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U3 Has Too Many Counts


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7 replies to this topic

#1 RJE

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Posted 20 July 2012 - 04:26 PM

Greetings First asked at Azeotech, thinking it was a scripting problem - does not seem to be that. Tried the suggestions from LJ support and get same results. For about a 16 Hz signal I'm getting 86 - 90 counts and I just can't seem to find out why. Hope you can give me some ideas to look for. I've attached 2 screen shots of the signal, one from my Fluke Scope and the other from a NI USB analog unit feeding Signal Express on my laptop. Just not sure what to look for anymore. Thanks Randy

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  • Flow_copy.jpg
  • flow_cnt.JPG


#2 LabJack Support

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Posted 21 July 2012 - 09:40 AM

Are you using a timer or a counter? If you are using a timer what mode is it set to?

#3 RJE

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Posted 21 July 2012 - 03:15 PM

I'm still using a counter because I'd like to figure out why it can't count such a low frequency. It's become one of those little annoying things that you want to figure out. Guess I will try a timer and see what I get with that. Thanks

#4 RJE

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Posted 21 July 2012 - 04:18 PM

Interesting. Just tried a timer as firmware counter and I get the opposite result. For about a 16 Hz signal I only get 4 counts. This would point to the signal as being the culprit would it not?

#5 LabJack Support

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Posted 23 July 2012 - 07:31 AM

This is strange. You signal capture looks very clean. I think the counter and timer should both work fine. Perhaps there is a grounding problem. Verify that the LabJack's GND is well connected to the GND for the signal source.

#6 RJE

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Posted 29 July 2012 - 04:32 PM

After slapping myself up side the head for forgetting to tie the U3's gnd in I thought I would be good. Well not so. I scoped FIO4 pin with my script cycling through the flow sensors. Using FIO4 as a counter and I extended the capture time for each signal. Input signals are as shown before with Hz readings of 16, 8, 15, 18 and 18. Counts captured are 90, 30, 47, 45 and 21. For some reason the last one is pretty close to the scope Hz readings - can't figure out why. Interesting to note that when I use my Fluke meter and measure the Hz of the signals I get the same value as the U3's count value - can't explain that either. Since I'm only really concerned if I loose flow (0 counts) - tested this and it works - 0 counts flashes a red LED - I might just have to live with it. Problem is using counters in future. Still scratching my head on this one.

#7 LabJack Support

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Posted 30 July 2012 - 11:51 AM

This is strange. The LabJack usually reads frequencies with little or no difficulty. Here are a couple shots in the dark: 1) Check for GND loop issues. Connect a 1kΩ resistor between the two grounds and measure the voltage across that resistor. If you read more than a couple millivolts then there is power flowing over that GND connection which could be causing read errors. 2) Maybe there is some noise that gets filtered out when you try to probe. Try adding a filter to the signal line. Connect a capacitor from LabJack's input to GND. And a resistor from the LabJack's input to the signal source. For a starting point you might try 1kΩ and 0.1uF. From there up to 10uF should still allow your signal through. 3) Maybe the problem is related to the channel switching. Try reading just one input without changing the pin offset.

#8 RJE

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Posted 02 August 2012 - 05:45 PM

Greetings #3 - did that already, forced the address lines from control panel - got same readings #1 - 1k resistor to ground - I get 7.7 mV across it. When I tie the whole control circuit common to house ground I get 18.3 mV across it. This is where ground becomes really confusing DF is running on a computer in a HTPC type of case. From the USB port of that I go to a USB hub where the U3's are plugged in. The power supply for the hub has no ground pin. My whole control circuit has 5 switching psu's providing various levels of voltage - all their commons are tied together but left floating of real house ground. #2 - put a 0.01 uf ceramic from FIO4 to ground - BINGO! All counts are normal. I really have to figure out this computer grounding thing better for the future. Thanks for your help. Randy


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