PHP and Modbus TCP: Understanding the Modbus protocol and its related parameters

PHP and Modbus TCP: Understanding the Modbus protocol and its related parameters

Abstract:
In industrial control systems, Modbus is a commonly used communication protocol for data interaction between devices . This article will introduce the basic principles and related parameters of the Modbus TCP protocol, and provide PHP code examples to help readers understand and apply the Modbus TCP protocol.

1. Introduction to Modbus TCP protocol

Modbus TCP is a variant of the Modbus protocol based on the TCP/IP protocol, which allows multiple devices to communicate through Ethernet. Modbus TCP uses a simple command/response model, and devices exchange data by reading and writing registers.

The basic principles of the Modbus TCP protocol are as follows:

1. All devices are identified and communicated through IP addresses.
2. Modbus TCP uses default port 502.
3. Request-response model. The master device sends a request command and the slave device returns response data.
4. Supports function codes for reading and writing registers.

2. Modbus TCP parameters

When using the Modbus TCP protocol, you need to understand the following important parameters:

1. IP address: The IP address of the device. for identification and communication. For example, a Modbus server might have an IP address of 192.168.1.100.
2. Port number: Modbus TCP uses the default port number 502. You need to ensure that the port number of the device is set correctly.
3. Function code: Modbus TCP supports a variety of function codes. Commonly used ones include reading holding registers (function code 03) and writing multiple holding registers (function code 16).
4. Register address: Holding register is the basic unit of Modbus TCP communication. Reading and writing data requires specifying the address of the register.

3. PHP code example

Use the PHP library Phpmodbus (https://github.com/r-vnc/phpmodbus) to easily communicate with Modbus TCP devices. Below is a simple example that demonstrates how to read data from a holding register using the Phpmodbus library.

// Reference the Phpmodbus library file
require_once('phpmodbusModbusMaster.php');

// Create a ModbusMaster object and specify the IP address and IP address of the Modbus server Port number
$modbus = new ModbusMaster("192.168.1.100", "TCP");

//Specify the register address and quantity to be read, and read the data through function code 03
$address = 0; // Register address
$quantity = 10; // Register quantity
$data = $modbus->readMultipleRegisters(1, $address, $quantity);

// Output the read data
if(!$data){

echo "读取错误！";

}else{

echo "读取到的数据：

";

foreach($data as $value){
    echo $value . "

";

}

}
?>

In the above example, we created a ModbusMaster object and specified the IP address and port number of the Modbus server. Then, we use the readMultipleRegisters function to read the data in the holding register and print the result.

This is just a simple example. In actual use, more complex operations may be required based on specific needs, such as writing data or using other function codes.

Conclusion:
Modbus TCP is a commonly used industrial control system communication protocol that is simple to use and easy to implement. This article introduces the basic principles and related parameters of the Modbus TCP protocol, and provides an example of using the PHP library Phpmodbus to communicate with a Modbus TCP device. Readers can expand and apply it according to actual needs to achieve more complex Modbus TCP communication functions.

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