Digital acquisition affects the data

[abdel2]

I have 2 different VIs running simultaneously. First VI is reading analog data from 3 different channels at 24kHz. Second VI is reading digital data at a rate of 1Mhz. Physically, there is a GO signal that is coming from another machine. This signal is split 3 ways: goes to PFI0, P1, and ai3. Second VI is simply reading P1 to see when it is high and then it does something. First VI is reading ai3 and presenting it. Whenever the second VI is running, the GO bit I observe in the first VI changes from 1 to 0 randomly (it's suppose to be 1 all the time). I am checking the same signal on an oscilloscope and it looks fine. Is there something wrong with the DAQ internally?

I have attached the second VI's image. If I reduce the timed loop frequency to 10kHz, it seems to be ok. Why is that?  

Some additional information:

The first VI is actually acquiring data based on the the GO trigger. So as soon as GO goes to 0 it should stop data acquisition but it doesn't in the case of random behaviour. If I manually set the GO bit to 0 from the other machine, it will stop. It means the GO bit is not affected on the physical line but it is somehow changed internally onces it's read using analog read. 

[NI-Bob]

Hello Abdel,

Thank you for contacting National Instruments! This is very interesting behavior that you have encountered.  I am looking into some possible reasons.  Can you please let me know what hardware you are using in this system?  This will help me troubleshoot this issue further and help determine why this is occurring.  I look forward to hearing from you!   

[abdel2]

I am using DAQ card 6259 with BNC 2090A connector block. The software is running on Labview real-time on a desktop computer. It's communicating with a host (development) computer using TCP/IP. I have the latest version of everything.

Thanks. 

[NI-Bob]

Hello Abdel2,

Thanks for getting back to me!  After looking through your information with your hardware I am wondering if you are running into a bouncing issue with you signal.  Does your Go signal continue to randomly go from 0 to 1 continuously or does it eventually become consistent if left running?  If you can, please try to take a look at your signal and determine if bouncing could be occurring.  Also, please take a look at this knowledgebase article as it gives information about setting up the digital debounce filter on an M series card like your 6259.

Another question I have for you is what value are you sending out to the 6259 as your Go signal?  Can we verify that you are outputting 5V for a 1 signal and 0V for a 0 signal.  I have seen certain issues when trying to output 3.3V for a 1 signal and because that is so close to the edge of a TTL signal it sometimes will not give consistent results.  However, since you are not running into this issue at a lower frequency I would be more inclined to say that this is an issue with a bouncing signal.

Please post here if you have any further questions after reading through this information and I will be happy to assist you.  I hope this helps!  Have a great day!

[abdel2]

Thank you. I am not working until after about 2 weeks, but I will try both of these things. I'll get back to you on that.

[Matthew_Kelton]

I am curious as to whether you are getting an error that is being ignored in your first VI.  I see from your JPG for your second VI, you have error clusters which are not monitored.  If you get an error when you are trying to measure the signal, the output of the VI will be 0.