How to design the EMC of the RS485 interface for a power FTU?

The FTU RS485 interface features EMC design to ensure reliable long-distance communication. It employs three levels of protection: the first level is a 3R090L-6X8 gas discharge tube (90V/3kA), the second level is an SMBJ6.5CA TVS diode (6.5V/600W), and the third level is a dedicated ESDSM712 protection chip. The signal line is connected in series with a CMZ3225A-510T common-mode inductor (510μH). PCB layout: protective components are placed close to the interface, and the signal line impedance is matched to 120Ω. Shielded twisted-pair cable is used, with the shielding layer grounded at a single point. With this design, the RS485 interface can pass IEC 61000-4-5 4kV surge, IEC 61000-4-4 4kV EFT, and IEC 61000-4-2 8kV electrostatic discharge tests, with a communication distance of up to 1200 meters.