Hi,
I am using LabWindows 8.1 and a PXI-4461 with Vista (home) on a 2.81 GHz Dual Core Processor. I wrote code to do the following:
(1) output a ramp, input a signal concurrently (precise sychronization of these two is not important at this point)
(2) if the input signal rises above 0.5 V, hold the output constant for 2 seconds, while continuing the input
(3) finish the ramp
More info: I do a continuous sampling, with 4 samples per Write, to do the output. The input is read and analyzed with RegisterEveryNSamples (where N = 4). The communication between the tasks is through a global variable "trap," which is set to 1 when threshold is reached. I achieve holding the output constant by stopping the task (maintain existing value = true), and then restarting after the pause. At 40kHz and 4 samples per Write, the limit imposed by the discrete data blocks would be 0.1 msec, but in fact I am getting delays of about 2.5 msec. My write buffer is 120x4 samples, just so that there is space for me to write. I do need to save the input data after the
acquisition, preferably the whole sweep, but most importantly the data
while the output is held constant.
So my questions are:
(a) Does it really take 2 ms to stop a task? When I put in "Timer()" functions as below, I get a difference of 0.002, which is of course limited to the resolution of the Timer() function. Returning from a function (the EveryN callback) should not take that long, right?
(b) Can you suggest any better ways of doing this?
/*****begin excerpt *****/
DAQmxErrChk (DAQmxRegisterEveryNSamplesEvent(inputTaskHandle,DAQmx_Val_Acquired_Into_Buffer,4 ,0,EveryNCallback,NULL));
DAQmxErrChk (DAQmxWriteAnalogF64(outputTaskHandle,480,0,10.0,DAQmx_Val_GroupByChannel,outData,&written, NULL));
DAQmxErrChk (DAQmxStartTask(inputTaskHandle));
DAQmxErrChk (DAQmxStartTask(outputTaskHandle));
for (count =120;count<10000; count++)
{
if (trap==0) {
DAQmxErrChk (DAQmxWriteAnalogF64(gTaskHandle,4,0,10.0,DAQmx_Val_GroupByChannel,&outData[count*sampsToWrite],&written,NULL));
} else {
DAQmxErrChk (DAQmxStopTask(outputTaskHandle));
time2 = Timer();
done = 1;
ProcessDrawEvents();
ProcessSystemEvents();
Delay (2.0);
trap = 0;
count++;
DAQmxErrChk (DAQmxWriteAnalogF64(outputTaskHandle,480,0,10.0,DAQmx_Val_GroupByChannel,&outData[count*4],&written,NULL));
count = count+119;
DAQmxErrChk (DAQmxStartTask(outputTaskHandle));
}
}
/*****end excerpt *****/
int32 CVICALLBACK EveryNCallback(TaskHandle taskHandle, int32 everyNsamplesEventType, uInt32 nSamples,void *callbackData)
{
...
DAQmxErrChk (DAQmxReadAnalogF64(taskHandle,nSamples,10.0,DAQmx_Val_GroupByScanNumber,&readData[inCount*nSamples],nSamples,&numRead,NULL));
if (trap == 0 && done == 0) {
for (i=0; i<nSamples; i++) {
if (readData[inCount*nSamples+i] > 0.5) {
trap = 1;
time1 = Timer();
break;
}
}
} else done = 1;
inCount++;
Error:
...
return 0;
}
I would appreciate any help or advice! Thanks. (Sorry about oddities in the code.)
