How to make BackgroundWorker result consistent - vb.net

Question

I am using a background worker to poll digital IO state from my PLC with MODBUS through serial com port. The background worker is used for READ only operation with polling speed of 200 ms between each cycle. The number of I/O to read in one cycle is around 50 bits (1 io state = 1 boolean). I then simulate a WRITE command to the PLC by toggling all I/O to it's opposite state (ON -> OFF / OFF -> ON) from the UI thread. I put the Toggle() sub routine into a timer with an interval of 100 ms and execute for 3000 ms. The result is some of the ports are not consistent. The starting state is either all ON or all OFF. The expected result after the toggle is also either all ON or all OFF. Sometimes the end result is 45 io ON and 5 io OFF. Sometimes it is 40 io OFF and 10 io ON. The result is always random but never the correct result which is all io ON or all io OFF.
Interestingly, if I use Timer instead of background worker to do the polling, the toggle result is always consistent ( all 50 IO either ON or OFF ). I prefer to use background worker for the polling because the UI is much smoother. When I use timer, the UI feels a little bit "laggy". However, the background worker is causing inconsistent result.
Below is simplified version of my codes to illustrate the my situation. I hope somebody can point me to the right direction. The background worker is working fine polling the io state. But as soon as I manipulate the port from the UI thread, the command is not 100% guaranteed to be executed at all time which is a serious problem for me !
I put the MODBUS read operation in the DoWork sub routine and pass the read result to UI thread using the Backgroundworker.ReportProgress(0, objResult). Therefore, the read and write manipulation of the object are both happening in the same UI thread. Why the result is still inconsistent?
Public Class Bit
Private _LastValue As Boolean
Private _Label As Control
Private DictControl As New Dictionary(Of String, Control)
Sub SetCoil(val As Boolean)
'Do work here
'Update last value
SetLastValue(val)
End Sub
Function GetCoil() As Boolean
Dim result As Boolean = True
'Do work here
'Update last value
SetLastValue(result)
Return result
End Function
Sub Toggle()
Dim result As Boolean = Not _LastValue
'invert current value
'set current value = not current value
'Update last value
SetLastValue(result)
End Sub
Sub SetLastValue(val As Boolean)
_LastValue = val
Dim ctlText As String = ""
If _LastValue = True Then
ctlText = "ON"
Else
ctlText = "OFF"
End If
'Update label
For Each c As KeyValuePair(Of String, Control) In DictControl
c.Value.Text = ctlText
Next
End Sub
Sub AddControl(key As String, c As Control)
DictControl.Add(key, c)
End Sub
End Class
Public Class Main
Const BIT_COUNT = 50
Dim bgw As New System.ComponentModel.BackgroundWorker
Dim dictBit As New Dictionary(Of Integer, Bit)
Sub New()
'Create a bunch of label and bit instances
'Add label control to Bit
'Add Bit to dictBit dictionary
'Add Handler to Background Worker
'Run Background Worker
'Run Timer
End Sub
Sub bgw_DoWorkHandler(sender As Object, e As System.ComponentModel.DoWorkEventArgs)
Dim Coils(49) As Boolean
Dim result() As Boolean
Dim run As Boolean = True
Do
'Do some processing & assign Coils value
result = Coils.Clone 'Duplicate the variable
bgw.ReportProgress(0, result)
Sleep(300)
Loop While run = True
End Sub
Sub bgw_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs)
Dim results() As Boolean = DirectCast(e.UserState, Boolean())
Dim i As Integer
For i = 0 To UBound(results)
If dictBit.ContainsKey(i) Then
dictBit(i).SetLastValue(results(i)) 'Set last value and update Label
End If
Next
End Sub
Sub TimerTickHandler(ByVal sender As Object, ByVal e As EventArgs)
'Background worker alternative.
Dim Coils(49) As Boolean
Dim result() As Boolean
Dim run As Boolean = True
'Do some work & assign Coils value
result = Coils.Clone
bgw.ReportProgress(0, result)
End Sub
Sub ToggleBits()
Dim i As Integer
For i = 0 To BIT_COUNT - 1
dictBit(i).Toggle()
Next
End Sub
Sub TimerToggleHandler(ByVal sender As Object, ByVal e As EventArgs)
'Interval = 100 ms. Run for 3000 ms before stop.
ToggleBits()
End Sub
End Class

Answer 1

The Background Worker is reading the IO during the toggle. That's why all 50 points aren't the same value.
Actual hardware inputs can change at any time. Even the a split second before or after your read. If this toggle routine is only for testing, then ignore the discrepancies. Otherwise, update a flag or fire an event at the end of the toggle. Anything to force the BW to wait until the toggle is complete before reading.