The update rate is not infinitely adjustable. The update rate is set by using one of the on-board clocks and dividing it with a counter to achieve the frequency you request. For instance, if you have a 10MHz clock, dividing it by 16 gives you 625kHz and dividing by 17 gives you 588kHz. Neither is close to the 595kHz you are trying to achieve.
The best way to output sine waves of varying frequency with the 6731 would be to pick a constant update rate and vary the frequency by altering the data you output.
A better way to do function generation would be to get a function generator card such as the
NI-5411. It allows generating sine waves at any arbitrary frequency up to 16MHz. If you want
to get faster than that, you will need to look into special-purpose RF function generators.
Hope that helps.
- Tony