Out of control,the measurement auto started

[RichardWang]

I configured a measurement with a start trigger(rising edge) and a stop trigger(falling edge).
Both start trigger and stop trigger use PFI0 as the trigger source.
As I start the measurement with "DAQmx_StartTask " function, the program should wait until expected edge of the trigger signal on PFI0 occurs. But the fact is,after several seconds,the measurment automatically started while the trigger signal has not occured yet.
I inspected the trig singal and found some noise signal in the trig signal channel.but their amplitude is very small(about 50mV),can these small signal trig the measurment to start?

my DAQ card is PCI6220.

[RichardWang]

I use the demo code installed with DAQmx program, but get the same result.(the "timeout" parameter in DAQmx "Read" function is set to "wait_infinite".)

[AnujD]

The noise spikes you are seeing are around 50 mV. If you are using a DMM, the DMM is probably averaging your noise using RMS, so you could potentially be getting bigger spikes.

Try connecting the Trigger input to ground and see if you are still getting random triggers.

Regards,
Anuj D.

[RichardWang]

Thank you for reply.
I connect the trig signal(PFI0) to ai2,and use Measurement automation-->DAQmx Devices--->PCI6220--->TestPanel-->analog input to view the trig singal as an analog signal. Then I see the peak voltage of noise is about 50mV.

Is it possible that the "fake trig signal"'s frequency is high enough and the sample rate of "test panel" program is not high enough that it cannot capture the "fake trig signal" that really trigged the measurement to start?

[AnujD]

Richard,

I'm not following what you just responded. Can you explain your theory furhter?

Regards,
Anuj D.

[RichardWang]

I suspect that there are some random noise signals that have enough amplitude to trig the measurement.(The actual trig signal has not occured yet)

So I connect the trig signal channel(PFI0) to analog input channel 2(ai2),and want to use the test program in "Measurment Automation" to view the realtime signal waveform of the trig channel.

But from the waveform I cannot find the noise signal that has enough amplitude which can trig the measurement.So I suspect that "fake trig signal" has a very high frequency and the sample rate of analog channel is not high enough to capture the "fake trig signal".

[AlanArmstead]

Richard,
Check out this example program document, looks like it might be what you are looking for:
http://sine.ni.com/apps/we/niepd_web_display.display_epd4?p_guid=E5BC1A8028A1541AE034080020E74861&p_node=201189&p_source=External

-Alan A.