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Hi Jerry,

    It is very likely that you'll successfully have RS-485 communications under the conditions you've mentioned without connecting the different node's signal commons, though this will depend on the exact details of entire system - what are the other node(s) transceiver(s), do the different nodes have different reference potentials or ground shifts (and how much), etc.  There are also EMC considerations which are impacted by the signal common/grounding configuration that is implemented.  Some configurations can have more emissions and/or be more susceptible to external transients if this connection is missing (or improperly implemented) in the system, 

  The isolation barrier in the ADM2867E will help minimize some of potential problems if the signal common is open, however, for best system performance and reliability, I would still recommend making this connection.  Also, the RS-485 standard does specifically recommend making this connection in the informative Annex, A.4.1.

  Thus, GND2 of the ADM2867E should be connected to the other RS-485 node's signal common.  

Eric