RS485: What is the difference between RO and D_OUT?
"\$D_{OUT}\$ (Pin A1): General Purpose Logic Output. Logic
  output connected through isolation path to \$D_{IN}\$ . Under
  the condition of an isolation communication failure \$D_{OUT}\$
  is in a high impedance state."  


So this is not a copy of the received signal, but a loopback from the input it gets from your microcontroller. In normal operation conditions it will copy the data you put on the bus, but it won't give you data others are sending. It seems to be used to verify that the isolation communication is in order.