Fundamentals of Siemens PLC Communication: Easily Master Modbus and Profinet Configuration

Let’s Talk About PLC Communication

Hello everyone, I am Lao Lu, an engineer with over ten years of experience in the automation industry. To be honest, PLC communication is an unavoidable topic. Whether you are a beginner or a seasoned engineer, you will encounter data transmission issues between devices in your projects. Modbus and Profinet, as two very common communication protocols, can be considered a “required course” for us engineers.

In this article, I will discuss how to use Siemens PLCs (mainly S7-1200 and S7-1500) to configure Modbus and Profinet communication. The article includes both basic principles and detailed configuration steps and debugging experiences. I believe that whether you are a newcomer to communication or an experienced hand looking to optimize existing projects, you will find some practical help here.

Main Content

1. Hardware Configuration and Environmental Requirements

Before we begin, we need to clarify the hardware requirements and project environment for communication configuration. Below are the basic hardware requirements for Modbus and Profinet:

1. Modbus Hardware Requirements

• PLC Model: Siemens S7-1200 or S7-1500 supports Modbus RTU and TCP protocols.
• Communication Module:
• Modbus RTU requires an RS485 module (e.g., CM1241).
• Modbus TCP requires the PLC’s Ethernet interface (built into the CPU).
• Other Devices: The controlled devices that need communication (such as inverters, instruments, etc.) must support the Modbus protocol.

2. Profinet Hardware Requirements

• PLC Model: Siemens S7-1200/1500 (with built-in Profinet interface).
• Communication Devices: I/O modules, inverters, HMIs, etc., that support the Profinet protocol.
• Network Topology: Standard Ethernet switch or direct point-to-point connection.

Tip:

• Ensure that all devices’ hardware and firmware versions support the target communication protocol.
• When wiring on-site, Modbus RTU requires attention to the termination resistance of the RS485 bus, while Profinet requires the use of shielded cables to ensure signal interference resistance.

2. Core Principles and Design Ideas

1. Introduction to Modbus Protocol

Modbus is a master-slave protocol, simply put, it means “the master device issues commands, and the slave device responds.” In PLC projects, Siemens PLCs usually act as the master station (Master) and communicate with other Modbus slave stations (such as sensors, instruments, etc.).

• Modbus RTU: Based on serial communication (RS485), it has a long transmission distance but slower speed.
• Modbus TCP: Based on Ethernet communication, it is fast and easy to expand.

2. Introduction to Profinet Protocol

Profinet is the industrial Ethernet protocol promoted by Siemens. It is based on real-time communication (Real-Time) mechanisms, enabling high-performance data transmission and device control. The characteristic of Profinet is “plug and play”; with simple configuration, it can achieve automatic device recognition and data exchange.

Design Ideas:

• Modbus: Clearly define the roles of master and slave devices, determine register addresses and data formats.
• Profinet: Properly allocate device addresses and configure I/O mapping relationships.

3. Code Implementation and Technical Details

1. Modbus RTU Configuration Steps

Taking S7-1200 as an example:

1. Hardware Configuration:

• Add the CM1241 module in TIA Portal.
• Set communication parameters (baud rate, parity, stop bits, etc.).

Create Modbus Instructions:

• Call the <span>MB_MASTER</span> instruction in the program, configuring the slave address, function code (e.g., read/write registers), and data length.
• Example code:

  MB_MASTER(
    REQ := M0.0,
    MODE := 0,  // Modbus RTU mode
    ADDR := 1,  // Slave address
    DATA_ADDR := 40001,  // Starting address of the register
    DATA_LEN := 2,
    DATA_PTR := P#DB1.DBX0.0
  );
  

Debugging and Monitoring:

• Use debugging tools to check if data is correctly written to the DB block.
• Check the waveform to ensure normal RS485 bus communication.

2. Modbus TCP Configuration Steps

1. Hardware Configuration:

• Ensure that the PLC and slave devices are connected to the same Ethernet network.

Use TCP Communication Blocks:

• Call the <span>TCON</span> and <span>TSEND</span> instructions in the program to establish a TCP connection and send Modbus messages.

3. Profinet Configuration Steps

1. Hardware Configuration:

• Add Profinet slave devices (e.g., ET200SP) in TIA Portal.
• Set device name and IP address.

I/O Mapping:

• Allocate input and output addresses in the master PLC.

Test Communication:

• Confirm that input/output signals are updated in real-time through online monitoring.

4. Function Extension Description

The functions of Modbus and Profinet can be further extended, such as:

• Modbus: Use Modbus gateways to achieve multi-device communication or interact with cloud platforms via Modbus TCP.
• Profinet: Combine with Time-Sensitive Networking (TSN) technology to further enhance real-time performance.

5. Practical Application Cases

Case Study: Boiler Control System

In a boiler control project, we used S7-1500 as the master station to read temperature sensor data via Modbus RTU while controlling multiple actuators (such as valves and pumps) using Profinet. Through reasonable communication configuration, the system achieved:

• Real-time collection and alarm of temperature data.
• Fast response and remote control of actuators.

Tip:

• In complex projects, it is recommended to prioritize the use of Profinet to improve efficiency, while Modbus can serve as a supplement.

6. Debugging Methods and Suggestions

1. Communication Testing Tools:

• Use serial port debugging assistants or Wireshark packet capture tools to verify the correctness of Modbus and Profinet communication messages.

Common Problem Troubleshooting:

• Problem: Device not recognized.

• Solution: Check if the device name and IP address are set correctly.

• Problem: Communication failure due to mismatched baud rates.

• Solution: Ensure that the communication parameters of the master and slave devices are consistent.

• Modbus RTU:
• Profinet:

7. Common Problems and Solutions

1. Data Anomalies in Modbus Communication

• Check if the data format matches, such as the byte order of 16-bit and 32-bit data.

Profinet Device Disconnection

• Check if the network connection is stable and eliminate interference.

Conclusion: Summary and Insights

Communication configuration may seem complex, but once you master the methods, it is not difficult. Modbus and Profinet each have their advantages, and the specific choice should be based on project requirements. My personal suggestion is to prioritize the use of Profinet within controllable limits, as it is both efficient and stable; while Modbus is more suitable for scenarios that require compatibility with other devices.

Finally, if you encounter problems in actual projects, feel free to leave a message for communication. The advancement of technology relies on everyone’s sharing, and I hope this article can help more engineering friends. Keep it up!