Brian Dorey made this DIY USB to RS485 adapter, that is available at Github:
We looked for a full-duplex ready-made adapter but all the ones we found are only half duplex devices and as we needed to be able to supply 12 volts via the RJ45 connectors on the slave boards we decided to make our own USB to RS485 full duplex adapter using a USB converter chip from FTDI.
The board uses an FT230X with an RS485 converter chip which outputs to a set of header pins and also an RJ45 socket.
The new adapter board can supply power to the slave devices through the USB port or can be powered from an external supply by removing a power selector jumper. The board also has an on board 120R terminator resistor with selection jumper and LED’s to show serial activity.
“RS485 full duplex” is a nonsense. With 4 wires it is RS422… If you search for an USB to RS422 converter in google you would have had a better chance to find something… FTDI itself sells some…