Hi @AllanL,
From your description, I assume it's a wrong connection. The digital inputs on our units are not potential-free, but instead respond to voltages in the range of 7 to 35 VDC. At the same time, it is necessary that the negative pole of the voltage source is connected to DIGND.
More information can be found in the article dedicated to digital inputs:
https://kb.unipi.technology/en:hw:02-neuron:description-of-io:01-description-of-di
Best regards,
Antonin