Hi Javier,
Oops, I must not have noticed the e-mail notification about your previous reply. But there is. Sorry.
> Setting 50000 samples per second gives 1/50000 = 20 us. I
know that this happens to be the max 2-channel AI sample but I need a
clock of 20us for setting 50000. Therefore I do not understand when you
say that it meets the requirements for 30us. Am I missing something out?
As you say for 30 us I need 33334 at least but 50000 would not work!
When using an external clock, you have to tell DAQmx the maximum rate that your clock could go. It is okay if the clock is actually slower than that. If your clock is actually 33 kHz, telling DAQmx that the clock might be as fast as 50 kHz is fine. So the sample rate is probably not why your program does not work.
> I set it to 1 second now instead of -1.
Now the longest that this call to DAQmxReadAnalogF64() will take is 1 second. If you're calling it from your UI thread, you probably don't want it to wait even that long.
> Well what happens is that I send a char 'r' and then the MUC
sends the clock when it sees the char 'r' and then it receives data.
The problem is that I do not know if ,after setting char 'r' and
starting the clock, takes faster than for the next instruction of the UI
to execute in visual c++ which is READ.
Start the DAQmx task before sending the 'r' to the MCU, so that the DAQ hardware is already reading before the MCU sends any clocks. That way, it will not matter how long it takes to execute the DAQmxReadAnalogF64() function.
> Never used this one but may be I should have a look in
examples or how to use it?I would need an example to implement it. I
just want to read the 128 samples from both channels
Many of the DAQmx ANSI C shipping examples demonstrate how to call DAQmxRegisterDoneEvent(). Here's one:
C:\Documents and Settings\All Users\Documents\National Instruments\NI-DAQ\Examples\DAQmx ANSI C\Analog In\Measure Voltage\Cont Acq-Ext Clk-Dig Start
However, there is no example that does exactly what you want. Keep reading, I've posted some pseudocode at the end of this message.
> I am not expert in programming so I do not know how to make
threads, etc.
Then taking advantage of DAQmx's buffering is the easiest approach.
> One other thing: I do as you said: FIRST I start the ANALOG
task in a INIT procedure and then I start a timer that sends a char 'r'
and next instruction I READ but then after one read I stop the task. My
mistake was to stop the task. Is it necessary to stop a task? Can I
delete the part of stopping the task if it is not needed? I do start it
but I was not sure if I had to stop it too or just come out of the
application would be enough
You must clear the task, or your app will leak memory (until your app exits, at which point it will be freed). If you do not stop the task, DAQmx will stop it when you clear the task.
> I attach a printscreen of my latest ERROR.
This error says that you are reading too many samples. If you call DAQmxCfgSampClkTiming(..., DAQmx_Val_FiniteSamps, 128) and then you try to read 129 samples, or you try to read 128 samples twice, or you try to read 1 sample 129 times, you will get this error.
> So therefore do you have an example of how to do
what you describe here?: DAQmxRegisterDoneEvent() to register a callback
function, and call DAQmxReadAnalogF64()
in the callback.
I don't have an example, but here is some untested C-like pseudocode showing what I'm talking about. Just the basics--no error handling, etc.
static TaskHandle task = 0;
static UINT_PTR timer = 0;
/* Forward declarations */
int32 CVICALLBACK OnDoneEvent(TaskHandle, int32, void*);
VOID CALLBACK OnTimerExpired(HWND, UINT, UINT_PTR, DWORD);
/* This function gets called when a button on the UI is clicked. */
void OnStartButtonClick() {
DAQmxCreateTask(..., &task);
DAQmxCreateAIVoltageChan(task, ...);
DAQmxCfgSampClkTiming(task, "PFI7", 50000, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, 128);
/* When/if this task finishes acquiring 128 samples, OnDoneEvent() will get called. Since this is a UI app, you might want to replace the 2nd parameter (0) with DAQmx_Val_SynchronousEventCallbacks to have DAQmx use Windows messages instead of a separate thread. */
DAQmxRegisterDoneEvent(task, 0, OnDoneEvent, NULL);
/* If something goes wrong, this task might never finish. Set a Win32 timer to execute in 20 seconds, in case that happens. Since this is a UI app, you might want to have the timer send a Windows message instead of calling a callback, but I showed the callback because it's clearer. */
timer = SetTimer(..., 20000, OnTimerExpired);
/* Start the DAQmx task before sending the command to the MCU, so that the PCI-6031E is already waiting for sample clocks when the MCU starts sending them. */
DAQmxStartTask(task);
SendCommandToMCU('r');
}
/* If this function gets called, then there are 128 samples to
read, and calling DAQmxReadAnalogF64() should not have to wait at all. */
int32 CVICALLBACK OnDoneEvent(TaskHandle, int32 status, void *) {
KillTimer(..., timer);
if(status < 0) ReportErrorToUser();
DAQmxReadAnalogF64(task, 128, 0.0, DAQmx_Val_GroupByScanNumber, buffer, buffer size, &samplesRead, NULL); DAQmxStopTask(task);
DAQmxClearTask(task);
DisplayBuffer(buffer, samplesRead);
return 0;
}
/* If this function gets called, then the DAQmx task did not finish in 20 seconds. */
VOID CALLBACK OnTimerExpired(HWND, UINT, UINT_PTR, DWORD) {
KillTimer(..., timer);
DAQmxClearTask(task);
DisplayError("Task didn't finish in 20 seconds! Something is wrong.");
}
Brad
---
Brad Keryan
NI R&D
