How to design an industrial servo RS485 interface EMC?

The EMC design of the servo RS485 communication interface needs to consider long-distance, multi-point communication and industrial environment interference immunity. E-Tech Electronics' design solution includes:

1) Protection circuit: Transient suppression devices are connected in parallel between the A/B lines and ground. The first stage uses a gas discharge tube (e.g., 2R090L-6X8) to protect against surges; the second stage uses a TVS array (e.g., ESDSM712, integrating 7V and 12V bidirectional TVS) to protect against EFT and ESD; a front-end series thermal protection PTC (e.g., SMD2920-050-60V) is used.

2) Common-mode filtering: A common-mode choke (e.g., CML3225A-510T, 51μH) is used at the interface to suppress common-mode interference.

3) Wiring: Shielded twisted-pair cable (characteristic impedance 120Ω) is used, with the shield grounded at a single point at the driver end.

4) Termination matching: A 120Ω matching resistor is connected in parallel at both ends of the line.

This design enables the RS485 interface to pass ±4kV EFT, ±8kV ESD and ±2kV surge tests, with a communication distance of up to 1200 meters and a bit error rate of less than 10^-7, meeting the requirements of industrial buses such as PROFIBUS.