Helping to understand a c++ code

Question

Hello everybody.
I have a problem with an example provided by NI for DAQmx ANSI C programmation. I putted this question here, because I could not find a suitable board for it.
About the problem: I'm currently working with a NI PCI-6120 device and therefore I like to connect a trigger signal to PFIO and a AI channel at which the software should acquire data in combination with a trigger event (->PFIO). Serveral times.
I found a good example (http://zone.ni.com/devzone/cda/tut/p/id/5382) which should do the things I'd like to have, but I have problems to understand the code or maybe I don't really understand the way they realized it.
Here the code:
#include <string.h>
#include <stdio.h>
#include <NIDAQmx.h>
static TaskHandle  AItaskHandle=0,DItaskHandle=0;
#define DAQmxErrChk(functionCall) if( DAQmxFailed(error=(functionCall)) ) goto Error; else
static int32 GetTerminalNameWithDevPrefix(TaskHandle taskHandle, const char terminalName[], char triggerName[]);
int32 CVICALLBACK EveryNCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData);
int32 CVICALLBACK DoneCallback(TaskHandle taskHandle, int32 status, void *callbackData);
int main(void)
 int32   error=0;
 char    errBuff[2048]={'\0'};
 char    trigName[256];
 // DAQmx Configure Code
 DAQmxErrChk (DAQmxCreateTask("",&AItaskHandle));
 DAQmxErrChk (DAQmxCreateAIVoltageChan(AItaskHandle,"Dev1/ai0",​"",DAQmx_Val_Cfg_Default,-10.0,10.0,DAQmx_Val_Volt​s,NULL));
 DAQmxErrChk (DAQmxCfgSampClkTiming(AItaskHandle,"",10000.0,DAQ​mx_Val_Rising,DAQmx_Val_ContSamps,1000));
 DAQmxErrChk (GetTerminalNameWithDevPrefix(AItaskHandle,"ai/Sam​pleClock",trigName));
 DAQmxErrChk (DAQmxCreateTask("",&DItaskHandle));
 DAQmxErrChk (DAQmxCreateDIChan(DItaskHandle,"Dev1/port0","",DA​Qmx_Val_ChanForAllLines));
 DAQmxErrChk (DAQmxCfgSampClkTiming(DItaskHandle,trigName,10000​.0,DAQmx_Val_Rising,DAQmx_Val_ContSamps,1000));
 DAQmxErrChk (DAQmxRegisterEveryNSamplesEvent(AItaskHandle,DAQm​x_Val_Acquired_Into_Buffer,1000,0,EveryNCallback,N​ULL));
 DAQmxErrChk (DAQmxRegisterDoneEvent(AItaskHandle,0,DoneCallbac​k,NULL));
 // DAQmx Start Code
 DAQmxErrChk (DAQmxStartTask(DItaskHandle));
 DAQmxErrChk (DAQmxStartTask(AItaskHandle));
 printf("Acquiring samples continuously. Press Enter to interrupt\n");
 printf("\nRead:\tAI\tDI\tTotal:\tAI\tDI\n");
 getchar();
Error:
 if( DAQmxFailed(error) )
  DAQmxGetExtendedErrorInfo(errBuff,2048);
 if( AItaskHandle ) {
  // DAQmx Stop Code
  DAQmxStopTask(AItaskHandle);
  DAQmxClearTask(AItaskHandle);
  AItaskHandle = 0;
 if( DItaskHandle ) {
  // DAQmx Stop Code
  DAQmxStopTask(DItaskHandle);
  DAQmxClearTask(DItaskHandle);
  DItaskHandle = 0;
 if( DAQmxFailed(error) )
  printf("DAQmx Error: %s\n",errBuff);
 printf("End of program, press Enter key to quit\n");
 getchar();
 return 0;
static int32 GetTerminalNameWithDevPrefix(TaskHandle taskHandle, const char terminalName[], char triggerName[])
 int32 error=0;
 char device[256];
 int32 productCategory;
 uInt32 numDevices,i=1;
 DAQmxErrChk (DAQmxGetTaskNumDevices(taskHandle,&numDevices));
 while( i<=numDevices ) {
  DAQmxErrChk (DAQmxGetNthTaskDevice(taskHandle,i++,device,256))​;
  DAQmxErrChk (DAQmxGetDevProductCategory(device,&productCategor​y));
  if( productCategory!=DAQmx_Val_CSeriesModule && productCategory!=DAQmx_Val_SCXIModule ) {
   *triggerName++ = '/';
   strcat(strcat(strcpy(triggerName,device),"/"),t​erminalName);
   break;
Error:
 return error;
int32 CVICALLBACK EveryNCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData)
 int32       error=0;
 char        errBuff[2048]={'\0'};
 static int  totalAI=0,totalDI=0;
 int32       readAI,readDI;
 float64     AIdata[1000];
 uInt32      DIdata[1000];
 // DAQmx Read Code
 DAQmxErrChk (DAQmxReadAnalogF64(AItaskHandle,1000,10.0,DAQmx_V​al_GroupByChannel,AIdata,1000,&readAI,NULL));
 DAQmxErrChk (DAQmxReadDigitalU32(DItaskHandle,1000,10.0,DAQmx_​Val_GroupByChannel,DIdata,1000,&readDI,NULL));
 printf("\t%d\t%d\t\t%d\t%d\r",readAI,readDI,total​AI+=readAI,totalDI+=readDI);
 fflush(stdout);
Error:
 if( DAQmxFailed(error) ) {
  DAQmxGetExtendedErrorInfo(errBuff,2048);
  // DAQmx Stop Code
  if( AItaskHandle ) {
   DAQmxStopTask(AItaskHandle);
   DAQmxClearTask(AItaskHandle);
   AItaskHandle = 0;
  if( DItaskHandle ) {
   DAQmxStopTask(DItaskHandle);
   DAQmxClearTask(DItaskHandle);
   DItaskHandle = 0;
  printf("DAQmx Error: %s\n",errBuff);
 return 0;
int32 CVICALLBACK DoneCallback(TaskHandle taskHandle, int32 status, void *callbackData)
 int32   error=0;
 char    errBuff[2048]={'\0'};
 // Check to see if an error stopped the task.
 DAQmxErrChk (status);
Error:
 if( DAQmxFailed(error) ) {
  DAQmxGetExtendedErrorInfo(errBuff,2048);
  DAQmxClearTask(taskHandle);
  if( DItaskHandle ) {
   DAQmxStopTask(DItaskHandle);
   DAQmxClearTask(DItaskHandle);
   DItaskHandle = 0;
  printf("DAQmx Error: %s\n",errBuff);
 return 0;
Here my questions:
1: DAQmxErrChk (GetTerminalNameWithDevPrefix(AItaskHandle,"ai/Sam​pleClock",trigName));
I don't get this. Why don't they just put one specific channel name at var: trigName?
2.1: DAQmxErrChk (DAQmxRegisterEveryNSamplesEvent(AItaskHandle,DAQm​x_Val_Acquired_Into_Buffer,1000,0,EveryNCallback,N​ULL));
2.2: DAQmxErrChk (DAQmxRegisterDoneEvent(AItaskHandle,0,DoneCallbac​k,NULL));
Why do they setup the event on the AI channel? I thought I have to check for events on the trigger channel?!
3: int32 CVICALLBACK EveryNCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData)
Does this event occur for every N samples acquired?
In summary I don't understand how the program is realising the 'wait on trigger' and then starts to acquire the data. In my eyes they just plot the data on both channels without any trigger.
Thank you for reading that far. Maybe someone can help me to understand it correctly!
Bye
Denis

