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. 

Analog Inputs

The NSP Analog Inputs rely on a Transient Voltage Suppressor (PACDN042Y3R) and Schottky Barrier Diode (BAS40-04-7-F) for circuit protection. For continuous voltages, the Transient Voltage Suppressor would fail first because it's rated for 0.225 Watts which would allow a continuous input voltage of around 9V. 
For short spikes of voltage, the Schottky Barrier Diode would likely be the first thing to fail. For transient pulses that actually have some current drive, it is rated to 40V if the current is limited to 200mA or less. If the input current is around 600mA, then the pulse must be kept below 1 sec.  
For ESD events in which the amount of input current is very very small, the input can handle spikes well into the tens of kV. 

Digital Inputs

The NSP digital inputs also rely on a Transient Voltage Suppressor (SMDA05.TBT) that is rated for 5V continuous with infinite current drive. However, if the current is limited to 1mA, it can handle up to 6V. Short pulses in the 20us range can get up to 10V. For ESD events with very small amount of current, it is rated to a value in range of 25kV.

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.