ground connection for isolated rs485
RS-485 standards mandate that a separate signal ground (Not always the same as Earth ground) wire be used to connect signal grounds at remote nodes so that the common mode of the signal wires stays close to the center voltage of 2.5 volts, or 1/2 of the 5 volts that RS-485 IC's normally use.

Without a common signal ground wire hopping from node to node then nodes can drift away from the common center point and data corruption will occur. It will not hurt the RS-485 IC's as they are designed to be electrically 'tough'. The signal ground is defined as the RS-485 IC's own ground connection on the board it is soldered to.

A better design would have offered you a 5 pin connector or a separate ground screw. If just 2 nodes there should not be much drift, but 8 nodes spread out over a hundred feet would need a signal ground wire (18 gauge is good enough) from node to node.

As you can see from this drawing there can be only 1 Earth ground if it is used.
This is copied from page 6 of:


  Tyco Electronics RS485 and Modbus Protocol Guide


Link to Tyco Electronics