Asynchrono​us Call - collect values without waiting for sub VI to finish executing

Question

Hi All, I'm relatively new to LabVIEW and for the first time I'm attempting to make use of Asynchronous calls in a program.
What I would like to accomplish, generally, is this: At the beginning of the main vi, begin running a sub-vi in the background. This sub-vi is collecting streaming data from an instrument with a refresh rate of 50Hz. At various points in the main vi I would like to call the instantaneous value of that streaming data from the sub vi, passing it back into the main vi, while allowing the sub-vi (the streaming data collection) to continue running. The reason for this, is that I need to collect the data from this instrument many times wihtin a loop, and the connection initialization to the streamin data instrument is both slow and faulty, and takes too long if I have to initialize the connection, collect the data, close the connection, each time I wish to collect the data. I would prefer to initialize the connection once at the beginning, have a stream of continuous data being collected in the background, and then at certain points within the main vi loop, query that streaming data for its instantaneous value, but allow the streaming data sub vi to continue running.
I recognize that a method to allow the streaming data collection sub vi to run in the background is to use the Asynchronous call method - however, to collect the data I need to use an asynchronous wait, which will only collect the data if the streaming data sub vi finishes executing. How can I collect the data at a particular instant while still allowing the subvi to keep running (thereby minimizing the number of times the connection to the instrument needs to initialized)?
Any suggestions or alternative methods would be great!
Thanks
Jason

Answer 1

This is typically solved in a different way than what you envision. Have your parallel deamon post its current values to a buffer. This could be a global variable (gasp!!) or an intelligent global variable (a VI with loop that executes always once and has an unitialized shift register that can either be initized or read based on a control input). You can add extra code to the read and write selection such as scaling etc to turn the intelligent global into an action engine that does extra things on its different methods.
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions
Attachments:
Intelligent Global.vi ‏19 KB