vb串口通讯发送和接受的难处解决方法

vb串口通讯发送和接受的难处
各位好，现在正在做一程序，vb和西门子plc，s7-300通讯的项目（十六进制）。单独做上位机接受数据时可以正常接收到plc发过来的十六进制信息 ，但是然后续上上位机发送数据代码就不能通讯成功，不知道发送数据应该如何写。协议是：上位机接受40字节。发送25字节。第一位起始位255。中间的数据就忽略不写了。发送和接受时交叉进行的。哪位能给具体提示一下逻辑上需要如何改正啊,现在就感觉不对，但是不知道该修改哪里。谢谢！
具体代码如下 
[code=VB][/code]Private Sub MSComPLC_OnComm() 
  Dim buffer As Variant 
  Dim InString(0 To PLCRxCount - 1) As Byte 
  Dim i As Integer 
  Dim SupplyNum As Integer 
  Dim CheckInt As Integer 
  Dim CheckByte As Byte 
  Dim errTemp As Integer 
  
  Select Case MSComPLC.CommEvent 
  '================================================================== 
  ' 接收数据 
  '================================================================== 
  Case comEvReceive 
   
  MSComPLC.InputLen = 0 
  buffer = MSComPLC.Input 
   
  For i = 0 To PLCRxCount - 1 
  InString(i) = buffer(i) 
  Next i 
  '================================================================== 
  ' 检查数据完整性/ 计算校验和 
  '================================================================== 
  CheckInt = 0 
  For i = 0 To PLCRxCount - 2 
  CheckInt = CheckInt + InString(i) 
  Next i 
  CheckByte = CheckInt Mod 256 
  '================================================================== 
  ' 更新上纸台信息，并设置Refresh标志以便更新显示 
  '================================================================== 
  If (InString(0) = 255) And (CheckByte = InString(PLCRxCount - 1)) Then 
  '上纸台可以接受命令 
  ...
...
...(中间程序，可以忽略与本问题无关）

  End If 
End Select 
End Sub 

Private Sub TmPLC_Timer() 
  Dim OutString(0 To PLCTxCount - 1) As Byte 
  Dim CheckInt As Integer 
  Dim j As Integer 
  Dim i As Integer 
  Dim AndBit As Integer 
   
  ' 特征字节 
  OutString(0) = 255 
  ...
  ...
  ...(z中间字节，可以忽略，与本问题无关）
   
  ' 校验字节，由计算确定 
  CheckInt = 0 
  For j = 0 To PLCTxCount - 2 
  CheckInt = CheckInt + OutString(j) 
  Next j 
  OutString(PLCTxCount - 1) = CheckInt Mod 256 
  Next i 
   
  '========================================================================== 
  ' 发送通讯字节内容 
  '========================================================================== 
  MSComPLC.InBufferCount = 0 
  MSComPLC.Output = OutString 
End Sub 


[解决办法]
学习

[解决办法]

VB code

Option Explicit    Dim PLCRxCount As LongPrivate Sub Form_Load()    TmPLC.Interval = 0    PLCRxCount = 40    MSComPLC.Settings = "9600,n,8,1"    MSComPLC.InputMode = comInputModeBinary    MSComPLC.RThreshold = PLCRxCount    MSComPLC.PortOpen = TrueEnd SubPrivate Sub MSComPLC_OnComm()    Dim buffer As Variant    Dim InString(0 To 40 - 1) As Byte    Dim i As Integer    Dim SupplyNum As Integer    Dim CheckInt As Integer    Dim CheckByte As Byte    Dim errTemp As Integer    Select Case MSComPLC.CommEvent      ' 接收数据        Case comEvReceive            MSComPLC.InputLen = 0            buffer = MSComPLC.Input            Label1 = UBound(buffer)            For i = 0 To PLCRxCount - 1                InString(i) = buffer(i)            Next i            ' 检查数据完整性/ 计算校验和            CheckInt = 0            For i = 0 To PLCRxCount - 2                CheckInt = CheckInt + InString(i)            Next i            CheckByte = CheckInt Mod 256                Call TmPLC_Timer            ' 更新上纸台信息，并设置Refresh标志以便更新显示            '==================================================================            If (InString(0) = 255) And (CheckByte = InString(PLCRxCount - 1)) Then                '上纸台可以接受命令                ' ...(中间程序，可以忽略与本问题无关）            End If    End SelectEnd SubPrivate Sub TmPLC_Timer()    Dim OutString(0 To 25 - 1) As Byte    Dim CheckInt As Integer    Dim j As Integer    Dim i As Integer    Dim AndBit As Integer        ' 特征字节    OutString(0) = 255    '...    '...    '...(z中间字节，可以忽略，与本问题无关）        ' 校验字节，由计算确定    CheckInt = 0    For j = 0 To PLCTxCount - 2        CheckInt = CheckInt + OutString(j)    Next j    '缺少For 条件    'OutString(PLCTxCount - 1) = CheckInt Mod 256    'Next i        '==========================================================================    ' 发送通讯字节内容    '==========================================================================    MSComPLC.InBufferCount = 0    MSComPLC.Output = OutStringEnd Sub 
 
[解决办法]
更正1个错误:
VB code
Private Sub MSComPLC_OnComm()    Dim buffer As Variant    Dim InString(0 To 40 - 1) As Byte    Dim i As Integer    Dim SupplyNum As Integer    Dim CheckInt As Integer    Dim CheckByte As Byte    Dim errTemp As Integer    Select Case MSComPLC.CommEvent      ' 接收数据        Case comEvReceive            MSComPLC.InputLen = 0            buffer = MSComPLC.Input            Label1 = UBound(buffer)            For i = 0 To PLCRxCount - 1                InString(i) = buffer(i)            Next i            ' 检查数据完整性/ 计算校验和            CheckInt = 0            For i = 0 To PLCRxCount - 2                CheckInt = CheckInt + InString(i)            Next i            CheckByte = CheckInt Mod 256            ' 更新上纸台信息，并设置Refresh标志以便更新显示            '==================================================================            If (InString(0) = 255) And (CheckByte = InString(PLCRxCount - 1)) Then                '上纸台可以接受命令                ' ...(中间程序，可以忽略与本问题无关）                 Call TmPLC_Timer           End If    End SelectEnd Sub
[解决办法]
鼎云厉害