how to use DAQmxCfgAnlgEdgeStartTrig

[koala0910]

hello, all

     I want  AI to start acquiring data when the data is bigger than n(n is a value we defined).

     I use  simulated NI-USB 6251 to test.

     I have searched the NI-DAQmx C Reference Help and find the function DAQmxCfgAnlgEdgeStartTrig.

     My code is listed:

int AIsampleRate = 100;
double n = 4.0;
DAQmxErrChk (DAQmxCreateTask("",&AItaskHandle));
DAQmxErrChk (DAQmxCreateAIVoltageChan(AItaskHandle,"Dev17/ai0","",DAQmx_Val_RSE,-10.0,10.0,DAQmx_Val_Volts,NULL));
DAQmxErrChk (DAQmxCfgSampClkTiming(AItaskHandle,"",AIsampleRate,DAQmx_Val_Rising,DAQmx_Val_ContSamps,AIsampleRate));
DAQmxErrChk (DAQmxCfgAnlgEdgeStartTrig(AItaskHandle,"Dev17/ai0",DAQmx_Val_RisingSlope, n));//if the data bigger than n,AI start
DAQmxErrChk (DAQmxSetAnlgEdgeStartTrigHyst(AItaskHandle, 1.0));
DAQmxErrChk (DAQmxStartTask(AItaskHandle));
DAQmxErrChk (DAQmxReadAnalogF64(AItaskHandle,100,10.0,DAQmx_Val_GroupByScanNumber,data,100,&read,NULL));

 there is no error on my code.

and if I do not use DAQmxCfgAnlgEdgeStartTrig, the data AI acquired is like figure 1.

figure 1

and if I use DAQmxCfgAnlgEdgeStartTrig, the data AI acquired will like figure 2 (frome 4.0).

figure 2

but, now the two results are the same, they are all like the figure1.

So how can I achievethe purpose I wanted?