To access the Modbus settings, go to the sidebar and click on Network → Modbus.
The configuration options will appear in the Modbus settings box.

🔓 Modbus functionality is optional and must be unlocked before use.
You can purchase the activation on grnbms.de. Once activated, the features will be available immediately.

Enable this checkbox to turn on the Modbus server.
If disabled, all other fields are ignored.

Modbus address of this device (used for RTU-style communication).
Default is usually 1.
⚠️ Must be unique if multiple Modbus devices share the same bus.

TCP port used for Modbus communication.
Default: 502.
Only change if your system requires a non-standard port.

Choose between:
• RTU over TCP – binary Modbus RTU frames inside TCP packets
• TCP – pure Modbus TCP protocol
💡 Use RTU over TCP if your client expects RTU framing.
Use TCP for standard Modbus TCP clients.

Select the serial interface to use for Modbus RTU.
Only shown if a serial port is available (e.g. RS485 1).

Enable this to forward Modbus traffic between Ethernet and the selected serial hardware port.
This allows external Modbus clients to talk to devices on the serial line through the BMS.
⚠️ Use with caution – this enables full passthrough between interfaces.

grnbmsmk3modbus_v1.pdf