
  "endpoint": "/dev/ttyUSB0",
  "transport": "RTU",
  "baudRate": 9600,
  "dataBits": 8,
  "parity": "None",
  "stopBits": 1,
  "timeout": 1000
}

Parameters:

• baudRate: 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200
• dataBits: 7 or 8
• parity: None, Even, Odd
• stopBits: 1 or 2
• timeout: Read timeout in milliseconds

Unit ID (Slave Address)

Modbus supports multiple devices on the same network using Unit IDs:

{
  "defaultUnitId": 1  // Default for all variables
}

Per-variable override:

{
  "key": "device2_temp",
  "address": 0,
  "registerType": "HoldingRegister",
  "dataType": "Float32",
  "unitId": 2  // Override for this variable
}

Common Use Cases

1. Energy Meter Reading

{
  "type": "ModbusReader",
  "config": {
    "endpoint": "192.168.1.50:502",
    "transport": "TCP",
    "pollingRate": 5000,
    "defaultUnitId": 1
  },
  "variables": [
    {
      "key": "voltage",
      "address": 0,
      "registerType": "InputRegister",
      "dataType": "Float32"
    },
    {
      "key": "current",
      "address": 2,
      "registerType": "InputRegister",
      "dataType": "Float32"
    },
    {
      "key": "power",
      "address": 4,
      "registerType": "InputRegister",
      "dataType": "Float32"
    },
    {
      "key": "energy",
      "address": 6,
      "registerType": "InputRegister",
      "dataType": "Float64"
    }
  ]
}

2. PLC Control

{
  "type": "ModbusWriter",
  "config": {
    "endpoint": "192.168.1.100:502",
    "transport": "TCP",
    "defaultUnitId": 1
  },
  "variables": [
    {
      "key": "motor_speed",
      "address": 100,
      "registerType": "HoldingRegister",
      "dataType": "UInt16"
    },
    {
      "key": "motor_start",
      "address": 0,
      "registerType": "Coil"
    },
    {
      "key": "motor_stop",
      "address": 1,
      "registerType": "Coil"
    }
  ]
}

3. Multi-Device Network

{
  "type": "ModbusReader",
  "config": {
    "endpoint": "/dev/ttyUSB0",
    "transport": "RTU",
    "pollingRate": 1000,
    "baudRate": 9600
  },
  "variables": [
    {
      "key": "device1_temp",
      "address": 0,
      "registerType": "HoldingRegister",
      "dataType": "Float32",
      "unitId": 1
    },
    {
      "key": "device2_temp",
      "address": 0,
      "registerType": "HoldingRegister",
      "dataType": "Float32",
      "unitId": 2
    },
    {
      "key": "device3_temp",
      "address": 0,
      "registerType": "HoldingRegister",
      "dataType": "Float32",
      "unitId": 3
    }
  ]
}

Troubleshooting

Connection Timeout

Solutions:

• Verify endpoint address and port
• Check network connectivity
• Increase timeout value
• Verify device is powered and responsive

Invalid Data

Solutions:

• Check byte order (BigEndian vs LittleEndian)
• Verify word order (HighFirst vs LowFirst)
• Confirm data type matches device specification
• Verify register address is correct

RTU Communication Errors

Solutions:

• Check serial cable connections
• Verify baud rate matches device
• Confirm parity and stop bits settings
• Check for proper termination resistors
• Reduce polling rate to avoid bus collisions

Exception Codes

Common Modbus exception codes:

• 01: Illegal Function
• 02: Illegal Data Address
• 03: Illegal Data Value
• 04: Slave Device Failure

Best Practices

1. Start with Read-Only: Test with Input Registers before writing
2. Use Appropriate Polling: Don’t poll faster than device can respond
3. Group Consecutive Registers: More efficient than scattered reads
4. Document Byte Order: Keep notes on device-specific configurations
5. Test Unit IDs: Verify each device responds to its Unit ID

Related Connectors

• OPC UA - Alternative industrial protocol
• Siemens S7 - Direct Siemens PLC communication
• Filter - Filter Modbus data
• SQL Writer - Store Modbus data in databases