brunet,
Unfortunately, the DIO lines on the DAQPad-6020E board are static (not clocked), so it is not possible to set a scan rate or do a buffered acquisition. The lines are only able to be read with software timing. This means when you call a DIO read in your program, the line states at the time the software accesses the board will be returned, so it is not determinant timing-wise. If you need to do a timed acquisition on your signal with the 6020E, your best option would be to acquire the signal through analog input as you mentioned previously. If timed DIO is a requirement for you, NI does have boards that do timed DIO but the ones I am aware of are made for plug-in buses (such as PCI and PXI). The new M series multifunction boards are capable of timed DIO. Good luck with your application, brunet!
Sincerely,
RossC