Problem of time on a chart during an acquisition with pretrigger

[nono973]

Hello,
I develop under labview 8.5 and I use the device NI USB 6210 to carry out acquisitions.

I carry out acquisitions with pretrigger (vi DAQmx trigger: reference digital edge) like the example: acq & graph voltage int clk dig ref.vi (search NI example finder: acquisition).

I observe problems of time during my acquisition. Indeed the chart indicates a time to me advances some over real-time. The values of acquisition are good but not times.
I think that the problem is related to the position of reading in the circular buffer.
Somebody could say to me if it observes the same problem.Is there a solution?

The example:

example:

 thanks.

Arnaud

[Norbert_B]

Arnaud,

timestamps are not generated by the ADC device, but when the data is transferred into the application memory. So timestamps can be off for quite some time.

It seems in addition to this, that the timestamping does not take into account the offset which is generated by the pretrigger scans..... but you could take this into account in your software.

Why the chart is off by four seconds on the left in comparison to the system timestamp is a bit unclear to me if the chart displays all data....

Norbert 

[nono973]

Thank you Norber for your answer.

you carried out the test on the example of labview. Because I did not carry out any modification, and the example should function normally.

the chart posts me a time which did not pass yet in reality.

I think that it is necessary that I act on the position of reading in the circular buffer but there are no possiblities.
excuse me for my English I am French.

Can you carry out the experiment on the same example and say to me if you have the same problem please

thank you

Arnaud

[Norbert_B]

Arnaud,

please see attached VI...

hope this helps,

Norbert

[Edit] Added wrong version of VI, updated with VI version LV 8.5 

Message Edited by Norbert B on 07-17-2009 09:20 AM