I have been trying to write a digital pulse train out to the 0 ctr on a 6221 with labview (8.5). I had this working off of a digital out the the frequency does not stay stable about aboe 30Hz once I have other programs running. I want to be able to put the digital pulse at up to 200Hz. It would be nice if I could change the frequency or stop it at my convienence. I what I think I am missing right now is something to do with how the pulse is clocked.
Why don't you use the Pulse Generation Ability of the Counter?
There are numerous examples, just use the NI Example Finder to search for them.
For changing the frequency, you can take a look at the example "Gen Fig Pulse Train-Buff-Implicit-Cont.vi" in the Example Finder. To access the Example Finder in your VI, go to "Help" and then "Find Examples..." This example shows you how to create a counter task, set up the timing and write a frequency. This example shows you how to write one frequency, but you can modify it to change the frequency during your program operation.
For pausing the counter, you may want to take a look at the example "Gen Dig Pulse Train-Continuous-Pause Trigger.vi" in the Example Finder. This example shows you how to set up pause triggers which allow you to start and stop the counter. Basically, a counter typically has two main inputs: the gate and the source. The gate input is used to toggled the enable and disable property of the counter. Pause triggers work by changing the signal that goes to this gate input.
I hope this helps!