Recommended circuity by Robotis. AX-12 is a half duplex uart system, the direction port pin is used to set transmission or receiving mode.
Method 2:
The diode helps to transfer incoming/outgoing signal to AX-12, taking the job of the direction port pin. Do not try this circuit if you daisy chain more then 2 AX-12 to Arduino. Arduino TX can not sink enough current to fan out the signals to several AX-12. This design is only good for prototyping.
