How to Log MODBUS RTU to MySQL Database

A complete guide to polling MODBUS RTU serial devices and exporting the register data directly into a MySQL database.

1. Protocol Configuration: MODBUS RTU

Communication Type: Active Polling (Master/Slave over RS485/COM)

Because MODBUS transmits data in binary, the parser must extract specific Holding Registers (e.g., 40001) or Input Registers. If your sensor uses 32-bit floating-point numbers, configure the parser to read two adjacent 16-bit registers and apply the correct byte-swapping (endianness) to reconstruct the float.

Apply this base configuration for the serial connection:

{
  "port": "COM3",
  "baud_rate": 9600,
  "data_bits": 8,
  "stop_bits": 1,
  "parity": "none",
  "protocol": "modbus_rtu_serial",
  "poll_interval_ms": 1000
}

2. Database Setup: MySQL

You will need the MySQL Connector/ODBC installed on the machine running the Data Logger. Create a System DSN pointing to your MySQL server instance.

Use the following SQL script to create your target table. We recommend using the InnoDB engine to handle high-frequency concurrent inserts without table locking.

CREATE TABLE sensor_data (
  id INT AUTO_INCREMENT PRIMARY KEY,
  log_timestamp DATETIME DEFAULT CURRENT_TIMESTAMP,
  device_id VARCHAR(50),
  register_value FLOAT
) ENGINE=InnoDB;

3. Protocol-Specific Troubleshooting

• Timeout or CRC errors: Unlike TCP, serial MODBUS RTU relies on strict timing. Verify that your RS485 A/B wires are not inverted, and that the baud rate and device ID in the software exactly matches the slave device.
• The device returns the "Illegal address" error code: Try to use the function #3 instead of the function #4 in your MODBUS request. Otherwise, verify that you are using the correct registe address. Try to specify an absolute offset (not logical address like 40001).
• I have several values that I want to export in one data row. Now, the logger writes one value per row: You need to enable the "Export all data at once" option in the MODBUS plugin settings.

4. Database-Specific Troubleshooting

• MySQL server has gone away: If your serial device is idle overnight, MySQL will drop the connection. Enable the 'Keep connection alive' setting in the Data Logger's ODBC export module.
• How can I insert or update a data row for my device?: You can use any SQL query in our 'SQL Database Pro' plugin. In MySQL, you can easily execute the following SQL: 'INSERT INTO ....... ON DUBLICATE KEY .... UPDATE ....'.
• I want to write data from each of my devices to a different table. How can I do it?: In the 'SQL Database Pro' plugin, you can use a placeholder in your SQL. For example, 'INSERT INTO SerialData%DEVICE_ID% (....)'. Before executing SQL, the logger replaces %DEVICE_ID% with the corresponding value from parsed data, and the real destination table will vary (SerialData1, SerialData2, etc.).