如何将串口通信读入缓冲区并解析出完整的消息

Question

我使用以下代码从COM端口读取值:

Private port As New SerialPort("COM13", 9600, Parity.None, 8, StopBits.One)

Private Sub port_DataReceived(ByVal sender As Object, ByVal e As SerialDataReceivedEventArgs)
    Debug.Print(port.ReadExisting())
End Sub

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    AddHandler port.DataReceived, New SerialDataReceivedEventHandler(AddressOf port_DataReceived)
    port.Open()
End Sub

这工作得很好,但是偶尔它不会获得所有数据,并且返回结果是两个字符串而不是一个.

一个例子是如果com端口发送单词"HELLO2YOU",它看起来像:

HEL
LO2YOU

要么

HELLO2
YOU

如何在其中放置一个缓冲区,以确保在显示之前读取所有数据？

谢谢!

Answer 1

您必须将串行端口通信视为流数据.无论何时收到数据,您都必须预期它可能是完整的消息,只是部分消息或多条消息.这一切都取决于数据进入的速度以及应用程序从队列中读取的速度.因此,你认为你需要一个缓冲区是正确的.但是,您可能还没有意识到的是,没有办法严格通过串行端口来了解每条消息的开始和结束位置.这必须通过发送方和接收方之间达成一致的协议来处理.例如,许多人使用标准的文本开头(STX)和文本结束(ETX)字符来指示每个消息发送的开始和结束.这样,当您收到数据时,您可以知道何时收到完整的消息.

例如,如果您使用STX和ETX字符,则可以创建如下类:

Public Class DataBuffer
    Private ReadOnly _startOfText As String = ASCII.GetChars(New Byte() {2})
    Private ReadOnly _endOfText As String = ASCII.GetChars(New Byte() {4})

    Public Event MessageReceived(ByVal message As String)
    Public Event DataIgnored(ByVal text As String)

    Private _buffer As StringBuilder = New StringBuilder

    Public Sub AppendText(ByVal text As String)
        _buffer.Append(text)
        While processBuffer(_buffer)
        End While
    End Sub

    Private Function processBuffer(ByVal buffer As StringBuilder) As Boolean
        Dim foundSomethingToProcess As Boolean = False
        Dim current As String = buffer.ToString()
        Dim stxPosition As Integer = current.IndexOf(_startOfText)
        Dim etxPosition As Integer = current.IndexOf(_endOfText)
        If (stxPosition >= 0) And (etxPosition >= 0) And (etxPosition > stxPosition) Then
            Dim messageText As String = current.Substring(0, etxPosition + 1)
            buffer.Remove(0, messageText.Length)
            If stxPosition > 0 Then
                RaiseEvent DataIgnored(messageText.Substring(0, stxPosition))
                messageText = messageText.Substring(stxPosition)
            End If
            RaiseEvent MessageReceived(messageText)
            foundSomethingToProcess = True
        ElseIf (stxPosition = -1) And (current.Length <> 0) Then
            buffer.Remove(0, current.Length)
            RaiseEvent DataIgnored(current)
            foundSomethingToProcess = True
        End If
        Return foundSomethingToProcess
    End Function


    Public Sub Flush()
        If _buffer.Length <> 0 Then
            RaiseEvent DataIgnored(_buffer.ToString())
        End If
    End Sub
End Class

我还要提一下,在通信协议中,通常有一个校验和字节,通过它可以确定消息在发送方和接收方之间传输过程中是否被破坏.