How to prevent interference at the I/O module dry contact DI?

Improving the noise immunity of dry contact digital inputs in I/O modules requires addressing key levels, filtering, and protection. First, provide a defined logic level to the input using pull-up or pull-down resistors. The resistance value is typically between 4.7kΩ and 47kΩ to provide sufficient noise suppression current without excessively increasing power consumption. Connect a capacitor in parallel to ground at the input, forming an RC low-pass filter with the pull-up/pull-down resistors to filter out high-frequency noise. The capacitor value is selected based on the signal speed, typically between 1nF and 100nF.

Transient voltage suppression devices, such as bidirectional TVS diodes (ESD5V0D3B) or Zener diodes, must be added to handle potential overvoltage and electrostatic discharge (ESD) issues. For long-line applications, a resistor of several tens of ohms can be connected in series at the input to dampen reflections. In software, multiple sampling and debouncing algorithms are used to further filter glitches. On the PCB layout, filtering and protection devices should be placed close to the input terminals. For grounding, ensure the ground of the dry contact input circuit is clean and avoid direct connection to noisy grounds. The effectiveness of the design was verified by conducting immunity tests using simulated on-site noise.