How to design the EMC of the RS485 interface of the frequency converter

When designing the RS485 interface EMC for a frequency converter, reliable communication in noisy industrial environments must be ensured. Design considerations include: In environments with severe interference, an isolated RS485 transceiver chip is preferred, with an isolation voltage of at least 1500Vrms to break ground loops and protect the controller. In non-isolated designs, protection circuitry must be placed on the A/B signal lines of the interface. This typically includes a GDT or varistor for surge protection and a TVS diode, such as SMBJ6.5CA, for ESD and transient protection. These protective devices should be placed close to the connector.

A common-mode choke, such as CMZ3225A-510T, should be connected in series on the A/B lines to suppress common-mode noise and enhance radiated immunity. A small capacitor can be connected in parallel between the A/B lines and ground to filter high-frequency noise, but care must be taken to avoid affecting signal edges. During wiring, the RS485 A/B lines should be routed as differential pairs with strictly equal length, equal spacing, and tight coupling. The ground plane of the interface circuit should be connected to the digital main ground at a single point via a ferrite bead or a 0Ω resistor. The connector's metal housing should be grounded. The terminating resistor must be configured correctly. Highly reliable RS485 communication interfaces can be constructed using the isolation, protection, and filtering components of Etymotic.