Server simulation implementation based on Modbus protocol, supporting TCP, TCP-PI, and RTU modes

In the field of modern industrial automation, Modbus protocol, as a simple and widely used communication protocol, is widely used for communication between various devices and systems. This article will explore how to use the Modbus protocol to build a simulated server that can handle various requests from clients and simulate abnormal or special behavior under specific conditions, providing a practical tool for development and testing environments.

Complete code (Chinese comments):

1. Program Overview

This program is a server simulation implementation based on Modbus protocol, supporting three modes: TCP, TCP-PI (TCP Protocol Independent), and RTU (Remote Terminal Unit). The server sets the working mode and communication port or device by parsing command-line parameters. Its main functions include listening to requests from clients, processing standard read and write register operations, and responding to specific exceptions or performing special operation simulations when needed.

2. Program Structure and Key Implementation

2.1 Initialization and Configuration

The program first determines the communication mode (TCP, TCP-PI, RTU) through command-line parameters, and creates the corresponding Modbus context based on the selected mode. This context is used to configure the network or serial communication parameters of the server, such as IP address and port number or serial port configuration.

2.2 Mapping and Memory Allocation

The program allocates and initializes memory for different types of registers (such as coils, discrete inputs, hold registers, and input registers) through themodbus_mapping_new_start_addressfunction. The values of these registers can be accessed and modified through Modbus commands, and are the core of server client interaction.

2.3 Request Processing Loop

The server runs in an infinite loop, constantly callingmodbus_receivefunction is used to wait and receive requests from clients. After receiving the request, the server will process the request based on the requested function code and data. For standard read and write requests, the server will modify or return the register values in the mapping. For special test commands, such as intentionally replying with an incorrect number of registers or simulating delayed responses, the server performs corresponding special actions.

2.4 Special Behavior Simulation

In order to enhance the testing capability, the server can recognize specific requests and trigger predefined behaviors accordingly, such as

• replying to abnormal messages, simulating device busy or communication errors.
• Intentionally delaying for a certain period of time before responding, simulating network latency.
• Returns an error message for illegal request addresses or quantities. These features make the server not only a regular Modbus server, but also a powerful tool for testing client or other server implementations.

These features make the server not only a regular Modbus server, but also a powerful tool for testing clients or other server implementations.

3. Application Scenarios

The application scenarios of this simulation server include but are not limited to:

• Development PhaseDevelopers can use this server to simulate various devices, verify and debug client applications.
• automated testingTesters can enhance the robustness testing of Modbus client software by utilizing its ability to simulate anomalies.
• Teaching and demonstrationEducators can use it to demonstrate to students the working principle and debugging methods of the Modbus protocol.

Conclusion

By building a feature rich Modbus simulation server, developers and testers can identify and solve potential issues in the early stages of development, which is crucial for ensuring the quality and stability of the final product. In addition, such tools greatly facilitate the in-depth understanding and application learning of the Modbus protocol.