Frequency/Period measurement with USB-6218

I have been trying to get a USB-6218 to measure either frequency or period and have been running in to issues. I have attempted to connected to each counter gate (PFI1 or PFI2) and have not had any luck. I am attempting to meausre lower-frequency signals (1Hz-1KHz) so the Low-Frequency, 1 counter approach seems to be the one I need. The end goal is to get it working in Labwindows. I have made attempts using both sample LabWindows programs as well as MAX. In each of them, it appears that the frequency measurement is actually coming back with the pulse width of both the low and high portions of the signal. For instance, if I am trying to meausre the period of a 100 Hz signal, with a 75% duty cycle, it will toggle between 0.0075 and 0.0025 seconds for the period. If I increase the frequency of the signal to 10 KHz, it measures the frequency well. Is there something I am missing? Based on the manual, I should be able to meausre lower frequency signals using this method. Any help is greatly appreciated.
Thanks.

Can you upload a screenshot of your Task in MAX is setup? Is this happening at all lower frequencies or just at 100 Hz? Lets make sure we can get this working in MAX before adding LabWindows/CVI on top of it.
Sometimes this can happen if the rise/fall time of the signal is not within the specifications of the device. This KnowledgeBase article shows the acceptable rise/fall times.
Applications Engineer
National Instruments

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Hello;
I can't think of any way to accomplish that task. Once the counters have only two input pins, and both are in use when one configures the counter to do buffered measurements, there isn't a way to trigger that operation in hardware.
I'm not sure if I understood your second question, however, the buffered operation on a counter is hardware controlled, meaning that a pulse on its gate input triggers the transfer of the count value from the counter register to the PC memory buffer. That is therefore a continuous measurement.
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Help with DAQmx tension measurement and saving to spreadsheet file with NI USB-6218 (very urgent !)

Hello people,
I've been given at the last minute a project at school where I need to do some data acquisitions (tensions) with a DAQ NI USB-6218 and it has
been miserable ever since, I first tried the "easy" way with DAQ assistant to save a given number of samples per channel to a text file
in order to use it in matlab, well...it isn't working : every time in my text file I have a different number of samples, I used the "Write to spreadsheet" function within a while loop where I wired the path outputs to a shift register in order not to have it asking me the path all the time,
but I'm still not getting the X number of samples per channel as specified at the beginning.
So I've been using another method with the DAQmx where I get each individual waveform into a vector of samples, when I look for the size
of the vector it looks Ok, 100 in my case but the problems start when I try to have a text file where each column will represent a channel for
subsequent matlab treatment, what I'm doing doesn't seem to be working, it's really unbearable, please help me sort this mess out.
I'll add both VIs as attachment so it can help you help me, thank you.
Basically what I need is to acquire 15 tensions with a given number of samples per channel (the same for each channel) and then plot it in matlab.
Note the first VI has switches to "unblock" each channel but it doesn't seem to be working, please look at both VIs.
It's very urgent, I could fail my school year if I fail this project.
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Hi Thank you very much for your help, in the meantime, I've changed the project to (see attachment), now
it is at least writing the waveforms to the txt file, but again there is an another problem, it seems to be ignoring the
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Problem with pin reference (USB 6218)

Hi evryone,
I am confused concerning the pin references for USB-6218. MAX says to use pin 15 and 16 for a differential AI (AI0/AIground) on the first channel. This seems correct because my measure looks ok. But when I look at the data sheet (http://www.ni.com/pdf/manuals/372085b.pdf) these pins correspond to PFI 6/P1.2 and D ground. The AOs don't match either.
I must be making a mistake when reading the data sheet. Would anybody know what I am doing wrong?
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User
Solved!
Go to Solution.
Attachments:
Forum.PNG ‏56 KB

Hi User_1979,
Did you use a simulate device in MAX?
These Connections refers to this datasheet of the USB 6218 (P.15). Your measurement seems ok between pin 15-16 (AI0-8), the behaviour is normal.
Also you can find the device pinout within MAX by right click on the device and click to view the pinout.
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Last edited by esdrasbeleza (2009-09-02 01:16:09)

Frequency/​Period measuremen​t with USB-6218

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Frequency/Period measurement with USB-6218