Defining Maximums

The maximum input characteristics information provided below is directly extracted from the Maximum Ratings which is normally mentioned inside the technical datasheets associated with the installed electronics behind the Analog and Digital input ports. When looking at maximum voltage ratings, there are two pieces of information that should be considered, the continuous voltage that can be sustained and the short spikes. In both cases, it is generally the amount of excess wattage (or the generated heat) that will primarily damage the electronics. Therefore by controlling the amount of input current, the damage could be easily avoided. 

Digital Inputs

On the CerePlex Direct, the digital inputs go through a resistor and then an isolation IC. The maximum ratings for the Cereplex Direct are all higher than the NSP in all conditions. So it is safe to observe the same input limitations that are listed above for the NSP digital inputs.  

Analog Inputs

The analog inputs for the Direct are the same as the NSP.  

Recommendations

To protect the analog or digital inputs from excessive amount of voltage, one can simply put resistors in a series to limit the input current. If the input is being used just as a trigger pulse where the exact value of the input is not important, then adding a 10K-Ohm resistor in series to the input can let extremely large input voltages (theoretically up to 100V) without damaging the inputs on the Cereplex Direct or the NSP. 
In some cases, the amplitude of the input signal which is being measured is important. For example, an externally amplified ECG signal connected to the analog input. In this case, the best way to protect the inputs is to construct a voltage divider using two 1 K-Ohm resistors. Such voltage divider has the benefit of a low amount of background noise and reduced signal distortion compared with the above method of using a series resistor.