Continuous Waveform Generation in C/C++, example request

[David Marshburn]

i'm looking for one or more analog-output examples in which a waveform is generated on the fly.  for comparison, all the examples shipping with NI-DAQmx continuously and repeatedly output the same data; i'd like to be able to change the output values.  non-renegeration may be the (non-obvious) phrase that i'm missing in my searches.  any suggestions, please?

this seems to be about what i'm looking for:

http://zone.ni.com/devzone/cda/epd/p/id/4872

only in c/c++ instead of labview.

[John_P1 · Accepted solution]

Hi David,

This example should be helpful--it sounds like what you want to do.

DAQmx - Continuously Generate Voltage - Non Regeneration - LabWindows/CVI

The DAQ part is:

//Configure

DAQmxCreateTask("",&taskHandle);

DAQmxCreateAOVoltageChan(taskHandle,chan,"",min,max,DAQmx_Val_Volts,NULL);

DAQmxSetWriteAttribute (taskHandle, DAQmx_Write_RegenMode, DAQmx_Val_DoNotAllowRegen);
DAQmxCfgSampClkTiming(taskHandle,"",rate,DAQmx_Val_Rising,DAQmx_Val_ContSamps,1000);


// Write Data

DAQmxWriteAnalogF64(taskHandle,bufferSize,0,10.0,DAQmx_Val_GroupByChannel,data,&written,NULL);

// Start Task

DAQmxStartTask(taskHandle));

// Continuously Write Data in a Loop

while( gRunning ) {
            DAQmxWriteAnalogF64(taskHandle,bufferSize,0,10.0,DAQmx_Val_GroupByChannel,data,&written,NULL);
        }

// Clear task when finished

 DAQmxClearTask(taskHandle);

The data written to your task must keep up with the rate that it is being clocked out or you will receive a buffer underflow error (i.e. the array of data must be large enough so that you write enough data per loop iteration).  The DAQmx Write is a blocking call, so if you try to write more data than the space that is available in your output buffer the loop will wait here until space becomes available and it completes its Write or the specified timeout is reached.

Best Regards,

Message Edited by John P on 01-26-2010 09:25 PM

[David Marshburn]

 thanks very much for the link and assistance!

 

---

 John P wrote:

The DAQmx Write is a blocking call, so if you try to write more data than the space that is available in your output buffer the loop will wait here until space becomes available and it completes its Write or the specified timeout is reached.

---

this is the important detail that wasn't apparent from the documentation -- it was unclear to me, without any example, how to time the addition of new data.  this c-based example would be a great addition to the examples installed with ni-daq!

thanks, again!

-david