CLAD DAQ question

[mayur95]

Hello everyone. I am new in the community. I was having trouble understanding the above vi.

Why do I get 2 rows ?

When I ran this vi , while loop count was 1. Then why did I get 2 rows? 

Thank you.

[gaving · Accepted solution]

The way I see it is the first time the loop runs the "Elapsed Time" VI will return a false, but the DAQmx Read vi will wait for 5 samples (i.e. 5 seconds since the rate was set at 1Hz). The next loop the "Elapsed Time" VI will return true (i.e. we've been running longer than 4 seconds), but we'll still wait for 5 samples from the DAQ device. Since the output of the Read VI is indexed on the while loop boundary we will see two iterations with 5 samples each.

[crossrulz · Accepted solution]

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@mayur95 wrote:

When I ran this vi , while loop count was 1. Then why did I get 2 rows?

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Remember that the loop iteration counter starts at 0.  So a count of 1 means you iterated 2 times.  Therefore, 2 rows.

[Kevin_Price · Accepted solution]

There's a little tidbit I wanted to emphasize.  The first call to Elapsed Time will result in an auto-reset behavior and consider that instant as t=0.  It will run in parallel to DAQmx Read and finish pretty much instantly while DAQmx Read has to wait around 5 seconds for its samples.  The next iteration starts about 5 seconds later and that's the reason Elapsed Time will return a True (at least 4 seconds have passed) and terminate the loop.

Even if you sequenced the functions in the loop so that DAQmx Read was called first, waited a full 5 seconds, and *then* you called Elapsed Time, it would *still* return a False on the first call.  It wouldn't set t=0 to be the beginning of the loop iteration, it would still be the instant that Elapsed Time first executed.

-Kevin P

[mayur95]

Thanks for the reply.

So , in all this vi should take almost 10 sec to complete , right?

[crossrulz · Accepted solution]

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mayur95 wrote:

So , in all this vi should take almost 10 sec to complete , right?

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It will take at least 10 seconds.

[mayur95]

Thank you Sir.

[cobbPP]

This algorithm is not robust as you keep playing with the rate and number of samples per channel it fails to make sense. Please have a look at the attached file, if rate is set at 5 as I should have 6 rows then ?

The first run makes a valid result, now try playing with the user inputs. As you see the algorithm failing to be consistent. 

It's as if its failing to reset....well anyone could give it a try !

[crossrulz]

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@cobbPP wrote:

if rate is set at 5 as I should have 6 rows then ?

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It is looking for 4 seconds to have passed since the first loop iteration began.  If you are running at 5 Samples/second, then each loop iteration should be 1 second ( (5 Samples) / (5 Samples/second) = 1 second).  Assuming everything is ideal, you will have read elapsed times of {0, 1, 2, 3, 4}, which will give you 5 rows.  Of course, you have weird software timing in the mix as well, which could add some time.  Probably not enough to mess this up though.