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Output Always High

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#1 Peter J Fischel

Peter J Fischel
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Posted 25 December 2012 - 10:43 PM

Happy Holidays folks! I have a U3-LV and I am trying to monitor the output of a CPU. The issue I am having is that no matter what I do the IO ports on the U3 are always high and must be grounded to drive them low. I have verified the CPU is indeed driving that line low with a simple LED. I use a when I check the IO from the U3 I get a high signal. I would suspect that since this is a IO port, that for input there would be no voltage at all. Another thing is when I test this using the control panel I see the same effect. Must ground it to get a signal. Is Anyone else having this issue, and how have you resolved it? thanks, Peter J. Fischel

#2 LabJack Support

LabJack Support
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Posted 26 December 2012 - 12:31 PM

The digital inputs on the U3 have a ~100k pull-up to 3.3 volts. So if you leave the terminal floating they will read high, but if you drive it to ground with an impedance of less than ~22k it will read low. You've seen that a short to ground causes it to read low, and you should also see that a 10k resistor to ground will cause it to read low.



Perhaps your CPU is providing a PNP style signal that changes between Vcc and open, not Vcc and 0? Basically the opposite of the typical NPN signal:


Can you provide a link to any detail or electrical specs for your signal?

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