如何使用vb6.0写modbusrtu通讯

你可以使用VB6.0调用Windows API函数实现Modbus RTU通信。以下是一个简单的示例代码，可以帮助你入门：

Private Declare Sub Sleep Lib "kernel32" (ByVal dwMilliseconds As Long)
Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As Any, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Long) As Long
Private Declare Function WriteFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, 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, lpOverlapped As Any) As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long

Const GENERIC_READ = &H80000000
Const GENERIC_WRITE = &H40000000
Const OPEN_EXISTING = 3
Const FILE_ATTRIBUTE_NORMAL = &H80
Const INVALID_HANDLE_VALUE = -1

Private Function ModbusRTU_Read(ByVal PortName As String, ByVal Address As Byte, ByVal FunctionCode As Byte, ByVal StartAddress As Integer, ByVal NumOfPoints As Integer, ByRef Data() As Byte) As Boolean
    Dim hCom As Long
    Dim Buffer() As Byte
    Dim CRC() As Byte
    Dim nWritten As Long
    Dim nRead As Long
    Dim i As Integer
    Dim j As Integer
    Dim k As Integer
    
    ' Open the serial port
    hCom = CreateFile(PortName, GENERIC_READ Or GENERIC_WRITE, 0, ByVal 0&, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0)
    If hCom = INVALID_HANDLE_VALUE Then
        ModbusRTU_Read = False
        Exit Function
    End If
    
    ' Construct the MODBUS RTU request
    ReDim Buffer(0 To 7 + NumOfPoints * 2)
    Buffer(0) = Address
    Buffer(1) = FunctionCode
    Buffer(2) = StartAddress \ 256
    Buffer(3) = StartAddress Mod 256
    Buffer(4) = NumOfPoints \ 256
    Buffer(5) = NumOfPoints Mod 256
    CRC = ModbusRTU_CRC(Buffer, 0, 6)
    Buffer(6 + NumOfPoints * 2) = CRC(0)
    Buffer(7 + NumOfPoints * 2) = CRC(1)
    
    ' Send the request
    If WriteFile(hCom, Buffer(0), UBound(Buffer) + 1, nWritten, ByVal 0&) = 0 Then
        ModbusRTU_Read = False
        CloseHandle hCom
        Exit Function
    End If
    
    ' Wait for response
    Sleep 10 * (NumOfPoints \ 8 + 1)
    
    ' Read the response
    ReDim Buffer(0 To 7 + NumOfPoints * 2)
    If ReadFile(hCom, Buffer(0), UBound(Buffer) + 1, nRead, ByVal 0&) = 0 Or nRead <> UBound(Buffer) + 1 Then
        ModbusRTU_Read = False
        CloseHandle hCom
        Exit Function
    End If
    
    ' Verify the response
    If Buffer(0) <> Address Or Buffer(1) <> FunctionCode Or Buffer(2) <> NumOfPoints * 2 Then
        ModbusRTU_Read = False
        CloseHandle hCom
        Exit Function
    End If
    CRC = ModbusRTU_CRC(Buffer, 0, 5 + NumOfPoints * 2)
    If Buffer(6 + NumOfPoints * 2) <> CRC(0) Or Buffer(7 + NumOfPoints * 2) <> CRC(1) Then
        ModbusRTU_Read = False
        CloseHandle hCom
        Exit Function
    End If
    
    ' Extract the data
    ReDim Data(0 To NumOfPoints - 1)
    k = 0
    For i = 0 To NumOfPoints - 1
        j = 3 + i * 2
        Data(i) = Buffer(j) * 256 + Buffer(j + 1)
        k = k + Data(i)
    Next i
    
    ModbusRTU_Read = True
    CloseHandle hCom
End Function

Private Function ModbusRTU_CRC(ByVal Data() As Byte, ByVal StartIndex As Integer, ByVal Count As Integer) As Byte()
    Dim i As Integer
    Dim j As Integer
    Dim Table() As Integer
    Dim CRC As Integer
    
    ' Initialize the CRC table
    ReDim Table(0 To 255)
    For i = 0 To 255
        CRC = i
        For j = 0 To 7
            If CRC And 1 Then
                CRC = (CRC \ 2) Xor &HA001
            Else
                CRC = CRC \ 2
            End If
        Next j
        Table(i) = CRC
    Next i
    
    ' Calculate the CRC
    CRC = &HFFFF
    For i = StartIndex To StartIndex + Count - 1
        CRC = Table((CRC Xor Data(i)) And &HFF) Xor (CRC \ 256)
    Next i
    
    ' Return the CRC in little-endian order
    ModbusRTU_CRC = Array(CRC Mod 256, CRC \ 256)
End Function

此代码使用Windows API函数打开串口，并构建MODBUS RTU请求。在发送请求后，它等待一段时间，然后读取响应。最后，它提取数据并关闭串口。请注意，此代码仅处理一个功能码-读取多个保持寄存器（功能码03h）。如果您需要执行其他操作，您需要修改代码。