Solution:

The detailed comparison is shown as below: 1. Auto: It is developped for RS-485. As you know, RS-485 supports half duplex, you can't send and receive messages at the same time, so it is necessary to control the direction of data flow. In the past, we use RTS signal to control driver/receiver open or close, now you can assign this task to hardware. That means you just need to send or receive messages, the flow control would be handled by hardware (There is a special electrical circuit to detect the data flow). 2. RTS: Just like the above paragraph, if you desire to control the data flow under your program, you should select this mode and set RTS signal to control it. 3. On: On means Always On, this mode is specially designed for RS-422. Compare to RS-485, RS-422 can send and receive messages simultaneously because of full duplex, however, you also can control the data flow. If you select RTS mode, that means you can use RTS signal to control driver ( Tx+/- ) open or close. If you select Auto mode, hardware would detect the data flow automatically. At this moment, if there is no message to send from Tx+/-, the circuit would leave Tx+/- floating, that means the remote site could receive noise (if there is no pull high/low on Rx+/-). In order to solve this situation, we allow you to set Tx+/- as Always On to avoid floating status when no message is sent.