The datasheet reference schematic shows that inverted RTS_N is used as RS-485 TX-enable. For OTP programming it refers to a dedicated programmer from Prolific.
FT232R offers RS-485 support by a dedicated pin without extra programming or an inverter.
another problem with prolific or FTDI is the driver can not reconnect or reopen the COM port automatically,if it disconnected by cable removal or reconnection of cable again or noise.