I need help with a problem I have to calculate the time of the samples I take from an NI cDAQ-9184 with an NI 9237 module.I work in a project in which we take samples of a load cell to calculate the weight of different vehicles, in this project we are continuously taking samples and calculating for each one of them the moment of time in which it was obtained.If I take samples at a speed of 1khz passing a buffer of 100 samples, the everyNcallback event should be fired every 100 milliseconds, however when I measure the time between shots I get about 62 milliseconds.How can this be?This is the code I use: #include <Windows.h> #include <stdio.h> #include <time.h> #include <tchar.h> #include "NIDAQmx.h" #define CANT_CANALES 4 #define CANT_BUFFERS 10 #define MUESTRAS_BUFFER 100 #define VELOCIDAD_MUESTREO 1000 #define MAGNITUD_MIN -0.025 #define MAGNITUD_MAX 0.025 char AI_Dispositivo[1024] = "cDAQ9184-1BFBD04Mod2"; float64 AI_Buffer[sizeof(float64) * MUESTRAS_BUFFER * CANT_CANALES]; TaskHandle AI_Tarea = NULL; LARGE_INTEGER t_ini, t_fin; int indice; double Tiempos[CANT_BUFFERS]; HANDLE Handle_Espera; double performancecounter_diff(LARGE_INTEGER *a, LARGE_INTEGER *b) { LARGE_INTEGER freq; QueryPerformanceFrequency(&freq); return (double)(a->QuadPart - b->QuadPart) / (double)freq.QuadPart; } int32 CVICALLBACK EveryNCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples, void *callbackData) { int32 readAI = 0, error = 0, i; QueryPerformanceCounter(&t_fin); if (indice < CANT_BUFFERS) { Tiempos[indice] = performancecounter_diff(&t_fin, &t_ini); indice++; } else { printf("Tiempos:\n"); for (i = 0; i < CANT_BUFFERS; i++) { printf("\t%f\n", Tiempos[i]); } SetEvent(Handle_Espera); } error = DAQmxReadAnalogF64(AI_Tarea, -1, 1.0, DAQmx_Val_GroupByScanNumber, AI_Buffer, MUESTRAS_BUFFER * CANT_CANALES, &readAI, NULL); if (error > 0) { printf("DAQmxReadAnalogF64 ADVERTENCIA %d\n", error); } else if (error < 0) { printf("DAQmxReadAnalogF64 ERROR %d\n", error); } if ((uInt32)readAI != nSamples) { printf("DAQmxReadAnalogF64 ERROR Muestras Leidas = %d Muestras disponibles = %d\n", readAI, nSamples); } QueryPerformanceCounter(&t_ini); return 0; } int main() { DWORD temp; Handle_Espera = CreateEvent(NULL, TRUE, FALSE, TEXT("Handle_Espera")); DAQmxCreateTask("AI_Tarea", &AI_Tarea); DAQmxCreateAIBridgeChan(AI_Tarea, "cDAQ9184-1BFBD04Mod2/ai0:3", "", MAGNITUD_MIN, MAGNITUD_MAX, DAQmx_Val_VoltsPerVolt, DAQmx_Val_FullBridge, DAQmx_Val_External, 9.0, 350, NULL); DAQmxCfgSampClkTiming(AI_Tarea, "", VELOCIDAD_MUESTREO, DAQmx_Val_Rising, DAQmx_Val_ContSamps, MUESTRAS_BUFFER); DAQmxRegisterEveryNSamplesEvent(AI_Tarea, DAQmx_Val_Acquired_Into_Buffer, MUESTRAS_BUFFER, 0, EveryNCallback, NULL); indice = 0; QueryPerformanceCounter(&t_ini); DAQmxStartTask(AI_Tarea); WaitForSingleObject(Handle_Espera, INFINITE); DAQmxStopTask(AI_Tarea); DAQmxWaitUntilTaskDone(AI_Tarea, -1); DAQmxClearTask(AI_Tarea); printf("Ingrese un numero para terminar: "); scanf_s("%d", &temp); return 0; } Result: Tiempos: 1.512656 0.061836 0.061815 0.062108 0.061618 0.064241 0.059541 0.062228 0.061497 0.062573 Ingrese un numero para terminar: From already thank you very much.

THIS is going to help make sense of it all. Your 9237 device isn't capable of a sample rate as low as 1000 Hz. When you request 1000 Hz, it will instead function at its lowest possible sample rate of 1613 Hz. And thus, 100 samples requires (100/1613) seconds, exactly matching the 62 msec you've observed. -Kevin P

Multifunction DAQ

Time between two shots of everyNCalback is shorter than it should be according to the sampling rate

Solved!

Author

Great.

Looking at the specifications, I see that this is indeed the case.

I do not understand why, but at least I know it's not an error in my code.

Thank you

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Multifunction DAQ

Time between two shots of everyNCalback is shorter than it should be according to the sampling rate

Re: Time between two shots of everyNCalback is shorter than it should be according to the sampling rate