This looks suspiciously like an earlier post that Steve Bailey attempted to field.....but why not.
First, each type of PLC digital input card is specific to itself and then to the manufacturer. Some are 24VDC, some are 120VAC, and so on.
In turn, each has its own "minimum" voltage level where the PLC says "Gee, this input is ON." By the same token, each has a "maximum" threashold where it says "OK, now it is OFF."
When we wire up and plan to use these inputs, we simply make sure that our "high" voltage when we want to send an "ON" signal to the PLC is above that "minimum". Similarly, for an "OFF" signal, we make sure the voltage drops below the "maximum."
If absolutely MUST have the values for a specific input, you will HAVE to go the manufacturer's data sheets for your specific card to get it. These values vary from card to card.
Is this what you are looking for?
Steve