You could try the arduino's hardware UART (e.g. pin 0 and pin 1) - switch to external power (to disable the FTDI chip) and you can hook the dosonchip TTL level 232 to the arduino's TTL level 232.
What do you mean? Can I decouple the AVR from the FTDI chip and then use the FTDI to interface the dosonchip module to the USB port?