[leon84]

Ora ti posto tutto il codice completo . Chiaramente non guardare gli eventuali errori non gestiti . Ho fatto in modo che alla fine della lettura del file venga generato un pacchetto di dimensione 0 e numero di byte 0 . In ricezione scrivo fino a quando non mi giunge questo pacchetto : guarda e fammi sapere .

Const FILE_ATTRIBUTE_TEMPORARY = &H100
Const FILE_SHARE_READ = &H1
Const FILE_SHARE_WRITE = &H2
Const CREATE_NEW = 1
Const CREATE_ALWAYS = 2
Const OPEN_EXISTING = 3
Const GENERIC_READ = &H80000000
Const GENERIC_WRITE = &H40000000
Private Declare Function WriteFile Lib "kernel32" (ByVal hfile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, ByVal lpOverlapped As Any) As Long
Private Declare Function ReadFile Lib "kernel32" (ByVal hfile As Long, lpBuffer As Any, ByVal nNumberOfBytesToRead As Long, lpNumberOfBytesRead As Long, ByVal lpOverlapped As Any) As Long
Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, ByVal lpSecurityAttributes As Any, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
'La costante sotto la puoi cambiare per aumentare il numero masismo di byte
Const BYTE_PER_MSG = 16

Private Sub SendMsg(num As Long, buf() As Byte)
Dim tmp(4) As Byte, I As Long
    Dim v As Variant
    tmp(1) = (num And &HFF000000) / &H1000000
    tmp(2) = (num And &HFF0000) / &H10000
    tmp(3) = (num And &HFF00) / &H100
    tmp(4) = num And &HFF
    v = tmp
    MSComm1.Output = v
    'Non so se buf vada ridimensionato comunque lo faccio uguale
    v = buf
    MSComm1.Output = v
End Sub

Private Sub Command1_Click()
    Dim buffer(BYTE_PER_MSG) As Byte, f As Long, read As Long
    CommonDialog1.ShowOpen
    f = CreateFile(CommonDialog1.FileName, GENERIC_READ, FILE_SHARE_READ, ByVal 0&, OPEN_EXISTING, 0, 0)
    If f = -1 Then
        MsgBox "Errore nell'apertura del file !!!"
    Else
        Do
            ReadFile f, buffer(1), BYTE_PER_MSG, read, ByVal 0&
            'Ora richiamo la Sub che invia i dati
            SendMsg read, buffer
        Loop Until read < BYTE_PER_MSG
        SendMsg 0, buffer
        CloseHandle f
    End If
End Sub

Private Sub Form_Load()
MSComm1.CommPort = 1
MSComm1.PortOpen = True
End Sub

Private Sub Form_Unload(Cancel As Integer)
MSComm1.PortOpen = False
End Sub

Private Sub MSComm1_OnComm()
If MSComm1.CommEvent = 2 Then
    CommonDialog1.ShowSave
    Dim hfile As Long
    hfile = CreateFile(CommonDialog1.FileName, GENERIC_WRITE, FILE_SHARE_READ, ByVal 0&, CREATE_ALWAYS, 0, 0)
    Dim tmp(4) As Byte
    MSComm1.InputLen = 4
    MSComm.InputMode = comInputModeBinary
    tmp = MSComm1.Input
    Dim num As Long
    num = Int(tmp(1)) * &H1000000
    num = num + Int(tmp(2)) * &H10000
    num = num + Int(tmp(3)) * &H100
    num = num + Int(tmp(4))
    If num <> 0 Then
    MSComm1.InputLen = num
    If MSComm1.InputCounter < num Then
    Do
    While MSComm1.InBufferCount >= num
    Dim buf() As Byte
    'quindi ora siamo sicuri che ci siano i byte da leggere
    buf = MSComm1.Input 'ed hai letto il buffer
    WriteFile f, buf(1), num, read, ByVal 0&
    Else
    CloseHandle f
    End If
End If
End Sub