A digital input can use a pull-up resistor for true 1’s and 0’s and stop a pin from floating. All you need is a single resistor to make sure the digital input is either at 0v or 5v. Without the resistor, the input signal would float because it wouldn’t actually be connected to anything when the switch is in the open state.
Note: The IO-204 web gateway, Rev. C circuit and the Gamma include 25k pull-up resistors on board.