Comprehensive Guide to Configuring Modbus Communication with Siemens PLCs

Key Takeaways

What is Modbus?Modbus is a communication protocol used in industrial automation for transmitting information over serial lines between electronic devices.
Which Siemens PLCs support Modbus?Siemens S7-1200, S7-1500, and other models can be configured for Modbus communication.
What types of Modbus exist?Modbus RTU, Modbus ASCII, and Modbus TCP.
How to configure Modbus TCP in Siemens PLCs?Detailed step-by-step configuration using Siemens TIA Portal, including creating data blocks, setting parameters, and troubleshooting.
Common issues and solutions?Configuration errors, address mismatches, and connection problems are common issues. Solutions include parameter checks, proper address setup, and debugging tools.


Modbus is a widely used communication protocol in industrial automation, facilitating the transfer of data between electronic devices. Siemens PLCs, known for their reliability and versatility, are compatible with Modbus, enabling seamless integration into various automation systems. This guide delves into the intricacies of configuring Modbus communication with Siemens PLCs, specifically focusing on Modbus TCP.

Understanding Modbus Communication

Modbus Function Codes

Modbus communication involves specific function codes to perform tasks like reading or writing data. Here are the primary Modbus function codes:

  • Read Coils (1): Read the status of coils.
  • Read Discrete Inputs (2): Read the status of inputs.
  • Read Holding Registers (3): Read the contents of holding registers.
  • Read Input Registers (4): Read the contents of input registers.
  • Write Single Coil (5): Write a single coil.
  • Write Single Register (6): Write a single holding register.
  • Write Multiple Coils (15): Write multiple coils.
  • Write Multiple Registers (16): Write multiple holding registers.

Types of Modbus

There are three main types of Modbus communication:

  1. Modbus RTU: Used for serial communication, sending data in binary format.
  2. Modbus ASCII: Similar to RTU but uses ASCII characters for data transmission.
  3. Modbus TCP: Utilizes TCP/IP for communication over Ethernet networks.

Configuring Modbus TCP Communication in Siemens TIA Portal

Step-by-Step Guide

Creating a Data Block for Modbus Configuration

  1. Open TIA Portal: Start by opening the Siemens TIA Portal software.
  2. Create a New Project: Set up a new project or open an existing one.
  3. Add a Data Block: Navigate to the project tree, right-click, and select ‘Add new data block’.
  4. Configure Data Block Parameters: Set the data block to hold Modbus configuration parameters. Uncheck the ‘optimize block access’ under attributes to enable absolute addressing.

Setting Modbus Parameters

  1. Default Port Configuration: Siemens Modbus communication typically uses port 502, but this can be changed if needed.
  2. Interface ID: Obtain the HW-identifier of the IE-interface submodule from the hardware configuration window.
  3. Compile Data Block: After configuring the parameters, compile the data block to apply the changes.

Addressing Siemens PLCs

  1. Absolute Addressing Method: Siemens uses an absolute addressing method for data blocks. For example, P#DB3.DBX0.0 BYTE 22 where:
    • DB3: Data block number.
    • DBX0.0: Starting data offset in DB3.
    • BYTE 22: Endpoint of the data block.

Practical Example: Configuring Siemens S7-1500 as a Modbus TCP Client

Parameter Breakdown

  1. MB_MODE: Designates the type of communication request (0 for reading, 1 or 2 for writing).
  2. MB_DATA_ADDR: Location of the data’s start address.
  3. MB_DATA_LEN: Number of bits or words of data being handled.
  4. MB_DATA_PTR: Pointer to where the data is stored.
  5. CONNECT: Defines the connection between the PLC and the device.

Configuring MB_CLIENT Block

  1. Set Parameters: Configure the MB_CLIENT block in TIA Portal with the parameters outlined above.
  2. Connection Setup: Use either the TCON_IP_v4 or TCON_Configured structure for connection settings.

