Hi,
TxD idle is HIGH. And when idle, the driver needs to be switched OFF.
So this (not the best solution as FvM explained) is
* only driver ACTIVE when transmitting a LOW
* for the rest of the time (sending_HIGH or receiving or idle) the driver is OFF and the external resistors pull (too weak) a HIGH level on the bus.
***
If I had to do the job:
* I´d use a different USB_to_UART bridge with RS485 capability
* AND I´d choose external resistor values for valid levels.
Klaus