I use the PXI8431 card for RS485 communication: here has another usb to Rs485 card(not NI card), and I set the two 485 card both two wire mode,(for pxi8431,I set two wire auto mode, and I have short the pin 4 and pin 8, pin 5 and pin 9), then the usb card send message to pxi card, and the pxi card can NOT receive anything, however, the pxi card send message to usb card, the usb card can receive the correct message. What is more, I set the DTR (pxi card)enable, and then set the DTR unenable, then the pxi card can receive the correct message sent by usb card, but at the same time ,the pxi card can not send message; then I set the DTR enable, and the pxi card can send message, because the usb card receive the correct message; however at this time, the pxi can NOT receive the message. I have read the NI help document, if I set the mode as "two wire DTR Control" mode, then the result I have described above is very correct, but now I am sure I choose the mode as "two wire auto". I do not know why? It seems the two wire auto mode does not work!!! I have change the pxi card to a new slot, but the same result.
For the four wire mode, I have test and the result is very correct.