Troubleshooting Tips

  1. Common Configuration Errors: Check for address mismatches and parameter errors.
  2. Debugging Tools: Utilize status words and diagnostic tools in TIA Portal to troubleshoot.

By following these steps, you can effectively configure Modbus TCP communication for Siemens PLCs, enhancing their integration into industrial automation systems.

For more detailed guides on Siemens PLCs, visit our Siemens PLCs page. If you have any questions or need further assistance, feel free to contact us.

Practical Example: Configuring Siemens S7-1200 as a Modbus TCP Server

Configuring the Siemens S7-1200 PLC as a Modbus TCP server involves setting up hold registers and ensuring the correct communication settings are in place. Here’s how to do it:

Creating and Configuring Global DB for Hold Registers

  1. Create a Global Data Block:
    • Open TIA Portal and navigate to the project tree.
    • Right-click and select ‘Add new data block’.
    • Name the data block and configure it to include hold registers.
  2. Setting Up Hold Registers:
    • Define an array of Bools for the first Word (Register 40001).
    • Add data for subsequent registers, such as Integers and Reals, for Registers 40002 and beyond.
  3. Logic Rungs Configuration:
    • Set up a single rung of logic in the Modbus TCP server.
    • Use this logic to control the Modbus communication, allowing the PLC to read and write data.

Configuring Modbus Parameters

  1. Default Port and Address Settings:
    • Ensure the Modbus TCP server uses port 502.
    • Configure the start addresses for the registers correctly.
  2. Connection Setup:
    • Configure the connection parameters, including the IP address of the PLC and the Modbus server settings.

Practical Example Code Snippet

Here’s an example code snippet for setting up hold registers:

DATA_BLOCK "ModbusData"
        BOOL BoolArray[16];   // For Register 40001
        INT IntValue;         // For Register 40002
        REAL RealValue;       // For Register 40003-40004

Common Issues and Troubleshooting Tips

Even with precise configurations, issues can arise during the setup of Modbus communication. Here are some common problems and their solutions:

Common Configuration Errors

  1. Address Mismatch:
    • Ensure that the start addresses and lengths of the data blocks are correctly configured.
    • Double-check the Modbus address setup in the TIA Portal.
  2. Parameter Errors:
    • Verify all parameters in the MB_CLIENT or MB_SERVER blocks.
    • Use the correct data types and address pointers.

Debugging Tools and Techniques

  1. Status Words:
    • Utilize status words to diagnose issues. For example, a status word of 7006 indicates data is being received from the Modbus server.
  2. Logging and Diagnostic Tools:
    • Use the logging functions in TIA Portal to monitor communication.
    • Enable diagnostic tools to get detailed error messages and status updates.

Practical Applications and Expert Advice

Real-world Applications

Modbus communication with Siemens PLCs is widely used in various industrial applications, such as:

  • Manufacturing: For machine-to-machine communication and process control.
  • Building Automation: Integrating HVAC systems and lighting controls.
  • Energy Management: Monitoring and controlling power distribution systems.

Expert Tips

Here are some expert recommendations to optimize Modbus communication with Siemens PLCs:

  1. Network Configuration:
    • Ensure all devices on the network have unique IP addresses.
    • Use managed switches to improve network reliability.
  2. Data Block Optimization:
    • Use non-optimized data blocks for easier access and better compatibility with Modbus.
  3. Regular Maintenance:
    • Regularly update firmware and software to ensure compatibility and security.
    • Perform periodic checks and diagnostics to maintain communication integrity.


Configuring Modbus communication with Siemens PLCs enhances their versatility and integration capabilities in industrial automation systems. By following this comprehensive guide, you can set up and optimize Modbus TCP communication with Siemens S7-1200 and S7-1500 PLCs, ensuring efficient and reliable data exchange.

For more detailed guides on Siemens PLCs, visit our Siemens PLCs page. If you have any questions or need further assistance, feel free to contact us.


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