Can the DIO voltage of USB-6341 be maintained? As far as I know, the logic high will be varies in between 2.2V-5V.
That specification is for the input voltage to be considered a logic High. The output is a 5V TTL and will only drop from the 5V when the current limit on the line is reached.
So means that we cannot set a static output voltage for DIO is it?
Is it possible to set a value for DIO output voltage if we add pulldown resistor or potentiometer?
It'd be better to add some isolation by using your DO to operate something like a relay to control an independent supply of DC voltage.