Relationship between I/O module DI threshold and EMC

The threshold voltage of digital inputs in I/O modules is closely related to their EMC immunity. A higher threshold voltage provides greater tolerance to positive noise, but may fail to reliably identify low-amplitude valid signals. Buffers with Schmitt trigger inputs are typically used, as they have hysteresis voltage, effectively suppressing small-amplitude noise superimposed on the signal. In industrial environments, considering attenuation over long transmission lines and ground potential differences, the DI threshold should have sufficient margin; for example, a 10-15V threshold can be used for a 24V system. However, an excessively high threshold may make the circuit more sensitive to negative transient interference such as EFT, as negative voltage can cause the input to enter unpredictable states. Therefore, clamping protection for power and ground at the input is necessary, such as using bidirectional TVS diodes or Zener diodes. In software, adaptive thresholds or amplitude judgment of the input signal can be set. During design, the threshold should be determined comprehensively based on the worst-case scenario of the application environment, such as cable length and ground noise level, and its reliability in noisy environments should be verified through immunity testing.