vb.net SerialPort writing and receiving - vb.net

Question

I have an issue with reading the results of a serialport communication.
If have multiple functions, which are executing different commands on my device and also receiving and process the sent results on slightly different ways.
If i call the functions all at once, its not working 100% reliable. If i call only one of these functions, all of them working perfectly. Im assuming, thinks going a little to fast for the slow serialPort communication, so some functions dont get propper results when calling all "at once"
Code which calls 3 functions (Error on different Positions: "Index out of (array) range" or "CRC incorrect")
Private Sub btn_initialize_Click(sender As Object, e As EventArgs) Handles btn_initialize.Click
Dim actual_mw As Double
Dim attenuated As Integer
'open (virtual)COMport
If rfid.init_serialport(cb_com.SelectedItem.ToString) = True Then
flp_data.Visible = True
lbl_snr_data.Text = rfid.get_serial_number()
lbl_type_data.Text = rfid.get_reader_type
Dim collection As MatchCollection = Regex.Matches(rfid.get_attennuation, "\d+")
attenuated = collection(1).Value
If attenuated = 0 Then
lbl_leistung_data.Text = "100mW / 100mW"
Else
actual_mw = 100 - (10 ^ (attenuated / 10))
lbl_leistung_data.Text = Math.Round(actual_mw, 2) & "mW / 100mW"
End If
Else
MsgBox("Error! COM-Schnittstelle konnte nicht initialisiert werden!", MsgBoxStyle.Critical, "Error")
End If
End Sub
Intialize and Dispose SerialPort
Public Class RFE
Private RF_CHECKSUM As Byte
Private device_output() As Byte
Private data() As Byte
Private device_input() As Byte
Private sp As SerialPort
Public Function init_serialport(portName As String) As Boolean
Try
sp = New SerialPort
sp.PortName = portName
sp.BaudRate = 9600
sp.Parity = Parity.None
sp.DataBits = 8
sp.StopBits = StopBits.One
sp.Handshake = Handshake.None
sp.ReadTimeout = 500
sp.WriteTimeout = 500
sp.Open()
If sp.IsOpen Then
Return True
Else
Return False
End If
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Function
Public Function disp_serialport()
Try
sp.Dispose()
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Function
Functions
Public Function get_serial_number() As String
Try
Dim serial_number As String
ReDim data(8)
data(0) = RF_START_BYTE0
data(1) = RF_START_BYTE1
data(2) = RF_START_BYTE2
data(3) = RF_COMMAND_START_BYTE
data(4) = RF_READER_COMMON
data(5) = RF_GET_SERIAL_NUMBER
data(6) = RF_LENGTH_START_BYTE
data(7) = &H0
data(8) = RF_CHECKSUM_START_BYTE
RF_CHECKSUM = Hex(calc_xor(data, 1))
device_input = data
Array.Resize(device_input, data.Length + 1)
device_input(9) = "&H" & RF_CHECKSUM
sp.Write(device_input, 0, device_input.Length)
ReDim device_output(sp.BytesToRead)
sp.Read(device_output, 0, device_output.Length - 1)
If calc_xor(device_output, 3) <> device_output(14) Then
MsgBox("CRC incorrect!", MsgBoxStyle.Critical, "Error")
Form1.Close()
End If
serial_number = Hex(device_output(9)).PadLeft(2, "0") & "-" & Hex(device_output(10)).PadLeft(2, "0") & "-" & Hex(device_output(11)).PadLeft(2, "0") & "-" _
& Hex(device_output(12)).PadLeft(2, "0")
device_output = Nothing
data = Nothing
RF_CHECKSUM = Nothing
sp.DiscardInBuffer()
sp.DiscardOutBuffer()
Return serial_number
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Function
The other functions are structure-vise the same, only the sent bytes and the processing of the result is slightly different.
As I said, im assueming the problem is the transaction of the signals (possibly in both directions) so the SerialPort and/or Device cant handle things fast enough.
What do I have to do so that the communication runs reliably/sequentially?
Maybe DataReceivedHandler could fix the Problem? But for every function I call, the processing of the result is different, how do I implement this in the DataReceivedHandler then (if the Handler could fix the Problem)?
Thank you all!