How to: Receive Strings From Serial Ports in Visual Basic

This topic describes how to use My.Computer.Ports to receive strings from the computer's serial ports in Visual Basic.

To receive strings from the serial port

1. Initialize the return string.

Dim returnStr As String = ""

2. Determine which serial port should provide the strings. This example assumes it is COM1.

3. Use the My.Computer.Ports.OpenSerialPort method to obtain a reference to the port. For more information, see OpenSerialPort.

The Try...Catch...Finally block allows the application to close the serial port even if it generates an exception. All code that manipulates the serial port should appear within this block.

Dim com1 As IO.Ports.SerialPort = Nothing
Try
com1 = My.Computer.Ports.OpenSerialPort("COM1")

Catch ex As TimeoutException
returnStr = "Error: Serial Port read timed out."
Finally
If com1 IsNot Nothing Then com1.Close()
End Try

4. Create a Do loop for reading lines of text until no more lines are available.

Do
Loop

5. Use the ReadLine() method to read the next available line of text from the serial port.

Dim Incoming As String = com1.ReadLine()

6. Use an If statement to determine if the ReadLine() method returns Nothing (which means no more text is available). If it does return Nothing, exit the Do loop.

If Incoming Is Nothing Then
Exit Do
End If

7. Add an Else block to the If statement to handle the case if the string is actually read. The block appends the string from the serial port to the return string.

Else
returnStr &= Incoming & vbCrLf

8. Return the string.

Return returnStr


Example

Function ReceiveSerialData() As String
' Receive strings from a serial port.
Dim returnStr As String = ""

Dim com1 As IO.Ports.SerialPort = Nothing
Try
com1 = My.Computer.Ports.OpenSerialPort("COM1")
Do
Dim Incoming As String = com1.ReadLine()
If Incoming Is Nothing Then
Exit Do
Else
returnStr &= Incoming & vbCrLf
End If
Loop
Catch ex As TimeoutException
returnStr = "Error: Serial Port read timed out."
Finally
If com1 IsNot Nothing Then com1.Close()
End Try

Return returnStr
End Function


This code example is also available as an IntelliSense code snippet. In the code snippet picker, it is located in Connectivity and Networking. For more information, see Code Snippets.

Compiling the Code

This example assumes the computer is using COM1.

Robust Programming

This example assumes the computer is using COM1. For more flexibility, the code should allow the user to select the desired serial port from a list of available ports. For more information, see How to: Show Available Serial Ports.

This example uses a Try...Catch...Finally block to make sure that the application closes the port and to catch any timeout exceptions. For more information, see Try...Catch...Finally Statement.