RS485 is just a hardware interface, he just from the MCU UART signal, flip the level for transmission, and drive the cable. So RS485 is actually just a hardware interface driver chip. MODBUS is a software protocol that specifies a language. RS485 is equivalent to the speaker. MODBUS can run on RS485, can also run on RS232, can also run on POWERBUS. Same thing. MODBUS can be used on RS485, and other self-designed protocols can be used. Protocols are just software.