What is the impact of the RS485 terminating resistor on EMC in edge boxes?

The RS485 bus termination resistor for edge computing boxes is typically 120Ω, connected across the differential line to eliminate signal reflections and ensure waveform integrity. From an EMC perspective, the termination resistor reduces the bus differential impedance, making it easier for common-mode current to be converted into differential-mode voltage. If the termination resistor is insufficient, it may burn out during a surge. However, proper matching can reduce ringing and high-frequency radiation. It is recommended to use a 0.25W metal film resistor and connect a TVS ESDSM712 in parallel across the resistor for clamping protection. If the bus is not always at the end, a switchable termination or a smaller 120Ω resistor can be used. The 120Ω value must match the characteristic impedance of the cable. In actual tests, after adding the termination resistor, the RS485 signal rising edge overshoot decreased from 2V to 0.3V, and the radiated emission decreased by 5dB at 100MHz.