How can i deliver a single pulse of varying amplitude using Analog output and USB-6251?

Question

I am trying to generate an analog output for the following waveform:
Pulse width: 300 usec, Frequency 40 Hz, Amplitude is following a sine wave with a 2 second period.
My approach is: Use an async timer with 1/[40 Hz] sec  interval. I set the DAQ output rate at 1 MHz and the approach has been to generate and write 300 samples of the amplitude I need to send out, then wait for the next async interval callback , update amplitude etc etc.  The assumption is that once the 300 samples are written and the task started , a 300 usec pulse would be sent out , task then stopped and then the analog output would be set to 0 waiting for he next pulse.  The reality is that I get a stream of pulses of 300 usec which last the whole timer interval. 
So , trying to establish whether what I am trying to do is feasible without actually sending a large array to the USB-6251 which would write zeros for the period after 300 usec.
Here is my timercallback code for review:
    switch (event)
        case EVENT_TIMER_TICK:
            DAQmxErrChk(DAQmxStopTask(TaskAnalog));
Determine Voltage value algorithm here
            //Generate Waveform and start Task
            //Prepare AO arr
            for (k=0;k<300;k++)
                AOArr[k] = 0;
                if (k<G_PulseWidth)
                AOArr[k] =  VOLTAGE_OUTPUT;
            if (AOArr!=NULL)
                DAQmxErrChk(DAQmxWriteAnalogF64 (TaskAnalog, (int32) 300 , 0, DAQmx_Val_WaitInfinitely, DAQmx_Val_GroupByChannel , AOArr, &sampsPerChanWritten, NULL));
            DAQmxErrChk(DAQmxStartTask(TaskAnalog));
            break;
Thanks

Answer 1

I did resovle the issue I encountered. It was due to bad configuration of the Task.
My initial configuration was:
DAQmxErrChk(DAQmxCfgSampClkTiming(TaskAnalog, "",1.0E+06, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps , 300 * 1.0E+06));
which should be:
DAQmxErrChk(DAQmxCfgSampClkTiming(TaskAnalog, "",1.0E+06, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps , 300));      
THis did it. works very well.
THanks