PCI-6111 triggers on the wrong edge

[Angelotti]

Hello,

I want to measure with the data acquisition board PCI-6111 the running time of an analog signal. For the start of the measurement I use a digital signal with a period of 4 ms, where I trigger on the falling edge of the digital signal. My problem now is that the PCI-6111 sometimes triggers on the rising edge, so that my measurement is wrong. Why triggers the data acquisition board sometimes on the positive edge of the signal, if I set the trigger on the falling edge? The VI of the measurement task and a view of the digital signal I have attached this issue. Thanks for your answers.

[John_P1]

Hi Angelotti,

The likely case is that there is a small noise on your signal that is causing the rising edge to also register as a falling edge.  If you were able to zoom in enough on the trigger signal, you'd probably find something like this:

       

In the above case, the board would detect this signal as a rising edge, followed by a falling edge, followed by another rising edge.  There is no built-in hysteresis or schmitt trigger for digital triggering on the 6111.  Newer boards do have digital filtering that could prevent this behavior from occurring, but the 6111 unfortunately does not.

Having said this, the good news is that the 6111 does support analog triggering on PFI 0.  You can configure an analog start trigger and utilize the hysteresis setting so that you will not receive any false triggers.

Best Regards,