Single Output/Input rate - USB-6009 vs MyDAQ

[rodpp]

Hello!

I am working in a control system that will read the sensors, calculate the next output and send it to the output. Then read the sensors again and so on.

I have two DAQ boards, one USB-6009 and one MyDAQ. In the firsts tests I found the follow results (approximate):

================================
ANSI C NI-DAQmx
================================
Output        Input        Fa
--------------------------------
USB-6009    USB-6009    750Hz
USB-6009    MyDAQ        200Hz
MyDAQ        USB-6009    300Hz
MyDAQ        MyDAQ        150Hz

 where Fa is the sample rate archived. The signal was a sinusoid.

This test was in the same computer, using all USB ports combinations.

The boards specifications regarding AI/AO rates are:

USB-6009

Input: 48KHz

Output: 150Hz

MyDAQ

Input: 200KHz

Output: 200KHz

Why the USB-6009 was better in this test, despite your poor specifications related with the MyDAQ board?

[Dennis_Knutson]

You're going to have to provide some details. What DAQmx functions are you using? How are you calculating the rate? You say 'archived'. Does that mean saved to disk or did you mean to say 'acthieved'? Can you post your code?

[rodpp]

Thank you for your answer!

The DAQmx functions are DAQmxWriteAnalogScalarF64 and DAQmxReadAnalogScalarF64.

As the code is running in Windows 7 SP1, the rate is controlled using the QueryPerformanceCounter. This function seems more precise than the C clock() function.

The global variables DUR (signal length in secoonds) and FA (sample rate in Hz) are necessary for the correct memory allocation in the variables declaration.

I tested both boards (USB-6009 and MyDAQ) using this code, changing only the device name and the output range (0 to 5 for the USB-6009 and -10 to 10 for the MyDAQ). I added a 2V DC, so both boards can output the same signal.

Please see the code in the following lines:

#include "stdafx.h"
#include <stdio.h>
#include <NIDAQmx.h>
#include <time.h> 
#include <math.h>
#include <tchar.h>
#include <windows.h>

#define DAQmxErrChk(functionCall) if( DAQmxFailed(error=(functionCall)) ) goto Error; else
#define PI		3.141592653589793 // pi
#define DUR		10 // signal length (s)
#define FA		750.0 // Sample rate (Hz)

int main(void)
{
	int			error=0;
	TaskHandle	taskHandleOut=0;
	TaskHandle	taskHandleIn=0;
	char		errBuff[2048]={'\0'};
	float64     data[1] = {1.0};
	clock_t		ltime;
	clock_t		ptime;
	clock_t		ttime;
	int			i;
	float		f;
	float		Vp;
	float		Vdc;
	float64		vout[(int)(DUR*FA+1)];
	float64		vin[(int)(DUR*FA+1)];
	__int64		ctr1 = 0, ctr2 = 0, freq = 0;

	/*********************************************/
	// DAQmx Configure Code
	/*********************************************/
	DAQmxErrChk (DAQmxCreateTask("",&taskHandleOut));
	DAQmxErrChk (DAQmxCreateTask("",&taskHandleIn));
	DAQmxErrChk (DAQmxCreateAOVoltageChan(taskHandleOut,"Dev1/ao0","",0,5.0,DAQmx_Val_Volts,""));
	DAQmxErrChk (DAQmxCreateAIVoltageChan(taskHandleIn,"Dev1/ai0","",DAQmx_Val_Diff,-10.0,10.0,DAQmx_Val_Volts,NULL));

	/*********************************************/
	// DAQmx Start Code
	/*********************************************/
	DAQmxErrChk (DAQmxStartTask(taskHandleOut));
	DAQmxErrChk (DAQmxStartTask(taskHandleIn));

	/*********************************************/
	// Output Signal
	/*********************************************/
	f = 50; // Signal frequency (Hz)
	Vp = (float)1.42; // Signal Amplitude (V)
	Vdc = 2; // DC level (V)

	for(i=0;i<=DUR*FA;i++)
	{
		vout[i] = Vdc+sin(2*PI*f*(1/FA)*i);
	}
	
	QueryPerformanceFrequency((LARGE_INTEGER *)&freq);
	ttime = clock();
	for (i=0;i<DUR*FA;i++)
	{
		QueryPerformanceCounter((LARGE_INTEGER *)&ctr1);
	
		/*********************************************/
		// DAQmx Write Code
		/*********************************************/
		DAQmxErrChk (DAQmxWriteAnalogScalarF64(taskHandleOut,false,5,vout[i],NULL));
	
		/*********************************************/
		// DAQmx Read Code
		/*********************************************/
		DAQmxErrChk (DAQmxReadAnalogScalarF64(taskHandleIn,5,data,NULL));

		vin[i] = data[0];
		
		QueryPerformanceCounter((LARGE_INTEGER *)&ctr2);
		// Sample Rate control
		while((ctr2-ctr1)<((1/FA)*freq))
		{
			QueryPerformanceCounter((LARGE_INTEGER *)&ctr2);
		}
	}
	ttime = clock() - ttime;
	printf("Total time of %lf seconds\n",(double)ttime/CLOCKS_PER_SEC);

	getchar();

	for(i=0;i<DUR*FA;i++)
	{
		printf("%llf\n",vin[i]);
	}

Error:
	if( DAQmxFailed(error) )
		DAQmxGetExtendedErrorInfo(errBuff,2048);
	if( taskHandleOut!=0 ) {
		/*********************************************/
		// DAQmx Stop Code
		/*********************************************/
		DAQmxStopTask(taskHandleOut);
		DAQmxClearTask(taskHandleOut);
	}
	if( taskHandleIn!=0 ) {
		/*********************************************/
		// DAQmx Stop Code
		/*********************************************/
		DAQmxStopTask(taskHandleIn);
		DAQmxClearTask(taskHandleIn);
	}
	if( DAQmxFailed(error) )
		printf("DAQmx Error: %s\n",errBuff);
	printf("End of program, press Enter key to quit\n");
	getchar();
	return 0;
}

Thank you again!

Best Regards,

Rodrigo.

[Dennis_Knutson]

Your comparison to the stated specs is just silly. Except for the 6009 analog output, the specs all refer to hardware timed I/O. Your single point writes and reads are software timed (the 6009 analog output can only do this) and will not come close to the specs.

[rodpp]

Of course, I know that. I do not expect put one sample and after read it and gain..., and reach the specifications hardware timed sample rate.

What I want to know is why the MyDAQ is so inferior than the USB-6009, despite its better specifications.

[Dennis_Knutson]

There are no specifications for what you are doing. Neither is really designed or intended for single point reads. I believe the 150 Hz rate for the 6009 is a minimum. The rate could be different with another pc or OS. The point being, take all of your numbers with a grain of salt.

[rodpp]

All things equal, the USB-6009 is better for this application in my setup.

The problem is that it is not marginally better, it is a lot better, 5 times faster in the dedicated config.