4 Eliminating ground loops and GPDs

While the RS-485 and RS-422 standards specify that a data link without a ground wire can be operated with a GPD of up to ±7 V, it is advisable not to assume that these values represent the maximum GPD. Much higher values are often encountered in industrial plants, sometimes reaching several hundreds and even thousands of volts. Because GPDs largely depend on factors outside the system designer’s control, such as the electric installation and/or the number of electric motors and generators, the most secure way to prevent transceiver damage from large ground-potential variation sis to galvanically isolate any remote network node from the bus. The circuit in Figure 3 demonstrates this by having only the remote transceiver connected to the bus, while the data-converter circuit is galvanically isolated.

Also, to provide the input and output signals of the remote transceiver with a stable ground reference, the transceiver’s ground terminal as well as the digital isolator’s ground terminal (GND1) are connected to the master ground potential via a separate ground conductor. This form of grounding is known as a single ground reference.