Modbus RTU Communication: How to Write for PLCs and Easily Connect Device Data

Implementing Modbus RTU communication with Siemens PLCs to achieve data exchange among multiple devices, applicable in industrial sites. Today, I will guide you step by step!

Project Requirements and Functionality Description

The main requirements of this project are:

1. Enable data communication between Siemens S7-1200 PLC and devices that support the Modbus RTU protocol, such as inverters, instruments, etc.
2. The PLC acts as the master station, actively reading real-time data from slave devices, such as current, voltage, temperature, etc.
3. Support the PLC sending control commands to slave devices, such as starting and stopping operations.
4. Display communication status and data returned from the slave on the HMI for easy monitoring.
5. The system must handle communication exceptions, such as timeouts or CRC validation failures, with timely alarms or retries.

Decomposition of Implementation Logic

The entire Modbus RTU communication logic can be broken down into several parts:

• Communication Initialization Logic: Configure the PLC’s serial port parameters, such as baud rate, parity, data bits, etc., to ensure proper communication with slave devices.
• Data Reading Logic: The PLC uses Modbus function code 03 to read the holding register data from the slave device.
• Data Writing Logic: The PLC uses Modbus function code 06 to write data to a single register of the slave device, enabling the sending of control commands.
• Communication Exception Handling: Handle communication timeouts, data validation failures, etc., to prevent system operation errors.
• Status Display Logic: Real-time display of communication status and data returned from the slave on the HMI, allowing operators to quickly view and confirm.

Code Implementation

Below is the detailed ladder diagram code for implementing Modbus RTU communication with Siemens S7-1200. The logic is clear and easy to understand.

1. Input/Output Allocation Table

First, organize resource allocation for more structured programming.

2. Communication Initialization Logic

Use the PLC’s communication function block (MB_COMM_LOAD) to set serial communication parameters, such as baud rate, data bits, stop bits, and parity.

| MB_COMM_LOAD
|---| |-------( )---|   // Initialize Modbus communication module

In the MB_COMM_LOAD block, configure the PLC’s communication port, such as RS485 or RS232, and the baud rate (usually 9600) to ensure normal communication.

3. Data Reading Logic

Use the Modbus function block (MB_MASTER) with function code 03 to read the holding register data from the slave device, such as real-time current or temperature values.

| M0.0       MB_MASTER
|---| |-------( )---|   // Trigger reading slave data  

| MB_MASTER  DB1.DBD0
|---|MOV|------( )---|   // Write data returned from slave to data block

MB_MASTER needs to set the slave address (e.g., 1), the starting address of the register, and the number of registers to read, with the returned data stored in the PLC’s data block.

4. Data Writing Logic

Use the Modbus function block (MB_MASTER) with function code 06 to write data to a single register of the slave device, such as sending a start command.

| DB1.DBD4   MB_MASTER
|---|MOV|------( )---|   // Send control command to slave device via Modbus

The register address and data content to be written need to be set according to the slave device’s manual, such as the register address corresponding to the start signal.

5. Communication Exception Handling

Monitor the status of MB_MASTER to determine if communication is normal. If there is a timeout or validation failure, trigger an alarm signal and log the error information.

| NOT MB_MASTER_STATUS  Q0.0
|---| |-------( )---|   // Trigger alarm signal for communication exception  

| MB_MASTER_STATUS  DB1.DBX0.0
|---| |-------( )---|   // Update HMI status when communication is normal

6. Status Display Logic

Real-time display of communication status and data returned from the slave on the HMI, allowing operators to quickly view device status.

| DB1.DBD0   HMI_DISPLAY
|---|MOV|------( )---|   // Write slave data to HMI display  

| DB1.DBX0.0  HMI_COMM_STATUS
|---| |-------( )---|   // Write communication status to HMI

Optimization and Common Issues

1. Communication Timeout Issue: Slow response from the slave device may cause timeouts; consider increasing the timeout duration for MB_MASTER to avoid misjudging communication exceptions.
2. CRC Validation Failure Issue: Signal interference may cause data validation failures; it is recommended to use shielded twisted pairs and ensure proper grounding to reduce interference.
3. Register Address Error Issue: Incorrect register addresses and function code settings for the slave device can lead to data reading failures; carefully check the device manual before programming.
4. Multiple Slave Communication Issues: When communicating with multiple slaves, different slave addresses need to be set to avoid address conflicts, and the program must also distinguish and handle data from each slave.

Complete Code Framework

Below is the complete ladder diagram code framework:

| MB_COMM_LOAD                              // Initialize communication
|---| |-------( )---|  

| M0.0       MB_MASTER                     // Read slave data
|---| |-------( )---|  

| MB_MASTER  DB1.DBD0                      // Write slave data to data block
|---|MOV|------( )---|  

| DB1.DBD4   MB_MASTER                     // Write control command to slave
|---|MOV|------( )---|  

| NOT MB_MASTER_STATUS  Q0.0               // Alarm for communication exception
|---| |-------( )---|  

| MB_MASTER_STATUS  DB1.DBX0.0            // Display communication status
|---| |-------( )---|  

| DB1.DBD0   HMI_DISPLAY                   // Display slave data
|---|MOV|------( )---|

Conclusion

The key to Modbus RTU communication is the configuration and matching of function codes; debugging clearly will ensure stability! If you have any questions, feel free to leave a message, and we will solve it together!