What kind of transient protection do you need with isolated RS-485 devices?
Transient protection on an isolated RS-485 device depends on the type of disturbance - such as electrostatic discharge (ESD), electrical fast transient (EFT), or surge - expected in the end system and the level of protection needed. TI’s portfolio of isolated RS-485 transceivers will have some level of internal transient protection on the integrated transceiver bus terminals with respect to floating isolated ground. In addition to this, with proper system design, you can use the isolation barrier to present high impedance to these transients. If you aren’t expecting differential transients in your system and test all transients with respect to end-equipment earth, then connecting protective earth (PE) to the logic side of isolated transceiver will create a scenario where all high-voltage transients occur across the isolation barrier. Connecting PE to logic side can potentially eliminate external components such as transient voltage suppression (TVS) diodes or pulse-proof resistors.
The diagram below illustrates these techniques for enhanced transient protection using the ISO1410.