Answer 1

Hello,
it is me again. I modified the code you provided to me. So far I can say it is working =)
But there are some things I like to know and maybe you can explain it to me.
Right now my code acquires data from two channels. I removed the BufferInput-function, because he was not able to trigger or run corretly if I have serveral scans/triggers.
Questions:
Why does it run if I dont use a specific InputBufferSize?
// ---------------------------------------- code snippet ----------------------------------------------
// ChanSR - SampleRate
// ChanSmpPts - SamplesPerChan
// Channels = \Dev3\ai0:1 (2 channels)
// -- create Tasks
DAQmxErrChk (DAQmxCreateTask("", &AItaskHandle));
DAQmxErrChk (DAQmxCreateTask("", &COtaskHandle));
// -- create voltage chan, configure timing and optional the buffer size   
DAQmxErrChk (DAQmxCreateAIVoltageChan(AItaskHandle, Channels, "", DAQmx_Val_Cfg_Default, -ChanVolts, ChanVolts, DAQmx_Val_Volts, NULL));
DAQmxErrChk (DAQmxCfgSampClkTiming(AItaskHandle, "Ctr0InternalOutput", ChanSR, DAQmx_Val_Falling, DAQmx_Val_ContSamps, (int32) ChanSmpPts));
//DAQmxErrChk (DAQmxSetBufferAttribute (AItaskHandle, DAQmx_Buf_Input_BufSize, ChanSmpPts*2));
// -- create the clock for the acquistion
DAQmxErrChk (DAQmxCreateCOPulseChanFreq (COtaskHandle, "Dev3/ctr0", "", DAQmx_Val_Hz, DAQmx_Val_Low, 0., ChanSR, 0.5));
DAQmxErrChk (DAQmxCfgImplicitTiming (COtaskHandle, DAQmx_Val_FiniteSamps, ChanSmpPts));
// -- create trigger and define the edge        
DAQmxErrChk (DAQmxCfgDigEdgeStartTrig(COtaskHandle,"PFI0", DAQmx_Val_Rising));    
DAQmxErrChk (DAQmxSetTrigAttribute (COtaskHandle, DAQmx_StartTrig_Retriggerable, TRUE));       
// -- start the tasks
DAQmxErrChk (DAQmxStartTask(COtaskHandle));
DAQmxErrChk (DAQmxStartTask(AItaskHandle));
// -- knowing the numbers of triggers we repeat the acquisition
for(i=0; i<TrigEvents; i++)
    DAQmxErrChk (DAQmxReadAnalogF64(AItaskHandle, (int32) ChanSmpPts, 2.0, DAQmx_Val_GroupByChannel, buffer, 
                                      (int32) (ChanCount * ChanSmpPts), &read, NULL));       
    for(k = 0; k < len; k++) 
         result[k + i*len] = buffer[k]; // just plot/save the data
Cleanup();
Bye
Denis