How Modbus Communication Protocol Works

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How does Modbus Communication Protocol Work?

Overview

Modbus is a widely used communication protocol in industrial automation, allowing devices to communicate with each other. This guide explains the structure and operation of Modbus messages and how Modbus communication works between devices.

Modbus Message Structure

Each Modbus message consists of four basic elements, which follow the same sequence and order in all messages, making it easy to parse the content.

1. Slave Address:

Identifies the device the message is intended for.

2. Function Code:

Specifies the action to be performed (e.g., read or write).

3. Data:

Contains the data to be read or written.

4. Error Checking Field:

Ensures the integrity of the message.

Communication Process

Master/Slave Communication:

In a Modbus network, the master device initiates communication by sending a message to a slave device. The slave then processes the message and responds.

Physical Addressing:

The message header contains the slave's address, ensuring only the intended device responds while others ignore the message.

Modbus Functions and Registers

Modbus functions perform read and write operations on the slave's internal memory registers, which are organized into four basic data types:

1. Discrete Inputs:

Single-bit inputs (e.g., status of a button).

2. Coils Outputs:

Single-bit outputs (e.g., status of a relay).

3. Input Registers:

16-bit read-only data (e.g., sensor readings).

4. Holding Registers:

16-bit read/write data (e.g., control setpoints).

Modbus Protocol Data Unit (PDU)

The PDU consists of:

Function Code:

Indicates the operation to be performed (e.g., read or write).

Data:

Includes the address of the memory register and the number of registers to read/write.

Common Modbus Function Codes

• Function Code 01: Read Coils

• Function Code 02: Read Discrete Inputs

• Function Code 03: Read Holding Registers

• Function Code 04: Read Input Registers

• Function Code 05: Write Single Coil

• Function Code 06: Write Single Register

• Function Code 15: Write Multiple Coils

• Function Code 16: Write Multiple Registers

Error Handling

If an error occurs, the slave device returns a message with an error code. The error code is the original function code with the most significant bit set to 1, along with an error description.