How to filter out active DDI interference in I/O modules?

To filter interference on active digital input signals of I/O modules, targeted measures must be taken based on the signal type. For 24V/0V level signals from sensors or PLCs, a resistor divider can be used at the input for level conversion, combined with RC filtering, as the resistor divider network itself has a certain filtering effect. A Schmitt trigger can then be connected for shaping to eliminate slow edges and small-amplitude noise. A TVS diode array such as ESDSV05-4H can be connected in parallel at the signal input to provide multi-channel ESD and transient protection. For high-speed pulse signals such as encoder outputs, RC filters with large time constants are not suitable; instead, low-capacitance TVS diodes should be used for protection, and impedance matching of the transmission line should be ensured to suppress reflections. Using a differential receiver, such as an RS-422 receiver, can provide stronger common-mode rejection. In PCB layout, active DI circuits should be kept away from noise sources, and their power supply should be isolated from the digital power supply through a ferrite bead. Observing the signal waveform with an oscilloscope can identify the type of interference, allowing for the selection of the most suitable filtering and protection scheme.