Multi channel acquisition problem - NI 4474

[f.strappini]

Hi all, i'm developing an appliction that use Accelerometer, i already used this card and Daqmx software to create an application (C) but always single channel , now when i 

a) create a speific task to read from a specific channel, i read normally

b) if i allocate another task for read another channel, i read an abnormal data, the readings are not standard

c) if the second reading is execuetd with 2 different tasks and afetr 500000 sampling, the reading is normal

here is the part of the code to allocate and execute a task:

                if(errCk(DAQmxCreateTask("",&m_taskHandle), call)){

m_mutex->unlock();

return false;

}

///Configura la Scheda

if(errCk(DAQmxCreateAIAccelChan(m_taskHandle,ch,"",DAQmx_Val_PseudoDiff,m_ni->dwl,m_ni->upl,DAQmx_Val_AccelUnit_g,m_ni->sen,DAQmx_Val_mVoltsPerG,DAQmx_Val_Internal,m_ni->curr,NULL), call)){

return false;

}

///Configura il campionamento

if(errCk(DAQmxCfgSampClkTiming(m_taskHandle,"",m_ni->fs/*10000.0*/,DAQmx_Val_Rising,DAQmx_Val_FiniteSamps,nsamp), call)){

return false;

}

///Registra l'evento func di callback

if(errCk(DAQmxRegisterEveryNSamplesEvent(m_taskHandle,DAQmx_Val_Acquired_Into_Buffer,nsamp,0,EveryNCallback,this), call)){

return false;

}

if((m_state = errCk(DAQmxStartTask(m_taskHandle), call) ? 1 : 0 )){

return false;

}

the reading is executed in this callback function:

static int  tRead=0;

int32       read=0;

int32       overloadDetectionEnabled=1;

bool32      overloaded=0;

///Quando e' true indica che i dati sono stati trasfereti al rec da data[]

bool  dc = false;

///In caso di errore non deve essere eseguito nessuno dei passi successivi

bool err = false;

float64     *data = (float64*)calloc(cbi->nsamp, sizeof(float64));

char        *overloadedChannels = (char*)malloc(sizeof(char)*cbi->nsamp);

///blocca la sessione critica

m_mutex->lock();

if(errCk(DAQmxReadAnalogF64(cbi->tk,cbi->nsamp,0.0,DAQmx_Val_GroupByScanNumber,data,cbi->nsamp,&read,NULL), call, false, true)){

return;

}

....overload control...returns data

any asuggestion? thanks

[dsb@NI]

add additional channels to the same DAQmx task. DAQmx supports multiple channels on a single task (even if those channels have different measurement types). DAQmx does not support parallel tasks that try to reserve/use the same subsystem.