Option Explicit
Public Function Code128(SourceString As String)
    'Written by Philip Treacy, Feb 2014
    'http://www.myonlinetraininghub.com/create-barcodes-with-excel-vba
    'This code is not guaranteed to be error free.  No warranty is implied or expressed. Use at your own risk and carry out your own testing
    'This function is governed by the GNU Lesser General Public License (GNU LGPL) Ver 3
    'Input Parameters : A string
    'Return : 1. An encoded string which produces a bar code when dispayed using the CODE128.TTF font
    '         2. An empty string if the input parameter contains invalid characters
    Dim Counter As Integer
    Dim CheckSum As Long
    Dim mini As Integer
    Dim dummy As Integer
    Dim UseTableB As Boolean
    Dim Code128_Barcode As String
    If Len(SourceString) > 0 Then
        'Check for valid characters
        For Counter = 1 To Len(SourceString)
            Select Case Asc(Mid(SourceString, Counter, 1))
                Case 32 To 126, 203
                Case Else
                    MsgBox "Invalid character in barcode string." & vbCrLf & vbCrLf & "Please only use standard ASCII characters", vbCritical
                    Code128 = ""
                    Exit Function
            End Select
        Next
        Code128_Barcode = ""
        UseTableB = True
        Counter = 1
        Do While Counter <= Len(SourceString)
            If UseTableB Then
                'Check if we can switch to Table C
                mini = IIf(Counter = 1 Or Counter + 3 = Len(SourceString), 4, 6)
                GoSub testnum
                If mini% < 0 Then 'Use Table C
                    If Counter = 1 Then
                        Code128_Barcode = Chr(205)
                    Else 'Switch to table C
                        Code128_Barcode = Code128_Barcode & Chr(199)
                    End If
                    UseTableB = False
                Else
                    If Counter = 1 Then Code128_Barcode = Chr(204) 'Starting with table B
                End If
            End If
            If Not UseTableB Then
                'We are using Table C, try to process 2 digits
                mini% = 2
                GoSub testnum
                If mini% < 0 Then 'OK for 2 digits, process it
                    dummy% = Val(Mid(SourceString, Counter, 2))
                    dummy% = IIf(dummy% < 95, dummy% + 32, dummy% + 100)
                    Code128_Barcode = Code128_Barcode & Chr(dummy%)
                    Counter = Counter + 2
                Else 'We haven't got 2 digits, switch to Table B
                    Code128_Barcode = Code128_Barcode & Chr(200)
                    UseTableB = True
                End If
            End If
            If UseTableB Then
                'Process 1 digit with table B
                Code128_Barcode = Code128_Barcode & Mid(SourceString, Counter, 1)
                Counter = Counter + 1
            End If
        Loop
        'Calculation of the checksum
        For Counter = 1 To Len(Code128_Barcode)
            dummy% = Asc(Mid(Code128_Barcode, Counter, 1))
            dummy% = IIf(dummy% < 127, dummy% - 32, dummy% - 100)
            If Counter = 1 Then CheckSum& = dummy%
            CheckSum& = (CheckSum& + (Counter - 1) * dummy%) Mod 103
        Next
        'Calculation of the checksum ASCII code
        CheckSum& = IIf(CheckSum& < 95, CheckSum& + 32, CheckSum& + 100)
        'Add the checksum and the STOP
        Code128_Barcode = Code128_Barcode & Chr(CheckSum&) & Chr$(206)
    End If
    Code128 = Code128_Barcode
    Exit Function
testnum:
        'if the mini% characters from Counter are numeric, then mini%=0
        mini% = mini% - 1
        If Counter + mini% <= Len(SourceString) Then
            Do While mini% >= 0
                If Asc(Mid(SourceString, Counter + mini%, 1)) < 48 Or Asc(Mid(SourceString, Counter + mini%, 1)) > 57 Then Exit Do
                mini% = mini% - 1
            Loop
        End If
        Return
End Function