Solution
If you want to use a digital output channel level up to 5V, you should set the digital output port as the open collector(open-drain) mode. The digital output port of USB-6501 which is set as active drive(Push-Pull) only generate output voltage up to 3.5V. This configuration are programmable at LabVIEW or NI MAX. Refer to following figures for each case.
If you use other digital output modules, please check the digital logic level in the data sheet.