1 Channel DAQ- Basic Doubt

[MogaRaghu]

The sample rate is 1 per sec. ; I need 5 samples ; I hold the acquiring loop for 5 sec.  Yet I get two sets of data ..  Tried to read Help but could find the reason for this. Even tried to set the loop timing to 1 sec and still I get two set of samples.  Why is this ?

[GerdW]

Hi Raghu,

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@MogaRaghu wrote:

The sample rate is 1 per sec. ; I need 5 samples ; I hold the acquiring loop for 5 sec.  Yet I get two sets of data ..  Tried to read Help but could find the reason for this. Even tried to set the loop timing to 1 sec and still I get two set of samples.  Why is this ?

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Because the loop iterates twice…

THINK DATAFLOW: the ElapsedTime gives FALSE in the first iteration, so the loop repeats…

Why don't you use "Continuous samples" when you need just 5 samples?

Why do you need ElapsedTime (and the loop) at all: DAQmxRead will wait for 5 samples on its own!

[Bob_Schor]

To learn how DAQmx works, open MAX (the Measurement and Automation Explorer).  Find your Device in the Devices list.  Open a Test Panel.  Find your A/D.  Now play with the settings.  Notice what the difference is between Finite Samples (where, say, "I want to take 5 samples") and Continuous Samples (where, say, "I want to take 5 samples at a time, over and over").  Learn what "Number of Samples" means.

If you want to take 5 samples and stop, then why do you need a loop?

Understand your Device and understand DAQmx a bit better -- MAX can give you the tools to explore and learn.

Bob Schor

[crossrulz]

This looks extremely similar to a CLAD example exam question.  Studying up for a CLAD?

[Kevin_Price]

A subtler point is that the call to DAQmx Read is probably going to complete in something more like 4 seconds rather than 5.

When the task is started, the 1st sample will be taken almost immediately, near t=0.  The 2nd sample will be taken at ~1 second and so on until the 5th sample is taken at ~4 seconds.

-Kevin P

[MogaRaghu]

@crossrulz

You are spot on.. yes its a lift off from a sample CLAD question bank ! I am not preparing but helping some one else who is. With my age well past 60, even assuming I get the certificate, it will be a personal joy and that's it !!  

The sample rate is 1/sec. And the read is set up for 5 samples.  And since the loop will anyway iterate once, for the Elapsed Time less than 5 sec, one would expect only to get one set of samples.  Why does the second iteration happen in that case ? 

[MogaRaghu · Accepted solution]

Just landed on this You tube video which answered my doubt.

While Loop iteration.. 

Thanks to all those who chipped in !