I managed to collect 16 channels of analog signals (write to measurement file VI) manually using a boolean start/stop control. I would like to allow the user to set a sample duration on the front panel (e.g., 20 seconds) and initiate the sampling by clicking a start button. The user can also terminate the data dump before the end of the set duration by clicking a stop button. I looked at several countdown timer examples but couldn't figure out how to incorporate it to my VI. Any suggestions and examples will be much appreciated.

You are making things far too complicated. For one, there is the Elapsed Time function on your palette. Second, as already mentioned, there is a basic arithmatic relationship between samples per second and number of samples read. Use that, you are going to get more samples than requested anyway. Lastly, your code shows two analog read tasks. You can't do that unless there are two separate devices.

LabVIEW

How to control sample duration during data acquisition

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It's the latter. The user can select both the total duration and sample rate. The file attached (converted from lvm to xlsx) is the output when I set the duration to 1 sec and the rate to 1,000 Hz. But I ended up with 1.099 sec of data. The extra .099 sec is not a big deal for my kind of applications.