pxi external trigger timing

[GlenS]

Hi,

I am using a PXI-5122 to record an externally triggered event.  When I run my setup using an oscilloscope the trigger and response are aligned (see first attachment, response is the noisier, channel 1 curve).  When transfer the setup to LabVIEW the trigger is mis-timed and seems to occur before the trigger is actually received (see second attachment).  The trigger point in the second curve should be at the beggining of the rise and this should also be at t = 0.  The trigger I am using is the NI-SCOPE External Edge Ref Trigger (VI included as third attachment).

I assume this problem is caused by the trigger being a reference trigger but I can see no other sorts of trigger to use and I don't know what to do about this problem.  Can someone help?

[John_P1 · Accepted solution]

Hi Glen,

It looks like you're correct that the problem is that the Scope is configured to use a reference trigger.  By default, the reference position is 50%, meaning half of the samples in a fetch will be before the trigger and half will occur after.  The Scope accomplishes this by continuously storing data as it is acquired in a circular buffer so that the most recent samples are available when the reference trigger is received.

With this in mind, the fix is really quite simple--you can specify the Reference Position as an input to the niScope Configure Horizontal Timing VI:

Just set the reference position to 0 (default is 50) and you will receive all of your data after the trigger occurs.  A value of 100 would give you an entire record of pre-trigger samples.

Best Regards,

John

[GlenS]

Brilliant, thanks!  Didn't think to look at that VI. 

Message Edited by GlenS on 10-20-2009 05:18 PM