After connecting the RS485 bus, according to the "Slave" hardware documentation, you can now activate the Modbus. The Modbus operates in "Master" mode. 

To enable Modbus, go inside the web interface of the Didactum monitoring unit and go to the Preferences menu >> Modbus RTU.

Set up the bus according to the documentation for the "Slave" equipment.

Please note:

The Modbus extension board only supports 1 stop bit.