There seems to be some confusion with the term EARTH.

For this application (battery powered) nothing needs to be connected to 'EARTH'.

The cable does not need to be shielded in most cases - CAT5 cable would probably be suitable. If shielded cable was used, the shield would connect to 0Volts at one end. In the case of a fixed installation say with PLC's and PC's, the shield would connect to the electrical earth. It would probably be best that the 0V does not get connected to EARTH as we really don't want any current flowing through the 0V line (which is really a reference).

It was mentioned that power was being sent down the cable also - beware of voltage drops across the cable if all units are being powered from the one source. If the units are separately powered, it is unecessary and not a good idea to connect all the VCC (5volts) of all the units together. Therefore in most installations, one should only need 3 wires to connect 485 devices together.

0V
485+ ,485- twisted pair