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
Output Always High
Started by
Peter J Fischel
, Dec 25 2012 10:43 PM
1 reply to this topic
#2
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.
http://labjack.com/s...users-guide/2.8
http://labjack.com/s...s-guide/2.8.1.1
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:
http://labjack.com/s...s-guide/2.8.1.2
Can you provide a link to any detail or electrical specs for your signal?
http://labjack.com/s...users-guide/2.8
http://labjack.com/s...s-guide/2.8.1.1
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:
http://labjack.com/s...s-guide/2.8.1.2
Can you provide a link to any detail or electrical specs for your signal?
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