Writing an Every N Samples Event for AO on NI-DAQ in C

[Astro Mark]

Hi there,
 
I'm trying to write some code that writes out to my NI-6731 board quickly (1 kHz) but I need to be able to change the output voltage in real time.  I.E. I don't know what voltage I need to output until just before I need to send it out.  Later, I will read these values from a file, but for now, I can write it internally in the callback routine.  I wrote some code based on the example ContAcq-IntClk-EveryNSamplesEvent.c so that every N Samples it writes a different set of samples.
 
The problem is that my code does not work.  I get the error:
 
DAQmx Error: Measurements: DAQmx Every N Samples Event is not supported within non-buffered tasks.
 
To receive Every N Samples Event notifications, configure your task to use buffering.
Task Name: _unnamedTask<0>
 
Status Code: -200848
 
I am pretty sure my code is set up for buffering tasks, so I am quite confused.  The code is designed to be slow right now, but eventually I will speed it up.  For now, I just want it to work!  If you could take a look at it and help me sort out what the problem is, I would really appreciate it.
 
Cheers
 

#include <stdio.h>
#include <math.h>
#include "C:\NI-DAQ\DAQmx-ANSI-C-Dev\include\NIDAQmx.h"
#define DAQmxErrChk(functionCall) if( DAQmxFailed(error=(functionCall)) ) goto Error; else
int32 CVICALLBACK EveryNSamplesCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData);
void Cleanup (void);
static TaskHandle taskHandle=0;
static int32 totalWrite=0;
static float64 data[4000];
int main(void)
{
int error=0;
char errBuff[2048]={'\0'};
int i=0;
for(;i<2000;i++) {
data[i] = 4;
data[i+2000] = 4;
}
/*********************************************/
// DAQmx Configure Code
/*********************************************/
DAQmxErrChk (DAQmxCreateTask("",&taskHandle));
DAQmxErrChk (DAQmxCreateAOVoltageChan(taskHandle,"Dev1/ao0:1", "",-10.0,10.0,DAQmx_Val_Volts,NULL));
DAQmxErrChk (DAQmxCfgSampClkTiming(taskHandle,"",2000.0,DAQmx_Val_Rising,DAQmx_Val_ContSamps,2000));
/*********************************************/
/* Tell the code to do another event once a certain
number of samples have been written*/
/*********************************************/
DAQmxErrChk (DAQmxRegisterEveryNSamplesEvent(taskHandle,DAQmx_Val_Transferred_From_Buffer,2000,0,EveryNSamplesCallback,NULL));
/*********************************************/
// DAQmx Start Code
/*********************************************/
DAQmxErrChk (DAQmxStartTask(taskHandle));
/*********************************************/
// DAQmx Write Code
/*********************************************/
DAQmxErrChk (DAQmxWriteAnalogF64(taskHandle,2000,0,10.0,DAQmx_Val_GroupByChannel,data,NULL,NULL));
printf("Generating samples continuously. Press Enter key to interrupt\n");
getchar();
DAQmxErrChk (DAQmxStopTask(taskHandle));
printf("\nWrote %d total samples.\n",totalWrite);
/*********************************************/
// DAQmx Wait Code
/*********************************************/
DAQmxErrChk (DAQmxWaitUntilTaskDone(taskHandle,10.0));
Error:
if( DAQmxFailed(error) )
DAQmxGetExtendedErrorInfo(errBuff,2048);
if( taskHandle!=0 ) {
/*********************************************/
// DAQmx Stop Code
/*********************************************/
DAQmxStopTask(taskHandle);
DAQmxClearTask(taskHandle);
}
if( DAQmxFailed(error) )
printf("DAQmx Error: %s\n",errBuff);
return 0;
}
int32 CVICALLBACK EveryNSamplesCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData)
{
int32 error=0;
int32 write;
char errBuff[2048]={'\0'};
int j=0;
float64 data2[4000];
for(;j<2000;j++) {
data2[j] = 2;
data2[j+2000] = 2;
}
/*********************************************/
// DAQmx Write Code
/*********************************************/
DAQmx

[Wallace_F]

Hello Astro Mark,
The NI-DAQmx C Reference Help file states that if you configure timing for your task, that the task is a buffered task.  Furthermore, if you don't manually configure the buffer size using DAQmxCfgOutputBuffer, DAQmx automatically configures the buffer when you configure sample timing.  I recommend explicitly configuring the output buffer using the DAQmxCfgOutputBuffer function, and give it the appropriate parameters to see if you can eliminate the error.
Best wishes,
Wallace F.

[Zach Hindes]

I tested out my theory of configuring an output buffer size but not
writing any data, and it turns out you get an error: "-200462:
Generation cannot be started, because the output buffer is empty. 
Before starting a buffered generation, write data. ..."  There you have
it!