等候呗，等5秒，直到读取到数据，如果是多次接收的问题，你可以判断接收的字符长度嘛。

Ports.Add("COM3")
With Ports("COM3")
    .Close()
    .Parity = Parity.None
    .Handshake = Handshake.None
    .StopBits = StopBits.One
    .BaudRate = 9600
    .DataBits = 8
    .ReadBufferSize() = 4096
    .WriteBufferSize() = 2048
    .Open
End With
Dim b_sent() As Byte = {&H01, &H03, &H00, &H00, &H00, &H01, &H84, &H0A}
Ports("COM3").Write(b_sent, 0, b_sent.length)
Dim s As String
Dim s1 As String

Dim cnt As Integer = 0
Dim sdt As Date = Date.Now
Do While Date.Now < sdt.AddSeconds(5) \'等到5秒
    cnt = Ports("COM3").BytesToRead
    If cnt > 0 Then Exit Do \'或者读取到数据
    application.doevents
Loop

If cnt > 0 Then \'缓冲区是否有数据
    Dim Val(cnt - 1) As Byte
    Ports("COM3").Read(val,0,cnt)
    Dim i As Integer
    For i = 0 To cnt-1
        s1 = Val(i)
        If s1.Length = 1 Then
            s = s & 0 & DecToHex(Val(i))
        Else
            s = s & DecToHex(Val(i))
        End If
    Next
End If
Output.Show(s)

兄弟，把你的思路分析一下呗

1、认认真真看代码，最多5秒，如果读取到数据，里面就退出循环了啊

Dim cnt As Integer = 0
Dim sdt As Date = Date.Now
Do While Date.Now < sdt.AddSeconds(5) \'等到5秒
    cnt = Ports("COM3").BytesToRead
    If cnt > 0 Then Exit Do \'或者读取到数据
    application.doevents
Loop

2、代码执行有什么问题？效果是怎样的？