What is Modbus ?
Modbus is a protocol.
Modbus protocols defines the messaging structure that is used to exchange data.
Modbus can use many different types of electrical standard and to tied to RS 485.
Modbus communication can take place over RS 232,Rs 485,RS 422,Radio,Satelite,TCP/IP.
Modbus does not define the physical medium which it communicates but rather defines a messaging structure.
What is RS 485?
RS485 is an electrical standard.
RS 485 defines the physical electrical signal level and wiring that allow data to be transferred.