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Is the U3 right for my time measurement project?

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#1 spackler73

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Posted 02 May 2014 - 12:58 PM

Hi All,


I work with industrial automation equipment and need to accurately measure the time interval between two separate inputs going high (rising edge). Each of the inputs is 24 VDC and each event lasts anywhere from 1 to 40 milliseconds. We are currently using an expensive Ethernet packet monitoring tool that supports physical inputs with a resolution of 20 nanoseconds. The software that comes with this device can record and plot successive events and compute the average of all the samples. We generally run the test for 1,000 samples.


Is there a way of using Labjack hardware and software (or third party stuff) to do the same thing without this becoming a science project for me? I'm really looking for a lower cost solution that me and others in my group can easily use.


Thanks in advance,

#2 LabJack Support

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Posted 05 May 2014 - 03:18 PM

All our newer devices (not the U12) have line-to-line timers, but the U3's clock is only 1.5% accurate, so I would look at the U6 or T7.


The U6 has 4 timers that can support up to 2 line-to-line readings.  The values are 16-bit, so you need to take care that the values do not overflow.




The T7 supports 1 line-to-line reading.  The value is 32-bit so little concern about overflow even with 12.5 ns resolution:





You say the signal is 24 volts when high.  Do you know if it is driven or PNP?  You will need a 22k series resistor to limit current for the 24 volts, and if it is PNP you will also want a 4.7k pull-down (which works with the driven signal also).  I would be a little concerned about the 22k resistor interacting with the input capacitance of the LabJack and slowing down the signal, so we might have to test that if want to pursue the U6 or T7.  The slow-down should be the same on both lines and thus mostly cancel out, but worth looking at.



For software, we need to read each value before a new value occurs.  You say each event lasts up to 40 ms, but what is the minimum time between subsequent events?

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