form1.frm

串行口通信程序

VERSION 5.00
Begin VB.Form Form1 
   Caption         =   "Simulated FIFO Buffer Handling"
   ClientHeight    =   4590
   ClientLeft      =   60
   ClientTop       =   345
   ClientWidth     =   7110
   LinkTopic       =   "Form1"
   ScaleHeight     =   4590
   ScaleWidth      =   7110
   StartUpPosition =   3  'Windows Default
   Begin VB.TextBox Text2 
      Height          =   1455
      Left            =   300
      MultiLine       =   -1  'True
      ScrollBars      =   2  'Vertical
      TabIndex        =   5
      Text            =   "Form1.frx":0000
      Top             =   2400
      Width           =   6375
   End
   Begin VB.OptionButton Option2 
      Caption         =   "2 Buffer System"
      Height          =   195
      Left            =   3180
      TabIndex        =   4
      Top             =   1620
      Width           =   1515
   End
   Begin VB.OptionButton Option1 
      Caption         =   "1 Buffer System"
      Height          =   195
      Left            =   1560
      TabIndex        =   3
      Top             =   1620
      Value           =   -1  'True
      Width           =   1455
   End
   Begin VB.Timer Timer2 
      Interval        =   250
      Left            =   840
      Top             =   1560
   End
   Begin VB.Timer Timer1 
      Interval        =   1
      Left            =   300
      Top             =   1560
   End
   Begin VB.TextBox Text1 
      Height          =   1035
      Left            =   300
      TabIndex        =   0
      Text            =   "Text1"
      Top             =   420
      Width           =   6375
   End
   Begin VB.Label Label1 
      Caption         =   "Simulated Output"
      Height          =   255
      Index           =   1
      Left            =   300
      TabIndex        =   2
      Top             =   2040
      Width           =   2295
   End
   Begin VB.Label Label1 
      Caption         =   "Simulated Incoming Data"
      Height          =   255
      Index           =   0
      Left            =   360
      TabIndex        =   1
      Top             =   60
      Width           =   2295
   End
End
Attribute VB_Name = "Form1"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = True
Attribute VB_Exposed = False
'This source code simulates how to use a 2 buffer
'system to prevent overflows and other problems
'dealing with async modem communications.  A 16-byte
'hardware FIFO (present in serial communications) is
'simulated by Text1.  If it ever gets above 16
'characters, the Text1_Change event notifies us.
'To simulate serial communications, simply type in
'Text1 as fast as you can.  This simulates your
'serial FIFO buffer filling up with data from
'the serial port (modem, straight connection, etc).
'
'If the first option box is checked, we use a 1 buffer
'and 1-timer system.  The empty for next loops
'in the control represent time eaten up by all of
'the serial parsing usually necessary in such software.
'You'll notice that if you type as fast as you can
'you can quickly fill the 16 byte simulated serial
'FIFO buffer.  This overflow would happen even faster
'in an actual serial communcations routine where data
'transmits many times faster than you can type.
'Overflowing the hardware buffer is never good, as
'AT THE MINIMUM it leads to dropped characters and
'data loss.
'
'If the second box is checked, you'll see that
'all of the routines get shunted to a second timer.
'At that point, the only thing that the first timer
'is supposed to do is to is to empty the simulated
'input FIFO buffer into buffIn as fast as possible.
'buffIN is a form-public variable length string.
'In VB6, a variable-length string can be up to 2gigs
'(2 billion bytes) in size or so, giving you
'worlds more leeway in handling this buffer than the
'16 byte serial buffer.
'
'In practice, you throw a DoEvents in the 2nd
'timer to handle things even more smoothly.
'That way, if the 2nd routine is eating up
'lots of processor time, it releases some of it
'to the first timer so it can continue to
'fill up the input buffer.  If you put a DoEvents
'in the first timeer, it would simply increase the
'amount of time necessary to complete the routine,
'causing an even faster buffer overflow.

'Notice that when we are using the 2-buffer system,
'processing may lag behind input by quite a bit
'but we never actually lose any information, nor
'do we overflow the simulated 16-byte FIFO.
Dim buffIn As String

Private Sub Option1_Click()
If Option1.Value = True Then
    Timer2.Enabled = False
Else
    Timer2.Enabled = True
End If
End Sub

Private Sub Option2_Click()
If Option1.Value = True Then
    Timer2.Enabled = False
Else
    Timer2.Enabled = True
End If

End Sub

Private Sub Text1_Change()
If Len(Text1) > 16 Then MsgBox "Simulated hardware buffer full!"
End Sub

Private Sub Text2_KeyPress(KeyAscii As Integer)
KeyAscii = 0
End Sub

Private Sub Timer1_Timer()
If Option1.Value = True Then
    For x = 1 To 400
        For i = 1 To 20000
         'just eat up time
        Next i
    Next x
    'the above loop simulates time eaten up because
    'of buffer handling code (parsing, etc)
    'do everything in this buffer
    Text2 = Text2 & Text1.Text
    Text1 = ""
Else
    buffIn = buffIn & Text1.Text
    Text1 = ""
End If

End Sub

Private Sub Timer2_Timer()
'all parsing and code timeouts happen here.
For x = 1 To 400
    For i = 1 To 20000
       'just eat up time
    Next i
    DoEvents
Next x
'the above loop simulates time eaten up because
'of buffer handling code (parsing, etc)
Text2 = Text2 & buffIn
buffIn = ""
End Sub