The problem of measure voltage

[34]

Halo,

Now, i am doing a project. i use the usb 6008 to measure the voltage for the  generator. i use the labview8 to make a progamme.
When i run the progamme, the waveform chart can not show the waveform.But there are no error to occur. How can i set the wareform chart?
i attach the photo for the progamme. Can anybody help me to solve this problem?
Thank.

[Wilfried_71]

You must post a screen picture of the front panel, and not of the diagram in order to have an idea of the problem.

[Matthew_W]

 
 Hi,

 A waveform graph needs an array of data before it will display anything. Currently you are acquiring a Single sample then placing it on the graph, the result will be effectively no information conveyed. Either change the DAQmx Read to N samples or use a Waveform Chart.  Observe in the following example the DAQmx Read is N Samples with 10 samples acquired before the Waveform Graph is updated.


 
Now, in this example I am using a Waveform Chart and calling a DAQmx Read Single Sample.

 

 
 





Message Edited by Matthew W on 11-13-2007 10:19 AM

[Dennis_Knutson]

Uh, according to the image, it is a chart.

34,

Is the chart set to autoscale? Also, you are acquiring a waveform data type. Do you see any difference if you switch to DBL datatype? What is the chart history set to?

[Matthew_W]

 To finish my post from earlier:

 This time I am using a DAQmx Read Single Channel Single Sample and displaying on a Waveform Chart.


 Hope this helps!

 Best regards,

 MatthewW
 Applications Engineer
 National Instruments

[Wilfried_71]

Even if there is just one point on the daqmx read, each iteration loop must give a point and trace a graph ?? So it must work like that ?

Is there a problem with the loss of hardware clock set up ?

[Matthew_W]

 Hi Wilfried,

 The waveform chart will update a single point at a time, each successive point will be added onto the chart with all of the previous points.

 The waveform graph will update the entire graph at a time. If this consists of a single point you will not see any visible data on the graph, if you have autoscaling
    turned on you will see the graph Y axis changing as the graph is redrawn.

 I'm sure what you mean by "Is there a problem with the loss of hardware clock set up?".
 I think your asking is software timed, such as seen in my previous posts, less accurate than hardware timed. Software timed data acquisition is non-deterministic, we cannot
guarantee that a particular sample will be taken at a particular time. With hardware timed data acquisition the data acquisition rate is determined by a hardware clock, not the iteration rate of Windows. Depending on your application that is ok. Please correct me if I have mis-understood your question.

Here is a KB with some loop benchmarks for different operating systems, I found it interesting.

 Have a great afternoon!

 Best regards,

 MatthewW
 Applications Engineer
 National Instruments