Is it possible to loop AI Voltage readings without creating a new task everytime?

[beepyou]

I would like to repeatedly make n Analog readings in a program, however, I am unable to accomplish this without creating a new task each time... perhaps I'm doing something wrong.

Here is my pseudo-code:

DAQmxErrChk(DAQmxCreateTask("", &taskHandleAI));
DAQmxErrChk(DAQmxCreateAIVoltageChan(taskHandleAI, ...));

DAQmxErrChk(DAQmxCfgSampClkTiming(taskHandleAI,...));

Loop start

DAQmxErrChk(DAQmxStartTask(*taskHandleAI));

DAQmxErrChk(DAQmxWaitUntilTaskDone(*taskHandleAI, 0.001));

DAQmxErrChk(DAQmxReadAnalogF64(*taskHandleAI, ...));

DAQmxErrChk(DAQmxStopTask(*taskHandleAI));

parse_data_and_do_something();

Loop end

The very first read seems to work fine, however, all subsequent reads gets the following error message:

DAQmx Error: Measurements: Task specified is invalid or does not exist.
Status Code: -200088

Does anyone have a clue what is happening?

[Steve289]

Hello,

Without actually seeing your block diagram it is hard to see what might be the problem. But from just looking at your pseudo-code I would move the StartTask, WaituntilTaskDone, and StopTask outside of the loop.  Just have the ReadTask inside the loop.

Hope this helps, Good-Luck.

Steven

[beepyou]

Sorry, I'm implementing this in C so I don't have block diagrams... 

I tried moving start/wait/stop outside of the loop, the exact same error occurs. 

I actually don't understand why you would do that though...

Let's say the loop only iterates 2 cycles, if we unroll the code I have, we get:

DAQmxErrChk(DAQmxCreateTask("", &taskHandleAI));
DAQmxErrChk(DAQmxCreateAIVoltageChan(taskHandleAI, ...));

DAQmxErrChk(DAQmxCfgSampClkTiming(taskHandleAI,...));

//first loop 

DAQmxErrChk(DAQmxStartTask(*taskHandleAI));

DAQmxErrChk(DAQmxWaitUntilTaskDone(*taskHandleAI, 0.001));

DAQmxErrChk(DAQmxReadAnalogF64(*taskHandleAI, ...));

DAQmxErrChk(DAQmxStopTask(*taskHandleAI));

parse_data_and_do_something();

//second loop 

DAQmxErrChk(DAQmxStartTask(*taskHandleAI));

DAQmxErrChk(DAQmxWaitUntilTaskDone(*taskHandleAI, 0.001));

DAQmxErrChk(DAQmxReadAnalogF64(*taskHandleAI, ...));

DAQmxErrChk(DAQmxStopTask(*taskHandleAI));

parse_data_and_do_something();

Your suggestion seems to imply the following:

DAQmxErrChk(DAQmxCreateTask("", &taskHandleAI));
DAQmxErrChk(DAQmxCreateAIVoltageChan(taskHandleAI, ...));

DAQmxErrChk(DAQmxCfgSampClkTiming(taskHandleAI,...));

DAQmxErrChk(DAQmxStartTask(*taskHandleAI));

DAQmxErrChk(DAQmxWaitUntilTaskDone(*taskHandleAI, 0.001));

//first loop 

DAQmxErrChk(DAQmxReadAnalogF64(*taskHandleAI, ...));

parse_data_and_do_something();

//secondloop 

DAQmxErrChk(DAQmxReadAnalogF64(*taskHandleAI, ...));

parse_data_and_do_something();

DAQmxErrChk(DAQmxStopTask(*taskHandleAI));

What triggers the read to start in this case?  

[Dennis_Knutson]

The DAQmxErrChk(DAQmxReadAnalogF64(*taskHandleAI, ...)); initiates a read. What did you think did?

Noticed the wait until task is done function. That does not belong anywhere. Have you looked at the C examples (i.e. Cont Acq-Int Clk)