Delay between real signal and acquired signal

[JCNT]

Hi everyone,

I  have a DAS-TC  pci card from measurement computing which acquires data from a thermocouple and these data are displayed in a XY Graph in LabVIEW. I am using the Universal Library  for LabVIEW from Measurement Computing to communicate LabVIEW with the DAS-TC card. The problem is that all the data that the thermocouple is acquiring are not being displayed in the XY Graph Waveform. Even if the data are displayed in an array they are not complete. For the acquisition I am using a While loop without any delay.

Thanks in advance and I really appreciate any help.

JC

[Dennis_Knutson]

An XY Graph is not designed to show historical data. Could that be your problem? A chart would seem to be much more appropriate to display data as it is acquired. A chart also has a history buffer to retain old data. Why don't you attach your code?

[RavensFan]

I'm confused.  The subject of your message seems to be completely different from the question you posted in the body of the message.

[JCNT]

Hi Dennis and Ravens,

Thanks for reply, I attached my code in order to clarify my problem. Let's say that I run this VI for 5 seconds and after that I got eight hundred data in the XY Graph and in the array "Data out", but in the reality the real data were one thousand.

Thanks again and I hope you help me, by the way the subject was totally wrong, because the problem it is different, I am sorry.

JC 

[J-M]

I have no experience with this PCI card from MCC but I have few devices from this company.  When you said the real data were one thousand, how did you estimated that number?

Jean-Marc

[JCNT]

I know becuase I have another device connected to the thermocuple and I can compare the data which displays  its screen and the XY Graph and I know there are some data which are missed because they are not neither in the waveform nor in the array.

[Dennis_Knutson]

Then I suspect this other device is sampling at a different rate. When I enable the random number generator in the subVI and run your program, the number of time the subVI is called is equal to the number of points in the graph and in the array. The subVI has to return something - even NaN,  and there is no way that elements of the array can be dropped.

[JCNT]

For sure the other device is sampling in a high rate  and I do not know if would it be possible to get the same in LabVIEW. I know that will depend of the DAS-TC board (where I have changed the number of samples/avg and think it is the sample rate I attached a figure) but I called to measurement Computing and told them about the problem and they said that their boards have never had that problem before and it could be the program (I've heard that many times and makes me upset becuase I consider LabVIEW is the best software)

Thanks for your support.

JC 

[Dennis_Knutson]

The only software that affects the sample rate is the dll that you are calling and there is no sample rate specified. It's returning a single sample and the LabVIEW code would not seem to be a factor at all unless and until the array you are building gets quite large and time is spent reallocating memory.

[JCNT]

 I got more than two hundred data for 5 seconds with no delay in the while loop so this could mean that the dll and the building array are consuming time and that is why I have not been able to get all the data? Should I change the board or the software in order to get a higher sample rate and get all the data in the array?

JC