AI/AO at different frequency

Question

Hi,
As a newbie, I met a problem when I tried to input and output analog signal at different frequency.
I followed PID-control-Multichannel.vi to build a control program, so input/output can be synchronized. However, the project requires that the AI frequency to be ten times of the AO. I could rewrite the while loop to make the output value constant for 9 of 10 cycles. However, I believe there is more straight forward way to do it.
Could anybody provide an example?
Thank you in advance.
Sincerely yours
Ming 
Solved!
Go to Solution.

Answer 1

lmuri wrote:
Hi,
As a newbie, I met a problem when I tried to input and output analog signal at different frequency.
I followed PID-control-Multichannel.vi to build a control program, so input/output can be synchronized. However, the project requires that the AI frequency to be ten times of the AO. I could rewrite the while loop to make the output value constant for 9 of 10 cycles. However, I believe there is more straight forward way to do it.
Could anybody provide an example?
Thank you in advance.
Sincerely yours
Ming 
Hello Ming!
Thank you for using the NI Forums. You'll be glad to know that DAQmx allows I/O tasks such as these to be ran not only concurrently but also at different rates.
The problem with the solution you've devised is that this implementation will remove the delegation of the tasks down to the hardware level and your program would become software driven; this becomes problematic when running data acquisition tasks at very high speeds as you become limited to the output speed of your Operating System (OS).
You can co-ordinate your tasks to operate synchronously and perform output and acquisition at different rates by creating a task master. This generally means that you configure a task through DAQmx that maintains a clock frequency and you create tasks which use this clock frequency, or a division of it, to operate at their own individual frequency. This will ease not only the implementation of synchronous DAQmx tasks but also provide an entirely hardware driven solution to maximimse performance.
Through LabVIEW, if you go to Help > Find Examples to open the NI Example Finder. If you browse through Hardware Input and Output > DAQmx > Synchronization > Multi-Function > Multi-Function-Synch Dig Read Write With Counter.vi, you will find an example of how to configure a Counter as a task master to control the operation of both a Read and Write operation. (This example shows a digital implementation but may be easily replaced with analogue.)
By setting the counter rate to the maximum frequency that you will require for your task (In this case, the speed at which you want to output values) and applying it to the output task SampleClock, you will drive the output task clock with the Counter as the clock source. You can then use the Counter as the source for the SampleClock for the input task, however set the rate to whatever division of the driving frequency you want. In the case of your example, you can set the input rate to 0.1 times the Counter Frequency to acquire at a 10th of the rate.
If you wanted to acquire at the same rate but only retrieve values at the 10th of the speed, this same solution could be configured to instead produce a trigger to return a buffered acquisition. With a master clocking task, the opportunities are endless!
I hope that you find this helpful, and if you need any more clarifcation don't hesitate to let me know. Have fun with your DAQ!
Alex Thomas, University of Manchester School of EEE LabVIEW Ambassador (CLAD)