I have had issues off and on with VFD noise coupling onto my limit switches and causing false trips on the inputs. I have used software filtering with some success and also a relay voltage level conversion circuit. But I wanted something more robust. Long story short, I built a solid state filtering board that eliminates noise three ways:
High voltage (24V) is used for all switch and input wiring. The 24V signaling is then attenuated down to 5V before being passed through step 2 below. The 24V to 5V conversion reduces all signals, including noise, by about 80%.
Low pass filter slows down the signal edge and eliminates all high frequency noise
Schmitt trigger buffering cleans up non-monotonic signal edges to prevent multiple triggers and produce a crisp clean signal. Using shmitt triggers instead of relays means no moving parts to fail over time.
The circuit logics, calculations, BOM, board, and some scope shots are included. I may do a PCB later to clean things up.