DAQmxRegis​terEveryNS​amplesEven​t used correctly?

Question

I have the following code :
'Add an analog input channel to the task.
DAQmxErrChk DAQmxCreateAIVoltageChan(taskHandle, "Dev1/ai0:7", "channel1", _
        DAQmx_Val_Cfg_Default, "-10", "10", _
        DAQmx_Val_VoltageUnits1_Volts, "")
'Configure task for continuous sample acquisition and read in data
DAQmxErrChk DAQmxCfgSampClkTiming(taskHandle, "OnboardClock", frmSettings.txtRate.Text, DAQmx_Val_Rising, _
             DAQmx_Val_AcquisitionType_ContSamps, CLng(frmSetMachine.txtSamplesPerChannel.Text))
'DAQmxErrChk DAQmxCfgSampClkTiming(taskHandle, "OnboardClock", frmSettings.txtRate.Text, DAQmx_Val_Rising, _
             'DAQmx_Val_AcquisitionType_ContSamps, CLng(frmSettings.txtRate.Text) / 10#)
'DAQmxCfgAnlgEdgeStartTrig taskHandle, "channel1", DAQmx_Val_Slope1_RisingSlope, #
DAQmxCfgAnlgEdgeStartTrig taskHandle, "channel1", DAQmx_Val_Slope1_RisingSlope, triggerVolume ' 0# ' triggerPressure
   'Initiate for Acquisition Event
DAQmxErrChk DAQmxGetTaskNumChans(taskHandle, numChannels)
etc etc
DummySub DAQmxRegisterEveryNSamplesEvent(taskHandle, DAQmx_Val_Acquired_Into_Buffer, CLng(frmSetMachine.txtSamplesPerChannel.Text), 0, AddressOf EveryNSamplesCallback, Null)
I get a compile error : Invalid use of AddressOf operator.
The callback is:
Public Function EveryNSamplesCallback(taskHandle As Long, everyNsamplesEventType As Long, nSamples As Long, callbackData As Long) As Long
If I just do:
DummySub DAQmxRegisterEveryNSamplesEvent(taskHandle, DAQmx_Val_Acquired_Into_Buffer, CLng(frmSetMachine.txtSamplesPerChannel.Text), 0, EveryNSamplesCallback, Null)
I get "argument not optional" and its pointing at EveryNSamplesCallback.
How do I fix this compile error? Also, is there anything else I need to add in code, or will this event fire automatically once I get past the compile issue?
thanks,
saroj

Answer 1

Hi,
The callback function must be declared in a .bas module, not in the form code.
The following code comes  from the  nidaqmx\Analog In\Measure Slow Varying Signal\One Sample sample and it works !!
(better than NI original code :-) )
good luck
Thierry
In the form code :
' stop task button
Private Sub bstoptask_Click()
    DAQmxErrChk DAQmxStopTask(taskHandle)
    taskIsRunning = False
End Sub
' start measurement
Private Sub startCommandButton_Click()
    Dim sampsPerChanRead As Long
    Dim errcode As Long
    On Error GoTo ErrorHandler
    If ValidateControlValues Then Exit Sub
    startCommandButton.Enabled = False
    '  Create the DAQmx task.
    DAQmxErrChk DAQmxCreateTask("", taskHandle)
    '  Add an analog input channel to the task.
    DAQmxErrChk DAQmxCreateAIVoltageChan(taskHandle, physicalChannelTextBox.Text, "", _
        DAQmx_Val_InputTermCfg_RSE,
CDbl(Val(minValueTextBox.Text)), CDbl(Val(maxValueTextBox.Text)), _
        DAQmx_Val_VoltageUnits2_Volts, "")
    DAQmxErrChk DAQmxCfgSampClkTiming(taskHandle,
"OnboardClock", 100, DAQmx_Val_Rising,
DAQmx_Val_AcquisitionType_ContSamps, 1000)
    DAQmxErrChk
DAQmxRegisterEveryNSamplesEvent(taskHandle,
DAQmx_Val_Acquired_Into_Buffer, 100, 0, AddressOf backfonc, Null)
    DAQmxErrChk DAQmxStartTask(taskHandle)
    taskIsRunning = True
    Exit Sub
ErrorHandler:
    If taskIsRunning = True Then
        DAQmxStopTask taskHandle
        DAQmxClearTask taskHandle
        taskIsRunning = False
    End If
    startCommandButton.Enabled = True
    MsgBox "Error:" & Err.Number & " " & Err.Description, , "Error"
End Sub
In the bas module :
Public total As Long
Public arraydata(0 To 1000) As Double
Public taskHandle As Long
Public taskIsRunning As Boolean
Public Function backfonc(ByVal Hwnd As Long, ByVal lParam As Long, ByVal nSamples As Long, ByVal callbackData As Long) As Long
Dim p1 As Long
    total = total + 1   ( number of calls)
    p1 = DAQmxReadAnalogF64(taskHandle, -1, 10, DAQmx_Val_GroupByScanNumber, arraydata(0), 100, 0, ByVal 0&)
    Mainform.acquisitionDataTextBox = "callback " & total & "  val=" & arraydata(0)
End Function