PLC and RFID System Communication: Writing PLC for Smoother Data Reading

Using PLC to control the RFID system, achieving data reading, authorization verification, applicable in logistics and warehousing. Today, I will guide you step by step!

Project Requirements and Functionality Description

The requirements for this PLC and RFID communication project are:

1. Read data from the RFID tag via PLC, such as item number, batch, etc.
2. Perform authorization verification based on tag information, such as allowing or prohibiting passage.
3. Display the data read from RFID and verification results in real-time on the HMI.
4. Trigger an alarm or stop operation when the tag is invalid or communication fails.
5. The program must be stable to avoid system logic confusion due to card reading failures or signal interference.

Decomposing the Implementation Logic

The entire system logic can be divided into the following parts:

• Communication Initialization Logic: Establish communication between PLC and RFID reader, setting parameters like baud rate and data format.
• Tag Data Reading Logic: Read data from the RFID tag via PLC and store it in a data block.
• Authorization Verification Logic: Determine whether to allow passage or trigger an alarm based on the read tag data.
• Status Display Logic: Real-time display of RFID read data and verification results on the HMI.
• Exception Handling Logic: Handle cases of communication interruption or invalid tags to prevent system errors.

Code Implementation

Below is the ladder diagram code for implementing PLC and RFID communication using Siemens S7-1200, with clear logic and stable operation.

1. Input/Output Allocation Table

First, organize the hardware resource allocation for easier programming later.

2. Communication Initialization Logic

Using PLC’s serial communication function, set the communication parameters of the RFID reader, such as baud rate and parity.

| RFID_INIT
|---| |-------( )---|   // Initialize RFID communication module

After initialization, the RFID reader is in standby mode, waiting for the PLC to send a trigger signal.

3. Tag Data Reading Logic

Send commands from PLC to the RFID reader to read the data on the tag and store it in the data block.

| Q0.0       RFID_READ
|---| |-------( )---|   // Trigger RFID reader to read tag data  

| RFID_READ  DB1.DBD0
|---|MOV|------( )---|   // Write the read tag data into the data block

4. Authorization Verification Logic

Verify the read tag data via PLC to determine whether to allow passage or trigger an alarm.

| DB1.DBD0   VALID_TAGS  Q0.1
|---|CMP|------( )---|   // If tag data is valid, allow passage  

| NOT DB1.DBD0  Q0.2
|---| |-------( )---|   // If tag is invalid, trigger alarm

5. Status Display Logic

Write the read tag data and verification results to the HMI for real-time display to the operator.

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

| Q0.1       DB1.DBX0.0
|---| |-------( )---|   // Write verification passed status to HMI

6. Exception Handling Logic

Trigger an alarm signal and stop system operation when communication is interrupted or card reading fails.

| NOT RFID_COMM_STATUS  Q0.2
|---| |-------( )---|   // Trigger alarm when RFID communication is interrupted  

| NOT RFID_COMM_STATUS  Q0.0
|---| |-------(R)---|   // Stop triggering the reader when communication is interrupted

Optimization and Common Issues

1. Tag Reading Failure Issue: May be caused by signal interference or tag damage leading to reading failures. It is recommended to add retry logic and trigger an alarm on consecutive failures.
2. Verification Logic Not Rigid: During verification, tag data should be strictly matched to avoid misjudging as valid tags.
3. Communication Delay Issue: If there is a delay in communication, sending trigger signals at intervals or batch reading data can improve efficiency.
4. Incomplete Exception Handling: Timely alarms should be triggered for communication interruptions or lost tags to prevent the system from continuing operation, causing safety hazards.

Complete Code Framework

Below is the complete ladder diagram code framework:

| RFID_INIT                                // Initialize communication
|---| |-------( )---|  

| Q0.0       RFID_READ                    // Trigger reader
|---| |-------( )---|  

| RFID_READ  DB1.DBD0                    // Read tag data
|---|MOV|------( )---|  

| DB1.DBD0   VALID_TAGS  Q0.1           // Verify tag validity
|---|CMP|------( )---|  

| NOT DB1.DBD0  Q0.2                    // Invalid tag alarm
|---| |-------( )---|  

| DB1.DBD0   HMI_DISPLAY                 // Data display
|---|MOV|------( )---|  

| Q0.1       DB1.DBX0.0                 // Verification result display
|---| |-------( )---|  

| NOT RFID_COMM_STATUS  Q0.2            // Communication exception alarm
|---| |-------( )---|

Conclusion

The core of RFID communication is data reading and verification. As long as the logic is clear, it will be stable! If you have any questions, feel free to leave a message, and we will resolve it together!