Why do I need to connect a ground line when using RS-422 or RS-485 communications?

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By connecting the ground wire, you avoid potential communication problems. Differential serial communications such as RS-422 and RS-485 use the ground as a reference. When there is no ground, the reference is basically floating. Communication failures can result if the floating reference causes the data line voltage to exceed the common mode range of the transmitter/receiver. When devices are on different power circuits there is a high probability of ground differences that can lead to communication failures or device damage.