vb编写应用程序与下位机通信出现掉包问题
近来写了个程序，通过modbus协议，读下位机的数据，
在读数据过程序，发现有时数据会跳到0的情况，不知这是不是读的过程序出现丢包？
如果是，怎么处理？
[最优解释]
modbus只是定义了数据格式，并没有定义出错处理，丢包是有可能的，在程序的OnComm代码中，至少应该监控"奇偶错"，估计可以发现大部分传输错误。另外，数据会跳到0不一定是数据错，你的读出目标如果为0，则就会读到0！
[其他解释]
Private Sub Timer1_Timer() '读存储区数据
.........
MSComm1.OutBufferCount = 0 '清空输出寄存器
MSComm1.Output = aa
'必须延迟一定时间再接受，当数据发到缓冲区后，发送是在后台的线程中进行，不会马上收到数据
FGetData = ReceiveData
........
End Sub
[其他解释]
这个的看看代码，分析一下才行。
[其他解释]
Private Sub Timer1_Timer() '读存储区数据
Dim CRC() As Byte
Dim FGetData As String
Dim aa() As Byte
Dim s As String
Dim str As String
Dim i As Integer
Dim bb, cc As String
ReDim aa(5) As Byte '定义动态数组
aa(0) = add
aa(1) = order
aa(2) = h_add
aa(3) = l_add
aa(4) = h_num
aa(5) = l_num
CRC = CRC16(aa)
str = CRC
s = ""
For i = 1 To LenB(str)
s = s + Hex(AscB(MidB(str, i, 1)))
Next i
bb = Right(s, 2)
cc = Mid(s, 1, 2)
If Len(s) < 4 Then
cc = Mid(s, 1, 1)
End If
ReDim Preserve aa(0 To 7) As Byte
aa(6) = Val("&H" & bb)
aa(7) = Val("&H" & cc)
MSComm1.OutBufferCount = 0 '清空输出寄存器
MSComm1.Output = aa
FGetData = ReceiveData
Text5.Text = FGetData
p11 = Val("&H" & Mid(FGetData, 7, 4))
p12 = Val("&H" & Mid(FGetData, 11, 4))
p13 = Val("&H" & Mid(FGetData, 15, 4))
p14 = Val("&H" & Mid(FGetData, 19, 4))
'For c_num = 0 To 10 Step 1
'm_data(c_num) = p12
'Next
'p12 = sortdata(m_data())
End Sub
Function CRC16(Data() As Byte) As String
Dim CRC16Lo As Byte, CRC16Hi As Byte 'CRC寄存器
Dim CL As Byte, CH As Byte '多项式码&HA001
Dim SaveHi As Byte, SaveLo As Byte
Dim i As Integer
Dim Flag As Integer
CRC16Lo = &HFF
CRC16Hi = &HFF
CL = &H1
CH = &HA0
For i = 0 To UBound(Data)
CRC16Lo = CRC16Lo Xor Data(i) '每一个数据与CRC寄存器进行异或
For Flag = 0 To 7
SaveHi = CRC16Hi
SaveLo = CRC16Lo
CRC16Hi = CRC16Hi \ 2 '高位右移一位
CRC16Lo = CRC16Lo \ 2 '低位右移一位
If ((SaveHi And &H1) = &H1) Then '如果高位字节最后一位为1
CRC16Lo = CRC16Lo Or &H80 '则低位字节右移后前面补1
End If '否则自动补0
If ((SaveLo And &H1) = &H1) Then '如果LSB为1，则与多项式码进行异或
CRC16Hi = CRC16Hi Xor CH
CRC16Lo = CRC16Lo Xor CL
End If
Next Flag
Next i
Dim ReturnData(1) As Byte
ReturnData(0) = CRC16Hi 'CRC高位
ReturnData(1) = CRC16Lo 'CRC低位
CRC16 = ReturnData
End Function
Function ReceiveData() As String '返回存储器区数据
Dim FGetData As String
Dim t1 As Long
Dim av As Variant
Dim i As Integer
Dim ReDataLen As Integer
FGetData = ""
t1 = GetTickCount() '取时间，做延时用
Do '循环等待接收数据
DoEvents
If Form1.MSComm1.InBufferCount > 0 Then '串口有数据了
ReDataLen = Form1.MSComm1.InBufferCount '取数据长度
av = Form1.MSComm1.Input '将串口数据取出来
For i = 0 To ReDataLen - 1
FGetData = FGetData & Right("00" & Hex(av(i)), 2)
Next i
End If
If Len(FGetData) >= 6 Then
If Len(FGetData) > Val("&H" & Mid(FGetData, 5, 2)) * 2 + 8 Then
ReceiveData = FGetData
Exit Function
End If
End If
If GetTickCount - t1 > 300 And c_num <= 20 Then '2秒没收完就不收了
c_num = c_num + 1
ReceiveData = Form1.Text5.Text
Else
ReceiveData = "0"
c_num = 0
Exit Function
End If
Loop
End Function
这是我读取的数据的代码，大虾们帮我看看是什么问题啊？
