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Why does my PXI-6509 exhibit unstable inputs?

Hi All,
I am setting up a self test application that is supposed to loopback test all 12-ports by connecting ports 0-5 to ports 6-11 (bit 0 to bit 0 etc).
The intent is to:
Set ports 0-5 as outputs and ports 6-11 as inputs.
Set each output bit individually
Confirm the generated bit pattern is reflected by reading the inputs
Reverse by setting the output ports as inputs and the inputs as outputs and repeat the test
 
The issue is that no matter which port I use as an input it always exhibits erratic behavior, ie I set bit 3 on the output but the input reflects bit 3 and 4 go active - very intermittently, also shows up on other bits but without much repeatability!
 
I think this may be because the inputs need external pull-ups but can find no documentation regarding the output circuitry of this card.
 
Any ideas?
 
Thanx in advance!
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Hi nkambe,

First I would like to confirm how you are testing this operation.  When testing a digital card, it is always a good idea to use Measurement and Automation Explorer and set up test panels to confirm that the board is reading correctly.  You can set the direction as input or output depending on the port and then drive the output line high or low.  If you have the lines connected correctly from one port to the next, you should be able to read on the input ports, what you are outputting in test panels.

There is a knowledgebase that describes connecting an external pull-up resistor to a DIO line.  However, this document specifies this value should equal the value of the internal pull-up resistor from the DPULL pin on the card.  This other knowledgebase shows those internal values.  Your card though, seems to not use internal resistors in order to driver the line since the DPULL pin is not on the pinout. I also couldn't find any information in the documentation about these values. 

First try testing your device and the connections thouroughly because this should not be an issue.  Then you can look into different resistor values based on what the other cards use and see if those correlate.  Hopefully this will help!
Regards,
Vanessa L.
Applications Engineer
National Instruments
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Hi,

Thank you for your response.

I found the problem and it was operator error - things work as expected now.

 

Thanx again

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