How to Set Tasks in separate forms from where they run

[walter-donovan]

MS 2008 in VBasic

 I am trying to write a data acq pgm that is portable between a few dif D/A systems I have. I have taken code from NI examples to set up channels (singl channel read ai). I choped up this code to attempt to separate it into 2 parts: 1) configure the tasks and 2) perform the tasks. I require this separation so I can have a separate config page where hdwr setups are loaded, saved and edited (for using different sensors, sig conditioners, etc). 

My attempts have so far been a failure. I cannot successfully define tasks on one form and  run them on another. I am sure this an issue with variable scope but I don't understand the workings of the code that assigns 'mytask'. I can't seem to pass the variable 'PhysChan' between forms. The button that clicks to call 'Config' opens the configuration page where the variable is defined. The problem arises when I try to debug or compile. The compiler does not recognise that the 'ConfigForm' is supposed to set the task the after execution of the ConfigForm, we return to the MainForm where the task is to be perfomed. I get a compiler error that the PhyChan variable is null. Compiler is trying to compile the MainForm in its entirety although I need it to see that its compilation is dependent on the ConfigForm. That is beyond my meager knowledge of VS. Is there a straightforward solution to this?

Module MainModule
    Public mytask1, mytask2, mytask3 As Task
    Public reader1, reader2, reader3 As AnalogMultiChannelReader
    Public data1(), data2(), data3() As Double
    Public PhysChan1, PhysChan2, PhysChan3 As String
    Public PassedConfig As Boolean
End Module

Public Class MainForm
    Inherits System.Windows.Forms.Form

    Public Sub MainForm_load(ByVal sender As System.Object, ByVal e _
    As System.EventArgs) Handles MyBase.Load
        mytask1 = New Task()
        mytask2 = New Task()
        mytask3 = New Task()
        reader1 = New AnalogMultiChannelReader(mytask1.Stream)
        reader2 = New AnalogMultiChannelReader(mytask2.Stream)
        reader3 = New AnalogMultiChannelReader(mytask3.Stream)
        PassedConfig = False

    End Sub

    Public Sub RunBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RunBtn.Click
        Try
            RunBtn.Enabled = False

            data1 = reader1.ReadSingleSample()
            data2 = reader2.ReadSingleSample()
            data3 = reader3.ReadSingleSample()
            TextBox1.Text = data1(0)
            TextBox2.Text = data2(0)
            TextBox3.Text = data3(0)

        Catch exception As DaqException
            MessageBox.Show(exception.Message)
        Finally
            'mytask1.Dispose()
        End Try
        RunBtn.Enabled = True
    End Sub

    Private Sub ConfigBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ConfigBtn.Click
        ConfigForm.Show()

        mytask1 = New Task()
        mytask1.AIChannels.CreateVoltageChannel(PhysChan1, "", _
            AITerminalConfiguration.Rse, Convert.ToDouble(-10), _
            Convert.ToDouble(10), AIVoltageUnits.Volts)
        reader1 = New AnalogMultiChannelReader(mytask1.Stream)
        mytask1.Control(TaskAction.Verify)

        mytask2 = New Task()
        mytask2.AIChannels.CreateVoltageChannel(PhysChan2, "", _
            AITerminalConfiguration.Rse, Convert.ToDouble(-10), _
            Convert.ToDouble(10), AIVoltageUnits.Volts)
        reader2 = New AnalogMultiChannelReader(mytask2.Stream)
        mytask2.Control(TaskAction.Verify)

        mytask3 = New Task()
        mytask3.AIChannels.CreateVoltageChannel(PhysChan3, "", _
            AITerminalConfiguration.Rse, Convert.ToDouble(-10), _
            Convert.ToDouble(10), AIVoltageUnits.Volts)
        reader3 = New AnalogMultiChannelReader(mytask3.Stream)
        mytask3.Control(TaskAction.Verify)
    End Sub

End Class

Public Class ConfigForm

    Public Sub ConfigForm_load(ByVal sender As System.Object, ByVal e _
        As System.EventArgs) Handles MyBase.Load
        mytask1 = New Task()
        mytask2 = New Task()
        mytask3 = New Task()
        reader1 = New AnalogMultiChannelReader(mytask1.Stream)
        reader2 = New AnalogMultiChannelReader(mytask2.Stream)
        reader3 = New AnalogMultiChannelReader(mytask3.Stream)

    End Sub

    Public Sub Done_click(ByVal sender As System.Object, ByVal e As System.EventArgs) _
        Handles DoneBtn.Click

        PhysChan1 = PhysChanCbo1.Text
        PhysChan2 = PhysChanCbo2.Text
        PhysChan3 = PhysChanCbo3.Text

        mytask1 = New Task()
        mytask1.AIChannels.CreateVoltageChannel(PhysChan1, "", _
            AITerminalConfiguration.Rse, Convert.ToDouble(-10), _
            Convert.ToDouble(10), AIVoltageUnits.Volts)
        reader1 = New AnalogMultiChannelReader(mytask1.Stream)
        mytask1.Control(TaskAction.Verify)

        mytask2 = New Task()
        mytask2.AIChannels.CreateVoltageChannel(PhysChan2, "", _
            AITerminalConfiguration.Rse, Convert.ToDouble(-10), _
            Convert.ToDouble(10), AIVoltageUnits.Volts)
        reader2 = New AnalogMultiChannelReader(mytask2.Stream)
        mytask2.Control(TaskAction.Verify)

        mytask3 = New Task()
        mytask3.AIChannels.CreateVoltageChannel(PhysChan3, "", _
            AITerminalConfiguration.Rse, Convert.ToDouble(-10), _
            Convert.ToDouble(10), AIVoltageUnits.Volts)
        reader3 = New AnalogMultiChannelReader(mytask3.Stream)
        mytask3.Control(TaskAction.Verify)
        PassedConfig = True
        Me.Hide()

    End Sub
End Class

 I apologize for the long code attachments. I could not see a way to pare it down and keep the intention of the code.
 

[pBerg]

Howdy walter,

If I understand your question correctly, it looks like your problem lies in the ConfigBtn_Click callback in MainForm.  In that function, you're passing the value of PhysChan1 to CreateVoltageChannel() before PhysChan1 is assigned a value. Currently, a value for PhysChan1 is assigned only in your ConfigForm, so if MainForm tries to use PhysChan1 before ConfigForm has a chance to assign it a value, you would receive an error like what you describe.

My suggestion would be to either (a) ensure that ConfigForm always runs before you try to read the value from PhysChan1 or (b) add some code in MainForm that will assign an appropriate value to PhysChan1 before it is read in CreateVoltageChannel().

[walter-donovan]

Thanks for the response. You are correct about the reading of a value that is not yet assigned. However during running of the app, you can see that the offending code is immediately after the line that calls the config procedure. This means that the variable cannot be called before it is assigned as you suggest. This error is not at runtime. It is because the compiler cannot understand that it is flagging a condition that cannot exist while the program runs. The error prevents the compiling of the program.

[walter-donovan]

Anyone? Anyone at all?

[pBerg]

hi walter,

Perhaps if you posted a ZIP of your very simplified version of your code that exhibits your problem, we could take a look at it.  (Basically, provide a ZIP containing a very simple project we can literally open and attempt to compile so we can see the problem you describe.)

Sorry for the confusion.