RS-485 is an electrical specification defining the characteristics of the signal line-drivers and receivers in a differential-balanced, multi-node system, no communications protocol is defined.
Modbus is the communication protocol that Garry suggested.
I know nothing of Fatek. I assume Garry does know - or took the time to investigate the documentation of your devices - and that is why he suggested ModbusRTU.I assume that Garry has determined that ALL of your proposed devices support ModbusRTU.
The first sentence in the overview that Garry linked says:
Modbus RTU is an open, serial (RS-232 or RS-485) protocol derived from the Master/Slave architecture
Check your device specifications. If they all support ModbusRTU, carry on!
If not, come back and let us know.
If you get bogged down in the idiosyncrasies of Modbus, come back.