For normal operation you would not want to exceed the peak voltage of the range you have selected on your digitizer (For example if you are using the 10Vpp range you should not exceed +/-5V DC because the input to the ADC will max out and the input circuitry will clamp at some point to prevent damage). The device is protected beyond the peak voltage of the selected range but this protection is dependant on how you have configured your input. More information on the maximum input voltages can be found in the spec sheet by going to Start -> programs -> National Instruments -> NI-Scope -> Documentation -> Specifications and selecting the 5114.
Protection for the 50 ohm and 1 Mohm ranges is somewhat different because in 1 Mohm the primary concern is breakdown voltages of parts in the input signal path. In the 50 ohm modes the board must also make sure that too much power is not dissipated in the 50 ohm termination resistor. If the board detects a power overload in the 50 ohm resistor it will trigger an overload error and switch back to 1Mohm mode to further protect itself. At this point 1Mohm limits will apply but it would not be a good idea to use the board in a system where you are expecting the 50 ohm terminator to be frequently overloaded.
Hope this helps.
-Matt
