How to design the EMC of the PMS RS485 interface for electrical force measurement switches?

The PMS's RS485 interface is used for long-distance communication and is highly susceptible to surges, EFTs, and common-mode interference; therefore, comprehensive EMC design is essential. In the circuit, a TVS diode (such as SMBJ6.5CA) is connected between the A/B lines and power and ground respectively for differential-mode protection, and a bidirectional TVS diode (such as SMBJ12CA) is connected between the A/B lines and ground for common-mode protection. A series common-mode inductor CML3225A-510T (51μH) is used to suppress high-frequency common-mode noise. A 120Ω matching resistor and a 10Ω current-limiting resistor are connected in series at the transceiver front end. An isolated RS485 transceiver (such as ADM2587E) is selected, and its isolation power supply requires a DC/DC module. PCB layout: Protective and filtering components are placed close to the connector, and the transceiver is placed close to the connector. The connector uses a metal housing, and the housing is connected to the chassis ground. Shielded twisted-pair cable is used, with the shield crimped 360° to chassis ground at the PMS end, and the other end left floating (unless the other device has good grounding). With this design, the interface can meet the severe requirements of IEC 61000-4-5 surge ±4kV, IEC 61000-4-4 EFT ±4kV, and IEC 61000-4-2 ESD contact ±8kV, with a communication distance of up to 1200 meters.