How to acquire continuous data with a PXI-6551

[Bob_Albern]

I am attempting to acquire a continuous low voltage data stream with a PXI-6551. I followed the example for acquiring data using custom levels. The HW board is active for the timeout period but I don't get any data. The input signal is 0V - 0.5V and I can see the data stream on a scope while the 6551 is running. I am using a CB/SCB-2162 Digital I/O accessory to interface to my device. The VI I created is attached. Any advice?

Thanks in advance,

Bob

[CT]

I think you missed at least two key items in your acquisition, one is the trigger (niHSDIO Configure Trigger.vi) and the other is niHSDIO Initiate.vi, which actually starts the acquisition. Look at the dynamic acquisition examples with either hardware or software trigger, then you will see why yours is not working. Another thing you can do to test your setup is to use the dynamic generation AND acquisition example to generate similar signal with one channel on the same card, loop it back to the channel you want to use to acquire and see if you can get the pattern first before trying to catch the actual signal. Once that works, you only need to figure out how and when to trigger the acquisition for the actual signal.

[Bob_Albern]

I originally had a config trigger vi using PFI1 wired to one of my input signals and an Initiate vi and Wain Until Done vi to start the acquisition. This did not work. I then went to the NI help examples and found the attached example vi. For my application, I stripped out the save data functions. The example also did not work but I suspected it was because I had not configured the board properly. Not because the vi was missing components!

Perhaps an NI representative could answer this (CT, I don't know if you work fo NI.): Why does NI ship examples that are incomplete and do not work? I have rarely found examples that actually function. I suspect this example sent me down the wrong path.

Back to the original issue. I have been using this card for some time but only in a mode similar to the loopback you suggest. I can generate data using the board, export a trigger on, say PFI2, then start an acquisition on additional channels using PFI2 as the input trigger. This works fine. After a nights sleep, I'm wondering if I missed a connection on the 2162 board. I'll look into this when I get a chance. What I am trying to do should be trivial. I expect when I find the last setup component everything will fall into place.

[Tica]

Hello Bob,

I generated a square wave with one of our function generators with an amplitude of ±0.5V and was able to use both the “monitor TPs.vi” and the example “Acquire and Save to HWS File.vi” to measure the generated signal. The only necessary change to get the NI example to work was to include the function niHSDIO Configure Voltage to specify the high and low voltage levels since by default those levels are for a 0-5 V signal.   

If you are not getting any data, my suggestion is to check the data you are trying to measure. Maybe the amplitude is not within the specified range. On your second post you mentioned configuring an input trigger, but according to your description (acquire a continuous low voltage data stream) you don’t need to worry about a trigger since you are simply doing a continuous acquisition.

As far as shipping examples go, I find them extremely helpful. Something to keep in mind when you use them though is that they are built for specific hardware and have instructions on how to configure your hardware before running them as shown in the attached image.

Hope this helps. Please let me know if you have further questions about a specific example or the data you are trying to measure.

Good luck,

Tica T

Applications Engineer

National Instruments