Do I need to connect a ground line using RS-422, RS-485 communications?
The short answer is yes. Differential serial communications such as RS-422 and RS-485 use the ground as a reference. If there is no ground, the reference is basically floating. Many installations do not connect the ground line and have no problems. Electrically, it will work unless the floating reference causes the data line voltage to exceed the common mode range of the transmitter/receiver. If you have devices on different power circuits there is a high probability of ground differences that can lead to the same condition. If you are unsure of the logic ground potential differences you can measure the voltage level between logic ground and earth ground. The voltage on the logic ground line will be added to the signal voltage level and may exceed the common mode range of the transmitter/receiver. Using an optically isolated serial device alleviates this problem. Sealevel offers a variety of optically isolated serial products including the following: PCI – 7103, 7108 and 7203 USB – 210
The short answer is yes. Differential serial communications such as RS-422 and RS-485 use the ground as a reference. If there is no ground, the reference is basically floating. Electrically, it will work unless the floating reference causes the data line voltage to exceed the common mode range of the transmitter/receiver. If you have devices on different power circuits there is a high probability of ground differences that can lead to communication failures.
