Grounding

The MD485 has a ground lug. Connect this ground lug to earth ground with an 8 AWG wire. This connection should be as short as possible.

The differential signaling of RS-485 does not require a signal ground to communicate. The standard allows for a common-mode voltage (Vcm) of -7 to +12 V. As long as the MD485 local grounds do not exceed this common-mode voltage limit, the RS-485 communications will work fine.

Over a distance of hundreds or thousand of feet, there can be significant differences in the voltage level of “ground”; it can easily extend beyond the common-mode voltage limits of RS‑485. The signal ground wire serves to tie the signal ground of each node to one common ground, which is within the common-mode voltage of the RS-485 specification.

The RS-485 specification also recommends connecting a 100 ohm resistor of at least 0.5 W in series between each node signal ground and the network ground wire. This resistor is in the MD485, between the terminal block connection marked with the ground symbol and the MD485 power ground. This way, if the ground potentials of two nodes vary, the resistors limit the current in the ground wire.

See The MD485 has a ground lug that should be attached to earth ground, and LEDs that indicate traffic on the ports., RS-485 connections and grounding, and RS-485 cable for more information on ground connections.