USB 6008 shows digital signal with nothing connected

[nckeeley]

So I'm really hoping this isn't as bad as I expect it is.

I've been having problems reading from my USB 6008 digital input this morning. Everything was working fine last night. Now all the digital lines on the device read a signal whether there is anything wired to them or not. When I run the DAQ Assistant and test the line, it shows a signal in every line. Then I made the attached VI (which should do the same thing) and it shows a constant signal in every line also. And I have nothing wired to the device. Does it need reset somehow or is it broken? Or am I just missing something?

[johnsold · Accepted solution]

nckeeley,

Look at pages 21 and 22 of the Ni USB-6008/6009 User Guide and Specifications.  The digital lines have a 4.7 kilohm pull up resistor to +5 V.  That will make an opne input line look (and be) high.

Lynn

[nckeeley]

Thanks for the reply. Afraid I'm still a little confused though. If I plug the two lines of my rotary encoder in (for example), one should show as high and one should show as low. I tested the voltage in the lines and they are correct (reading ~0.01V and ~3.9V). But my DAQ still reads as both lines being high. How would my device know if the line is 'open' or not? When I define my input line for the digital signal is that supposed to 'close' the line?

I'm sure I have the correct lines specified in my program, but I still get all lines reading as high.

Thanks for your help on this.

[nckeeley]

I should know by now that when odd things start happening, start by checking the ground. Thanks for your response and sorry to waste your time...

[johnsold]

Grounds are important.  The current has to go somewhere.

I am glad you figured it out for yourself. Trouble shooting bad ground connections via the Forum is sometimes quite frustrating.

Lynn