If you search for SimpleModbusSlave library, there is a version for DUE.
It seems to work reliably and I have not had any issues with it for the past year or two.
For my solar system, I have a DUE set up as Modbus RTU slave, using SimpleModbusSlave_DUE, being polled by a small GNU/Linux board which runs Modbus RTU master, using Libmodbus, which is a library for GNU/Linux systems.
You will find a thread in Networking, Protocols and Devices: this thread
https://forum.arduino.cc/index.php?topic=176142.0Search in there and you should find the link to get the DUE version of the library.
I don't have the actual link to the library source unfortunately, but do have the library.
Essentially, there is little difference between what interface you use.
The only thing that is needed for TIA-485 is the data direction control line if you are using half duplex.
I use it over USB as a serial device.
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Paul - VK7KPA