How to use RegisterEveryNSamplesEvent?

[nickb]

Hello NewBe,

Here are a couple thoughts from reading through your posts. If you would like to retain your previous architecture of continually calling a function that checks if there are enough items in the buffer to perform a read, you could make use of the function  DAQmxGetReadAvailSampPerChan, which will return to you the number of available samples in a particular channel.  You could check the return of this until it matched some value you thought to be enough samples, and then perform the read.  

I will admit that I am having a little bit of trouble picturing the architecture of your application, and so apologize if I'm a little off base with my next suggestion.  If you would like to make use of the DAQmxRegisterEveryNSamplesEvent function, you could create a struct of the data that you will need to modify within the callback function, and pass this struct by reference to the callback function via the void *callbackData parameter.  If this doesn't seem helpful, could you perhaps post some pseudo code, or even the actual code, of the class you are trying to implement this within?

I also thought that I would mention that if you had Measurement Studio, you would have access to a fully object oriented DAQmx API, with examples written specifically for MFC applications.

NickB

National Instruments 

[NewBe]

Hi,

Thanks for your response. I had your second thought implemented pretty much the way you were suggesting, but with a little twist to it, in the call back function, I would dispatch to a member function in which I do my reading and processing just for the fact so I can have access to other data members of my class..... I haven't got it to work quite right yet.

 By the way, I downloaded a trial version of Measurmeant Studio for VC++, I didn't get any meanigful examples with it though....

I will take a look at your first thought and see if it is a better idea to implement that one.

Thanks

[NewBe]

Sorry, I meant to give you some idea about the code:

In summary I have an analog Task of 8 chs, and 2 counter tasks, one counter for very tooth of a revolution, and the other counter task for Once per Revolution.

class CCommunication

{

   .......

   int variable(s); //I have many variables declared here - like 50 of them

   int *m_Buffer; // I need this buffer to carry the data after I read NI data and process it, I wanna stuff it in this buffer to use it for the rest of the project

   int GetNIDAQData(int *Buffer); //This function is where I read the NI data

  ......

  DAQmxReadBinary16( ...)

  DAQmxReadCounterU32(...)..

 .....

};

// implementation in .CPP

 .......

while(1)

{

  .....

  int ReturnStringSize = GetNIDAQData(m_Buffer);   //function call to come back with Buffer and string size from underneath

   .......

  //After I filled the m_Buffer from reading NI from the function below, I will pass it on to the rest of the project

  ......more processing

  .......Garph data...

  .......etc.....

}

int CCommunication::GetNIDAQData(int *Buffer)

{

    if(Buffer is Ready to be Read)

    {

        DAQmxReadBinary(... 8 chs...)

        DAQmxReadCounterU32( .....)

        DAQmxReadCounterU32( .....)

       ........

       Process data ....

       ...........

      Buffer[0] = Time;

      Buffer[1] = RPM;

      Buffer[2] = Max Time;

      Buffer[3] = Degrees.

      Buffer[4 = AnalogCh1Value;

      Buffer[5] = AnalogCh2Value;

     ....etc .......

    }

 else

    return 0;

}

Q1) In the RegisterEverySamplesEvent function, Do I specify (#of SamplesPerChannel) or Total# of Samples....I currently do SampsPerChannel.

Q2) I am specifying SetInputBufferSize() even though its continuous acquisition, because it fails when the #of samples sepecified to issue the trigger varies and   becomes a non-multiple of the buffer size or vice versa (-20777)...Does this make sense?

like I said, I will give your thought #1 a go as well.

[FBurton]

I apologize for hijacking this thread with my previous question.

A solution to my problem (one that works) is to register a single callback function and to read data from the 4 tasks in that function. This technique is effectively illustrated in the NIDAQmx example "Synchronization\Multi-Device\Continuous AI\ContinuousAI.c".

Francis 

[Doiron]

Hello All,

I wanted to update everyone on the solution to NewBe's issue:

- First, call back function has to be declared as static data member to
match C signature.
- pass the call back function as a reference object
ClassName::&CallbackfunctionName.
- pass "this" pointer for call back argument.
-  In the call back function, Dispatch to another data member function
using the access object of call back data which was passed by "this"
pointer.
- In dispched function, you will be able to read NI buffers and have access
to rest of data members.

Thanks to all who participated! 

[pa1409]

Hi, are you able to show this explanation with an example please? I have a problem with using one of the NI callback functions as a member function of a class.

[AndyCandy]

Dear pa1409,

In the event that you don't hear back from this post soon, I would recommend starting a new post, since it's been many years since the creation of the post.