Both Modbus and RS485 are serial communication standards, but they regulate diverse things about it. The RS485 classifies how bits should be sent by the line of communication. In this case: the 2 twisted-pair wire communication, the maximum number of device on the same segment of network, baud rates of transmission and maximum lengths. Modbus classifies other details, like the method of communication between 2 nodes of the network. In this case: it describes that the network have to have one master and one or more slaves, what are the applicable commands for writing and reading data. It may be worth detecting that standard of Modbus enables more than one standard of communication.
RS485 is serial-interface standard hardware that identifies the hardware for sending a message. Modbus is a protocol of messaging that identifies the message content. So far both are good: the two do not be different at all. BUT, there is a hardware specification for Modbus over a SERIAL line: the message finishes when there is no character has been transmitted for 3.5 character times.
Another possible issue is that Modbus names a CRC for the messages while Ethernet has already have a CRC. You require knowing how your converter operates: how it identifies when an output-packet begins and when it finishes.
In fact, ModbusPlus is different with RS-485. It follows the majority of the rules of design for RS-485, but it is utilizing transformers of isolation so that the signal field is not a standard level voltage of RS-485 signal. If you open up the adapters of PLC, you will notice that a chip of RS-485 is utilized to trigger the transformer. This may assist encourage this legend of urban. It often hears words about using parts of RS-485 on ModbusPlus - but actually an RS-485 device never work on ModbusPlus, except surge protectors.