The i4xx cards have two kinds of digital inputs which are referred to as
"digital i/o" and
the later of which is more advanced.
In the typical case, a logic 0 corresponds to 0 to 0.8Volts,
and a logic 1 corresponds to 2V to 5V.
For details on the number of channels and the characteristics of each,
click on the model numbers below the above pictures.
For details on working with digital i/o channels,