Trigger problem with NI DAQPAD 6015

[stephenzhou74]

Hello:

I have a 0~5V square wave trigger signal (see figure 1). After I connect it to the PFI0 of DAQ card, it becomes 0.6V~5V (see figure 2). What could cause this problem, and how can I fix it? The DAQ card I am using is NI DAQPad-6015 (for USB) 200 kS/s, 16-Bit, 16 Analog Input Multifunction DAQ.

When I tried to use this trigger signal as external digital trigger (and set falling edge), the acquired data is not stable. Sometimes it is triggered from falling edge, sometimes it is triggered from rising edge. How can I solve this problem?

Any comments or suggestions will be greatly appreciated!

Stephen

[Sean_C.]

Hello Stephen,

How are you connecting your trigger signal?  Make sure that you have the positive reference wired to PFI0 and the ground reference wired to DGND. 
How are you determining on which edge the 6015 is triggering?  How fast is your square wave?

Please let me know these details and I will be happy to help you trouble shoot.

Regards,
Sean C.

[stephenzhou74]

Hello Sean

Thank you very much for your reply.

I connected my trigger signal to the BNC connector of PFI0.

At the same time, I also connected this signal to channel 1 of the DAQPAD 6015. I set the falling edge as the trigger signal. I expect to get signal like figure 3 all the time, but sometimes I get signal like figure 4. The square wave is about 4 Hz.

Thanks for your help!

Stephen

[Sean_C.]

Hi Steven,

I just ran the shipping example Acq&Graph Voltage-Int Clk-Dig Start.vi with a 4 Hz square wave wired to AI0 and PFI0.  I was able to trigger off the correct edge every time.  Try running this shipping example.  You can find it from LabVIEW by selecting Help >> Find Examples... Then browsing to Hardware Input and Output >> DAQmx >> Analog Measurements >> Voltage.  Do you get the same results?

You mention that you connected your signal to the BNC connector of PFI0.  Are you using a BNC connector block?  Do you have the Mass Termination DAQPad-6015?  If so, try using a different cable and connector block.

Finally, try using a different function generator to generate your square wave.

If you are still seeing the same problem after all of this, I recommend contacting National Instruments to send your device in for repairs.

Let me know your results.

Regards,
Sean C.

[stephenzhou74]

Hi, Sean

Thank you so much for your reply.

If I use the trigger signal from Stanford Research function generator, it works fine (and the trigger signal is still 0~5V).

I don't know why my trigger signal will float 0.6V after connecting the PFI0. But I found out that it will trigger off the correct edge every time when I use some "sampling rate (e.g. 12000 Samples/s)" and "number of samples (e.g. 12000)" combination, but does not work for other settings (e.g. 10000 Samples/s and 12000 number of samples). I don't know why, but at least it works fine under some conditions. It is OK for me.

Thank you!


Stephen



BTW below is the reply from NI.

Thank you for contacting National Instruments.

I'm not sure what might be causing your voltage levels to be higher,
but
0.6V still falls within the spec for a TTL "low" (0V to 0.8V), so I
don't
think that is causing your unstable data.

There are two things I recommend. First, upgrade to the latest DAQ
drivers
for your device. The NI DAQPad 6015 is compatible with NI-DAQmx 7.5,
which
is a free download from our website. Also, try using one of the
examples
from the Example Finder (Help>Find Examples...). The example named
"Acq&Graph Voltage-Int Clk-Dig Ref.vi" seems appropriate for your
application. Save a copy of this example, and edit it to fit your
needs.

[Sean_C.]

Hello Steven,

I am glad to hear that you can get this working.  It seems strange to me that it will work only for certain combinations of Sample Rate and Number of Samples...  The only other suggestion I can think of would be to try passing your trigger signal through a low pass filter.  It may be that your signal is noisy, and it is triggering off of the noise rather than the true falling edge.

I hope this helps.  Please let me know if you have any further questions.

Regards,
Sean C.

[Brian T]

I jumpered AI0 to PFI0 to observe the trigger input signal. I used a vaiable power supply to generate the input signal and ran the example program Acq&GraphVoltage-Int Clk-Deg Ref. This allows pre-trigger with PFI0 triggering. The display positions a cursor at the trigger point. Analog triggering is not supported with daqpads.

I found that there are frequent buffer overruns that are not apparent to the user. That is, some data taken at the end of the acquisition period apear at the beginning of the data buffer. You will observe data to the right of your plot appearing on the left with a discontinuity at the point where the overrun or overwrite ends. The next effect is that the data on the left of the plot, where you thing your trigger data resides has been over written with data at the end of the acquisition period and therefore become confused with regard to what voltage level or slope triggered your daq. In reality the PFI input with digital triggering induces an edge trigger at approximately 1.3 volts.  I used 10KHz sampling with 10K points. I suspect the problem may not appear for smaller amounts of data, say 10KHZ samping for 1000 points.

This seems to be a problem with buffer memory limitations in the USB daq devices and of course the use of serial USB to xfer the data. My solution is to return the daqpad and procure PCMCIA cards. The application works perfectly with an M-series PCI card AND IT SUPPORTS ANALOG TRIGGERING!

[Erbee]

Hi,

I am using DAQPad 6015 and I have a little problem with the buffer size. Actually, I am trying to triggerize an application that is used in my job. So I want to acquire a signal and save a part of it. My problem is that I am not used to use DAQmx and that I always have a problem of buffer when I try acquiring my data. I want to know if my problem is due to a wrong use of Labview or if I can't do all these with this device. I also want to triggerize my application without any signal of reference, only when it changes. Any idea? I look forward to hear from you and thanks you in advance for your answers.

[RT4CY]

Are you receiving a specific error which indicates you are overflowing your buffer?  It would be helpful if you could post the error code.  Are you doing any post-acquisition processing?

You also had questions about triggering.  The DAQPad 6015 only supports Digital Edge triggering.  If you are acquiring an analog signal, and want to trigger off of some change, you will need to implement this in software.

If you are receiving buffer overflow errors due to post-acquisition processing, I would suggest implementing the Producer/Consumer pattern.  In order to trigger off of some change in your signal, I would suggest implementing the State Machine design pattern.  You could combine the two to build the application that suits your needs. 

These are not basic concepts, but don’t worry!  We provide plenty of Developer Zone tutorials to help you build your application.  I would also recommend searching through the NI Example Finder.  Open LabVIEW and go to Help»Find Examples.  You can Browse by task, I would suggest taking a look at the following folders.

For examples using DAQmx Programming:
Hardware Input and Output»DAQmx»Analog Measurements

For examples using State Machines:
Industry Applications»Test Sequencer

I would also strongly suggest taking our LabVIEW Basics course.  You will learn how to apply the state machine design pattern, as well as more advanced topics such as event-driven programming.

www.ni.com/training

Message Edited by RT4CY on 07-18-2008 02:05 PM