I was looking at calibration procedures for a couple of NI boards, among which a few are digital. For those I haven't found any calibration procedure. Is it because I should consider the digital modules don't need calibration?
Static DIO do not need calibration. Hardware Timed DIO should probably have its on board clock calibrated.
I am afraid I have to respectfully disagree with crossrulz here, There might be times when you need to calibrate static digital I/O, however it comes down to exactly what you are trying to accomplish. It may be the case that there are some characteristics that are critical to your application, such as timing skew between channels, output rise time, etc. In that case it would be worthwhile to test for those parameters.
I would strongly recommend having some method of functional check, to make sure that the digital I/O is still doing what you want. I.E. digital inputs are actually read by the card on every input line, and the card generates outputs on each output line. Obviously you need to test each line separately, so you can detect inputs shorted together etc. Unless you design this into your hardware at an early stage, it may be difficult to accomplish in the absence of whatever your system is connected to, or in a reasonable time if there is a large number of I/O lines. "Record of Functional check of I/O lines" might be what an auditor would want to see.
Be cautious. Many of the DIO cards are not running in a real-time environment and will therefore not be able to checked for any particular timing as Windows will control the time slicing. You can still verify simple things with DIO cards.
Some boards may require some external circuitry for test (I.E. Open collector outputs)