Missing Samples in NI 9219

[Morya]

I created following VI to measure sensor (own developed) output (in voltage), the excitation voltage given to my sensor and temperature at the same time to compare it with my sensor. I m using cDAQ 9172 and module NI 9219. I need measurement in micro seconds. So, I use no.s of samples as 100K and sample rate as 100k. But I m missing samples after every 100K samples. I checked this error for different no.s of samples and found that, I miss samples after every cycle (no.s of samples) i.e. if I give 10k no.s of samples then I miss it after every 10K samples. If time at 100000 sample is 0.99999 seconds then I get directly 2.07751 seconds at 100001 sample. (I attached screenshot for this.) Please see this and give me suggestion what should I do.

[aputman]

The DAQ assistant, when configured for N samples, is going to run until it acquires N samples and then it stops.  Then the loop has to process the samples thru the formula and write them to a file and meanwhile, the DAQ assistant is not doing anything. Also the device is not doing anything because the DAQ assistant told it to stop.  It's not writing values to a buffer or anything.  So depending on how long it takes to write those values to a file, there will be a gap in your data. 

[Morya]

I also tried with continuous samples but same problem. If I remove all things and connect DAQ Assist directly to Write to Measure File, then will I get all readings ? Or any improvement in my VI to get all readings. 

[GerdW]

Hi Morya,

Or any improvement in my VI to get all readings. 

Get rid of all those ExpressVIs and use simple "plain" functions.

It's really easy!

[Morya]

Thank you sir. Very soon I will learn this and create for my project. I will upload here. But I have one doubt. I have to use 3 channels of NI 9219 at the same time. Then what should I do ? If you can upload any example VI for this then it will help me lot. Thanks again.

[GerdW]

Hi Morya,

LabVIEW also comes with a huge library of example VIs.

There are also examples explaining how to read more than one channel from a DAQmx device…

[Morya]

Thank you so much sir. I created one VI using examples given in LabVIEW. Still not sure it will work or not. But I have one doubt. I want to save data in .lvm file. All data should be saved in one file. When I run again that VI, readings should not overwrite old file. It should take next available file name. Is it possible in this VI ?  I also want to show voltage and temperature in different graphs. How to do this ?. Still this VI is not final. I want to improve and still learning.

[Morya]

Hello sir,

Please check my VI. I want to add time column in saved data. How to do that ? Please see my last reply also.

[GerdW]

Hi Morya,

When I run again that VI, readings should not overwrite old file.

Use a different filename, when you don't want to iverwrite old data…

I want to add time column in saved data.

You save your waveform data in a TDMS file, so there already is timing information included.

When you need a different format of the saved data file you need to implement your own data saving routine which doesn't depend in the DAQmx-builtin TDMS saving…

[Morya]

I want time column along with all readings in TDMS file. It means, it should show every fraction of time with respect to its readings. Because at last I have to combine all data and have to generate graph of resistance(which I will get from supply voltage and Wheatstone bridge output voltage)  v/s time. For example, if I run VI for 5 minute, then using all saved data I should able to generate graph of Resistance v/s Time.  I attached one file of my previous readings (I deleted samples after 251 seconds, as file is too large). I want time column like (this is example only), 0.0, 0.000005,0.000010,0.000015, etc.