Would you Open and Close DAQmx tasks within a function or Globally

Question

Hi Guys,
This should start something.
The subject is DAQmx Tasks.
If you were writing a function that is called many times, say MeasureVolts (double *Value) to obtain a reading from an Analogue Input Channel.
Would you.
1. open and start the task inside the function, do the measurement and then close the task inside the same function.
or
2. open the task globally and later do the measurement inside the function using the global task then close the task only when the application is closed.
I am mostly concerned with the overhead of task generation and closing, what do you guys and girls out there do?? and what is the general opinion?
I would appreciate your views.
Regards
Paul.

Answer 1

I would definitely NOT keep configuring and clearing (I think that's what you mean by open and close) the same task over and over.  I would configure once, read many times in my main app, then stop & clear once.
I typically like to package such data acq into an "Action Engine" that keeps track of its own DAQmx task id so I don't need to sling that task id wire around into multiple parallel loops / vi's.  Attached is a very stripped-down example for Digital Input. 
I would typically do more with error testing and would make the "Action" enum into a typedef custom control, but this gives you the basic idea.  The action engine approach works very nicely when there's a small # of different kinds of input parameters and output info needed to support all the necessary action cases.   When you start needing a large number of differnent kinds of inputs and outputs to support the desired actions, then you start having difficulty with (1) having enough terminals to wire and (2) remembering which inputs and outputs are meaningful for which actions.
-Kevin P.
Attachments:
DI Action Engine.vi ‏75 KB