Time delay problems with PCI 6024E and CB-68LP

[Gorman]

I am using the PCI-6024E with the CB-68LP on a Windows 2000 computer.  THe LabVIEW version is 7.0 and the processor speed of the computer is 548 MHz.  In LabVIEW, I am using the DAQ Assistant to measure temperatures.  In Properties of the DAQ Assistant there is Task Timing.  I want to collect N Samples at a certain rate.  The number of samples needs to be much larger than the rate so that when it is saved as a .lvm file there are no breaks in it.  Which means that when it completes the given number of samples it retypes the header, I want a continuous set of data (this occurs even with Run Continuously is chosen).  When I have the count rate very similar to or greater than the number of samples, the Save File prompt appears almost immideately when the program is run.  However, when the Number of samples is much larger than the count rate it takes a long time for the Save File prompt to appear.  "Long time" varies depending on the difference, but with 360 samples at 1 Hz it took about 10 minutes for the prompt to appear.  How do I get this to occur instantaneously (upgrade computer?) or how do I save the file so that it is in .csv file (essentially so I can open it into some sort of spreadsheet program)?  Right now i have the DAQ Assistant connected to the Write LabVIEW Measurement File.  If something is not clear enough please let me know and any help on this matter would be greatly appreciated.

Thank you

William Gorman

[devchander]

Hi William,

Probably your program flow is like that.

You say if sampling rate = Number of samples to acquire, you get prompt instantaneously.

This is because your acquisistion task  is over in 1 sec and program flow comes to 'write to lvm file' DAQ assistant and you get the pop up.

Where as in the next case, 360 samples @ 1Hz takes approximately 6 minutes to acuire all samples, so labview will finish this task ( acquire all 360 samples) and then go to 'write to lvm file' function and  now, you get your file dialog pop up

there are many ways to get around this. You could use 'File i/o' functions in your functions palette to open/ create a file before starting acquisition,  write into your file after acquiring data and then close the file

You could use 'write to spreadsheet.vi' function in your file i/o palette for creating a file that can be opened with a spreadsheet program!

Any doubts, do ask

Regards,

Dev

Message Edited by devchander on 10-11-2006 11:32 PM

[Gorman]

I recieve the prompt to save the file before it collects the data.  I understand that if I was collecting 360 samples at 1 Hz it would take 6 minutes to collect the data.  However, the prompt comes before the data is collected and then after I choose a file location it takes the 6 minutes and collects the data.  Once it has collected all 360 samples it will write a new header in the file and start counting again using the LabVIEW measurement file assistant.  What I essentially want is for it to collect an infinite amount of samles so that it won't rewrite a header and then I will manually abort the program when I have collected the amount of data I need.  But in order for this to happen I have to wait 10 minutes or some amount of time just to get the prompt before I can start collecting the data.  When using the write to spreadsheet idea, I still have to wait for the prompt to occur before the data is collected and the wait takes the same amount of time.  Using the WriteToSpreadsheet and the write as a LabVIEW Measurement file give the desired end format but the wait beforehand is still occuring.  With the WriteToSpreadsheet format, (at 360 samples: 1Hz) I pressed run and it took 5 minutes for the save file prompt to appear, then I would choose a file location, and then it would start collecting the data. 

[Gorman]

I think I figured it out.  Using the Write LabVIEW Measurement File.vi there is an option in the properties for only having one header.  With that checked I can use 2 samples and collect at 1 Hz.  This will cause the save file prompt to occur immediately and it will append the data to the file in a continuous manner.  The only downside to this is the fact that I was using a waveform as a visualization so that I knew the data was roughly correct and that there wasn't anything extra strange occuring but with the waveform connected directly to the DAQ Assistant it gets updated every 2 data points.  Is it possible to either create the waveform in such a manner that the data is added to the graph instead of each data point replacing the other or is there a way to read a file that is in the process of being written.

Thanks again for your help.

William Gorman

*A new post will be started on pinouts for the CB-68LP

[Neil_S.]

William,

Thanks for posting to the NI forums.  You should be able to use a waveform chart instead of a waveform graph to be able to keep a history of the data.  A waveform graph only graphs the data that is passed to it; however, the waveform chart also graphs the previous values passed to it.  Let me know if you have any additional questions.

Regards,

Neil S.
Applications Engineer
National Instruments