Acuire data using signal express!

[TVS]

really appreciate your help so far!! I have couple of more questions!

1. The log based text file is giving the needed 10 seconds data for the 10 seconds test run time but it is sampled at 0.0005s. Is there a way I can acuire the samples at the user defined rate i.e., in my project it would be 0.001s.

2. Also could you suggest me the sample rate and numebr of samples that I should put in for 100 second test run.

Thank you!

[--Chris]

The sample rate of the ASCII file that is generated from the log file should have the same sample rate as the acquisition, i.e. the dt value in the created ASCII file in the log file directory should match the "Rate" specified in the DAQmx Acquire configuration.  It is a little confusing that you specify a sample rate in Hz and the ASCII file contains a dt in seconds, but they should match.

When doing the 100s log you should be able to use the same rate and "Samples to Acquire" as you did for the 10s acquisition.  Just change the stop condition to 100s.

[TVS]

Hi!!

I have tried various runs for data logging but it still gets the data sampled at 0.0005 sec!!

[--Chris]

If you right click on a signal group in the log and select properties, does the dialog display the correct value for the dt?

You might want to attach your project again in it's current state so we can look at it.

[Ima U]

Hello TVS,

You mentioned that you were looking at the text file from the Save to ASCII step before. When you look at the number of samples in the log file are you seeing the same results as proud NI Engineer? I agree with proud NI Engineer, you no longer need the save ASCII step now that you are logging data.

[TVS]

Hi,

It displays 0.0005s for dt when I check the properties of the log. I have attached the project.

Thanks!

[TVS1]

Hi Ima,

Thanks for your help!! Well, I see that it is logging in the data for the required run time but why it is always logging the samples at time step dt 0.0005s even though I specify a different sample rate. What I mean to say here is it should log the samples (if rate is 100 samples per sec, dt should be 0.01s and not 0.0005s) at user defined rate and not the default 0.0005sec!

Thank you!

[--Chris]

I believe that your issue is that the device has a minimum rate of 2000Hz and that the rate you requested is being coerced to this minimum.  I verified that the NI9233 module does this.  What is the model of the device that you are using?

I have filed a bug report that the DAQmx Step is not informing that the rate value is coerced.

The logged data is correct in that the device sampled the signal at 2000 Hz and there is are no issues with the actual data, it is just sampled faster than you desire.

[TVS1]

Yes, I am using NI9233 module!

[TVS1]

Hi!

I have another question to you about logging data!

Well, I just want to know what happens if one uses discrete instead of continous way for logging in the data. I know it skips the samples but what is the pattern of the points that is written out to ascii file.

Say if the discrete block is 100 samples and sampling frequency is 100 htz, and the run is for 10 seconds. When it writes out the data to an ascii file, in which way are the samples wriiten out to the file, I mean does it skip the samples in between or does it write out the data sequentially by storing the data in a buffer and lose the last part of the samples when we stop the run.

Thanks for your help!