How to design the EMC of an RS485 interface for Human-Machine Delivery (HMI)?

Designing an RS485 interface for an HMI requires comprehensive consideration of four aspects: isolation, protection, filtering, and routing. First, in industrial environments with severe interference, it is strongly recommended to use an isolated RS485 transceiver chip with an isolation voltage of at least 1500Vrms to break ground loops. 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. Connecting a common-mode choke, such as the CMZ3225A-510T, in series on the A/B lines can 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 routing, the RS485 A/B lines should be routed as differential pairs with strictly equal length, equal spacing, and tight coupling, and kept away from power supplies and other noise sources. 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. For terminating resistors, a pluggable resistor network is recommended for configuration. A highly reliable RS485 communication interface can be built by employing Etymotic's isolation, protection, and filtering components and following good design principles.