What is the best way to access a serial port from VBA?

Sub Stinky()
Dim COM_Byte As Byte
Dim Received_Lines As Long
Dim Input_Buffer As String
Dim Output_Buffer As String
Dim Chars2Send As Long
Dim CharsRemaining As Long
Dim lfsr As Long
    Open "COM7:9600,N,8,1" For Random As #1 Len = 1
    Input_Buffer = ""
    CharsRemaining = 0
    Do
    Get #1, , COM_Byte
    If COM_Byte Then
        If COM_Byte = 13 Then           ' look for CR line termination
            Debug.Print Input_Buffer, Now   ' print it
            Input_Buffer = ""               ' and clear input buffer
        '   generate some output (9 characters)
            lfsr = &H3FFFFFFF - 2 ^ (Received_Lines And 15)
            Output_Buffer = "?@@@@@@@@"
            Chars2Send = 9
            CharsRemaining = 9
            For j = 0 To 2
                Mid(Output_Buffer, 2 + j, 1) = Chr(Asc(Mid(Output_Buffer, 2 + j, 1)) + (31 And Int(lfsr / 32 ^ (2 - j))))
            Next j
            Debug.Print Output_Buffer
        '   show what I generated
            Received_Lines = Received_Lines + 1 ' keep track of received line count
        Else
            Input_Buffer = Input_Buffer & Chr(COM_Byte) ' assemble output buffer
        '   process any characters to send
            If CharsRemaining Then
                CharsRemaining = CharsRemaining - 1
                COM_Byte = Asc(Mid(Output_Buffer, Chars2Send - CharsRemaining, 1))
                Put #1, , COM_Byte
            End If
        End If
    End If
    DoEvents
    Loop
    Close
End Sub

This works for me. I'm not sure if the OPEN actually sets up the Baud rate, as I first used TeraTerm. My COM port is a USB connection to a BASYS3 prototyping kit. It is spewing characters at 9600, records of 36 characters ending with CR. I can randomly send commands of 9 characters. In the above code, I generate these command strings every time I have received a new line. The way I chose which character to send is a little clunky: perhaps a better way is to have a character pointer and a number of characters, and when those go equal to set them both to zero.

The Win32 API handles the serial port as a file. You can access the serial ports directly by calling these API functions from within VBA. I had to do this for an old .NET application but VBA is no different.

Rather than hash it out for you on this site, here's a reference I've hung onto over the years. How to perform serial port communications in VBA

Here is a brief module of VBA code that can send and receive messages on a PC serial port. This is not very elegant, but it is simple and should work on modern versions of Excel and Windows.

You are left on your own to expand the functionality and store or parse the messages. This just shows the low-level stuff to deal with the serial port.

The first 5 lines declare the millisecond "Sleep" library function (based on Excel version).

The SerialPort() subroutine outlines the steps to open the port, transmit some data, receive some data, try again to receive some data (to show that it really does not run afoul of the "end of file" error), and close the port.

---

#If VBA7 Then ' Excel 2010 or later
    Public Declare PtrSafe Sub Sleep Lib "kernel32" (ByVal Milliseconds As LongPtr)
#Else ' Excel 2007 or earlier
    Public Declare Sub Sleep Lib "kernel32" (ByVal Milliseconds As Long)
#End If

Public Sub SerialPort()
    ' open a COM port, transmit a message, gather results, close the port.

    ' open the COM port as file #1
    Debug.Print "Open COM port 4"
    Open "COM4:115200,N,8,1" For Binary Access Read Write As #1

    transmit$ = Chr(2) + "Hello, World." + Chr(13)
    receiveDummy$ = "~~~"

    ' transmit a message
    Put #1, , transmit$
    Debug.Print "Message sent."

    ' wait a bit for a response
    Sleep 100

    ' check for received message
    Debug.Print "Look for incoming message."
    On Error Resume Next
    Do While True
        receive$ = receiveDummy$  'dummy value
        Input #1, receive$
        If receive$ = receiveDummy$ Then Exit Do  'the string didn't change, so move on
        Debug.Print receive$
    Loop
    On Error GoTo 0

    ' do it again to show that the empty input queue doesn't stop the flow
    Debug.Print "Look again for incoming message (should not stop on error)."
    On Error Resume Next
    Do While True
        receive$ = receiveDummy$  'dummy value
        Input #1, receive$
        If receive$ = receiveDummy$ Then Exit Do  'the string didn't change, so move on
        Debug.Print receive$
    Loop
    On Error GoTo 0

    ' close the serial port
    Debug.Print "Close COM port."
    Close #1

    Debug.Print "Done."
End Sub