PCI-6115, 10 MS/sec acquisition, 5 MS/sec counter

[TJR_741]

I am using a PCI-6115 DAQ board. I have just upgraded to LabView 7.1 and the DAQmx drivers, and so I am in the process of learning how to make them do what I want.

I want to use a digital trigger, pulsing at about 20 kHz, to initiate the collection of about 500 samples at 10 MS/sec. Since I want to collect this number of samples each time the trigger arrives, I believe the example program 'Multi-function Counter Retrigg Pulse Train Generation for AI Sample Clock' basically points the way to doing what I want to do. By setting up a re-triggerable counter as my sample clock, and running the AI continuously, I will be able to collect the 500 points each time the trigger shows up. I believe I can pull these 500 points out of the buffer fast enough to use double buffered data acquisition for an extended period, on the order of 10 minutes.

THE PROBLEM: It seems that the maximum counter output is only 5 MS/sec. Since the counter controls the data acquisition rate, it appears that it is impossible to take advantage of the 10 MS/sec AI sampling rate. Am I right about this? I tried setting the AI rate at 10 MS, and it then simply ignored the setting and collected at the 5 MS rate of the counter. Is there another way to periodically collect the 500 samples without missing any of the triggers? I tried putting a triggered AI read in a loop, and it is way too slow to keep up with the 20 kHz trigger rate.

Thanks for any help you can offer,
TJ

[reddog]

In the DAQmx Export Signal Property Node, select the Events->Counter Output Event->Output Behavior property and set it to pulse instead of toggle. By setting this on the counter output pulse train task, it will effectively change the 5 MHz pulse train into a 10 MHz pulse train. The catch is the pulse train is no longer a 50% duty cycle pulse train. For what you want, this shouldn't be a problem.

The other thing to keep in mind is the 6115 uses a pipelined ADC with a pipeline of two. This means if you violate the minimum sampling rate of 20 KS/s (more than 50 microseconds between sample clocks, or in your case between the last sample and the next trigger), the points in the pipeline will degrade in quality and may not meet the accuracy specifications of the board. If this is a problem, you can sample an extra two points and throw the first two away. Good luck!

[TJR_741]

reddog:

This solution sounds doable, but my first attempt to implement it didn't pan out, so I think I need a few more details.

I found the DAQmx Export Signal Property Node, and I used my Counter task as the task input into it. Is this what you had in mind? Do I need to explicitly use the DAQmx Export Signal VI? I have not used these Export Signal VIs before, and I find them a bit confusing. What is the right Task In for them? The Counter? The AI? A new, dedicated task?

I have attached my modified VI so that it is explicit what I tried to do. The recorded signal is a pulse train where the low pulse is 5 microseconds wide. You can see that the AI collected this in 25 points, so that the sample was still only being collected at 5 MS/sec.


Thanks for the advice about the 6115 and slow triggering. A second application I'm writing may run into this problem, so I'll keep it in mind.

Thanks for any additional help you can offer,
TJR

[TJR_741]

I posted too soon. With just a bit more work, I got it. Thanks for the advice. This definitely looks like it is behaving the way I need.

I'll attach the working version of this VI in case anyone is interested.

[halcyon]

Hi,

Retriggerable pulse generation revisited. Thank you for the previous response many years back which was very useful. This retriggering pulse generation ties up  2 counters on a board. If I have a 4 counters board, does that mean I can still use the other 2 counters for generating the gate signal and if so which are the tied up counters?

Thank you.