I use this to connect to some AVR boards I'm working on (e.g. RPUno). The FTDI UART to USB bridge is working well with Linux (I use Ubuntu and Raspbian). The bridge also works on Windows but mostly I use SSH to the Linux machine and then do things from a terminal session (e.g. Picocom). There are two data paths, one is RS-422 and the other RS-485. The RS-422 path connects the FTDI RX to the MCU TX and FTDI TX to MCU RX. While the RS-485 is used for management (e.g. the DTR signal is to run a bootloader). RS-422 is full duplex and works with optiboot and avrdude once placed in a point to point mode. Looking at the MCU TX signal, it can be seen to go from the MCU to a transceiver input which drives the differential TX pair and then to a differential receiver (same U2 chip has both) that drives the FTDI RX. The differential pairs are shared over CAT5 to other boards with RPUadpt shield(s). The transceiver can also be disabled (e.g. a sort of lockout) by the bus manager to prevent the MCU or FTDI UART from using the differential pair.