Continuous data acquisition using NiDAQmx with a start and a stop trigger

Question

I'm sorry if this has been answered many times before, I can't quite seem to find the answer I'm looking for.
I am using LabWindows CVI version 7 and NiDAQmx with a PCI6023E.
I wish to acquire data continuously using an external clock as a timebase - I am happy with this.
I wish the acquisition to start when an external signal (say on PFI7) goes high. I am also happy with this.
What I also need is for the acquisition to stop when the signal on PFI7 goes low, or possibly when a signal on say PFI8 goes high. I'm not too concerned about which approach to use.
How do I stop the acquisition with an external signal?
Thanks in advance,
Crispie

Answer 1

I don't have CVI installed, but I've attached screen shots of a LabVIEW program that I believe accomplishes what you are trying to do. Translating it to the C API should be straight forward. I'll try to explain what the program is doing since it's using some of the more advanced features of the driver. Also, the DAQ device you are using doesn't support a true "stop" trigger so I'm using a reference trigger to get as close to the desired functionality as possible.
First, the program configures a finite acquisition that uses both a start trigger and a reference trigger. The acquisition is using an external sample clock and will acquire 4 samples (2 pre-trigger samples and 2 post-trigger samples). Four may seem like an odd number here, but it allows us to emulate the functionality of a stop trigger as close as possible. Given this configuration, you must acquire at least 2 samples before the "stop" trigger is recognized, and you must acquire 2 more samples after the "stop" trigger is recognized. Hopefully this restriction is acceptable. You can always discard the last two data points after the stop trigger if they're not of interest, but you're stuck always acquiring at least two points between when the start and stop triggers are recognized.
The program also overrides the default buffer size and read position. By default, the DAQmx driver will pick a buffer size exactly big enough to fit the pre-trigger and post-trigger data (4 samples in this case) and will begin reading data from the start of the pre-trigger data. Explicitly allocating a larger buffer will allow your acquisition to execute without receiving buffer overflow errors, and changing the default read position will allow you to read all of the data between the start and stop triggers as it is acquired and not just the pre-trigger and post-trigger data.
Finally, the while loop takes care of reading the data. In this case, the loop continues to read data until the task is done and there are no longer samples available for reading from the buffer. The number of samples read per iteration is the lesser of the user specified amount or the number of samples available for reading from the buffer.
I hope this helps. Good luck.
Attachments:
Stop_Trigger1.JPG ‏40 KB
Stop_Trigger2.JPG ‏43 KB