Go to websiteGet a demo

Modbus

The Modbus class provides a unified interface for communicating with Modbus devices. It supports the two most common communication methods:

  • Modbus TCP: For devices connected over an Ethernet network.

  • Modbus RTU: For devices connected over a serial line (e.g., RS-485 or RS-232).

You must create an instance of this class and then call one of the connect... methods to establish a session before performing any read or write operations.

Connection Management

You must use one of the following methods to connect to your device.

connectTcp

Establishes a Modbus TCP connection to a device over a network.

Parameters

  • options: An object containing the network details.

    • host: The hostname or IP address of the Modbus device.

    • port: The TCP port of the device. Defaults to 502 or 1502 typically.

    • unitId: The Unit ID of the target device. Defaults to 1.

    • socketTimeout: Connection timeout in milliseconds. Defaults to 5000.

Example

# options
host: 192.168.1.120
port: 502
unitId: 1

disconnectTcp

Closes the active Modbus TCP connection.

connectRtu

Establishes a Modbus RTU connection to a device over a serial port.

Parameters

  • options: An object containing the serial port configuration.

    • path: The system path of the serial port (e.g., COM3 on Windows, /dev/ttyUSB0 on Linux).

    • baudRate: The communication speed. Must match the device setting. Defaults to 9600.

    • dataBits: Can be 5, 6, 7, or 8. Defaults to 8.

    • stopBits: Can be 1 or 2. Defaults to 1.

    • parity: Can be none, even, or odd. Defaults to none.

  • address: The station address (from 1-247) of the device on the serial bus.

Example

# options
path: /dev/ttyUSB0
baudRate: 19200
parity: even
# address
10

disconnectRtu

Closes the active Modbus RTU serial port connection.

Data Read/Write Operations

read

A generic function to read data from the connected Modbus device. It supports standard read function codes and can interpret the resulting data into various types.

Parameters

  • options: An object specifying what to read.

    • fc: The Function Code for the read operation.

      • 1: Read Coils (single bits, read/write).

      • 2: Read Discrete Inputs (single bits, read-only).

      • 3: Read Holding Registers (16-bit words, read/write).

      • 4: Read Input Registers (16-bit words, read-only).

    • address: The starting address of the data point (zero-based).

    • length: The number of items to read (for registers, this is the number of 16-bit words).

    • dataType: An optional string specifying how to interpret the raw buffer data. Supported types include int16BE, uint32BE, floatBE, int16LE, floatLE, etc. BE stands for Big-Endian, LE for Little-Endian.

Example 1: Read a single boolean coil status

# options
fc: 1
address: 100
length: 1
dataType: boolean

Example 2: Read a single 16-bit signed integer

# options
fc: 3
address: 40001
length: 1
dataType: int16BE

Example 3: Read a 32-bit floating point number

Note that a 32-bit value occupies two 16-bit registers, so length must be 2.

# options
fc: 4
address: 30010
length: 2
dataType: floatBE

Output

The value read from the device, converted to the specified dataType.

write

A generic function to write data to the connected Modbus device.

Parameters

  • data: The data to write. This can be a single value, an array of values, or a Buffer.

  • addressInfo: An object specifying where and how to write.

    • fc: The Function Code for the write operation.

      • 5: Write Single Coil (writes true or false).

      • 6: Write Single Register (writes one 16-bit word).

      • 15: Write Multiple Coils.

      • 16: Write Multiple Registers.

    • address: The starting address for the write operation.

Example 1: Write a single coil ON

# data
true
# addressInfo
fc: 5
address: 100

Example 2: Write a single 16-bit integer value

# data
1234
# addressInfo
fc: 6
address: 40001

Example 3: Write an array of two 16-bit values

# data
[100, 200]
# addressInfo
fc: 16
address: 40050

String Helper Functions

These functions simplify reading and writing text strings, which are stored across multiple registers.

readString

A convenience function to read a text string from a sequence of holding registers.

Parameters

  • startAddress: The starting register address of the string.

  • length: The number of 16-bit registers the string occupies.

Example

# startAddress
40100
# lengthy
10

Output

The text string read from the device.

writeString

A convenience function to encode and write a text string to a sequence of holding registers.

Parameters

  • text: The string to be written.

  • startAddress: The starting register address.

Example

# text
New Product ID
# startAddress
40100

Last updated