USB scope triggering off wrong digital edge

[ASpektron]

All,

 I have setup my USB scope card (5133) to trigger of a positive digital edge on the PFI input. This then captures rising analgue connected to the two channels. This normally works fine, but sometimes the scope will start to trigger off a negative edge! (digital input signal is a digital pulse). To get the scope to work correctly again I have to set the scope to trigger off an analogue edge on ch0/1, then after runing the vi once return to the digtal edge trigger on PFI1. I have attached an image of the setup of my scope.

[Matt_E.]

Hello,

I don't immediatly see anything wrong with the screenshot of your VI.  I have a few questions about your setup that will help narrow this down.

1)  What are you using as the source of your digital trigger on PFI 1?  Have you verified that this digital signal meets the VIH and VIL levels specified in the 5133 spec sheet?  http://digital.ni.com/manuals.nsf/websearch/8D323D9E4F4B39288625737C0071E462  (Page 7)  Also make sure you are not exceeding -0.5V or +3.5V and that your pulse width is greater than 20 ns.  The easiest way to verify this would be to run the configured acquisition example VI with your digital trigger signal running into one of the analog input channels.

2)  How are you connecting your signals to your board?  Are you using BNC cables or probes?

3)  I am assuming that the signals you are digitizing on ch 0 and ch 1 are different than the signal you are feeding into PFI 1.  If you have a BNC splitter handy try splitting the signal you are feeding into PFI 1 so that it also runs into one of the input channels.  Leave the other analog input channel unconnected.  Then run your VI again and see if you see the same phenomena where it looks like the scope is triggering on the falling edge of PFI 1.  If the scope is triggering on the falling edge you should see the falling edge waveform captured on the analog channel.  This will help us determine if the scope is triggering on the wrong edge or if your input signal and digital rising edge are not always correlated.  If you are using probes instead of BNC cables you can do the same test, just connect both probes to the same signal.

Regards,
Matt