Counter counts twice as many edges as it should, but correct frequency. Why?

Hi everyone,
I am using a PCI 6122 (S Series, DAQ-STC) and Labview 7.1 on a Windows XP. There are only two counters on this card (Ctr 0 and Ctr 1) and I need to use both of them to count edges. While testing the counters, I am inputing a TTL signal of 10kHz to the Gate of each counter, to be used as an external sample clock, and a 20Hz TTL signal into the Source of each counter, to be used as the input signal. Ctr 0 counts 40 edges per second, i.e. twice as many as it should, while Ctr 1 is reading the correct number of edges, i.e. 20 edges per second.
The strange thing is that when I switch the connections of Gate and Source for both counters to measure the frequency (or period), both counters read the correct frequency, i.e. 20Hz. To the best of my knowledge, both counters were reading the
correct number of edges a couple of weeks ago. Also, I have reset the
device several times but it does not make a difference. I am trying to figure out why this is happening and how to fix it, but I am no closer to answering either question. Does anyone have any suggestions?
Thanks.
Solved!
Go to Solution.

After many tests on my own and after speaking with a NI engineer and repeating some tests over the phone, it seems like there is something wrong with the (one) counter on the board. I have sent it back for repairs (thankfully, it was still under warranty .
Message Edited by DanceWiz on 09-15-2009 09:58 AM

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Setting output pin to high while counter counts edges - 660x

Traditional DAQ allowed you to set the output of a counter while it counted, is there an equivalent for daqmx? The counter output for our current setup spins a motor until the counter reaches the proper number of counted edges from another pulse generator on the board. Perhaps I need to reconsider the setup, but there should be a method to do this action. I was only able to find an indicator of the output state in the Channel property nodes. Any ideas? Thanks.

Hello Roth,
An event is thrown whenever a counter reaches its terminal count, and you should be able to use that to your advantage here. Once that terminal count is reached, you can use DAQmx properties to set the counter to output a pulse or toggle the line. If you use this property in conjunction with the initial count property, you can have the counter cound X number of edges and then toggle the output line:
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Message Edited by Justin M. on 06-12-2006 01:05 PM
Attachments:
Terminal Count Toggle.JPG ‏11 KB

Why does my DAQ counter count more when a capacitor is used to eliminate noise

Hi,
In my application i need to measure the speed of the motor using an encoder. The output of the encoder is a bit noisy (around 200 mV of noise.).Actually we are using a Siemens variable speed drive and it is the switching noise even after perfect shielding. Also the ouput of the encoder has a DC shift in waveform fr 383mV.When i connect to the counter channel (source :to count the pulses or gate :to measure frequency) the readings are high than actual. Then we conected a 0.1 microfarad capacitor across the counter channel and the DIO Ground. Then checked the waveform using a 60 mHz tecktronic oscilloscope. The waveform was without noise but the shape was a bit distorted, meaning there was no sharp rising edges and has a slow rise.But still there was a dc level shift for 383 mv.When i measured using counter the count was abnormal.Actual reading should show 1000 pulses /second, but with a capacitor it was counting 4789,5000, 356 etc.
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While putting the capacitor between your signal and ground will git rid of any transients, it will also "round" out your signal. Your square wave provided by the encoder can be approximated by the sum:
y(t) = a_0 + sum(k=0...infinity, a_k*cos(k*f*t)) + sum(k=0...infinity, b_k*sin(k*f*t))
The capacitor has a frequency-depenent resitant (called reactance) which decreases with increasing frequency.
The reactance is:
X(f) = 1 / 2*PI*f*C
Where f is your frequency and C=1uF. What happens is the capacitor shorts the higher frequency harmonics to ground and makes your square waveform "ring" at the edges.
If this ringing causes you to swing high-and-low close enough to the switching point, then you could count pulses that really aren't there. What you really need is some sort of hysteresis to prevent this false switching. You should put this after the bypass capacitor and before the counter.
Also, another question is: What are the output levels of your encoder? The 383mV DC offset shouldn't affect a counter, but if your logic "high" voltage doesn't exceed about 2V then you might be hitting a region of metastability: where the counter randomly switches high/low.
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competitor A 20-2-2012.vi ‏139 KB
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