VB comm接收数据首字节丢掉疑难

VB comm接收数据首字节丢失疑难
大家好！
情况是这样的：
笔记本用的usb转485与arm通信，modbus协议（不了解也无所谓）

发送和接收基本没问题，问题是：
如果arm发送小于8字节，接收没问题
如果超过8个字节，第一个字节丢失，其余的可以收到
试了好多办法，还是不行
1.可能是程序问题，代码如下，大侠帮忙建议一下吧
2.如果是usb转串口设备的问题，有什么办法通过软件解决？

Private Sub Command4_Click()

ReDim t.data(2) As Byte
  t.data(0) = 49
  t.data(1) = 78
       Call uartsend(1, 3, 16, 0, 0, 11, 0, t.data())

end sub
Public Function uartsend(ByVal add As Byte, ByVal funcode As Byte, _
ByVal regah As Byte, ByVal regal As Byte, ByVal regnumh As Byte, _
ByVal regnuml As Byte, ByVal datalen As Byte, ByRef data() As Byte)

On Error GoTo uarterr

Dim pack() As Byte

            ReDim pack(0 To 8) As Byte
            pack(0) = add
            pack(1) = funcode
            pack(2) = regah
            pack(3) = regal
            pack(4) = regnumh
            pack(5) = regnuml
            pack(6) = CRC16(pack, 6) Mod 256
            pack(7) = CRC16(pack, 6) \ 256

form1.mscomm1.commport=4
  Form1.MSComm1.InputMode = comInputModeBinary
  Form1.MSComm1.InBufferSize = 512
  Form1.MSComm1.OutBufferSize = 512
  Form1.MSComm1.InBufferCount = 0
  Form1.MSComm1.OutBufferCount = 0
  Form1.MSComm1.SThreshold = 0
  Form1.MSComm1.RThreshold = 1
  Form1.MSComm1.InputLen = 0
  Form1.MSComm1.Settings = "19200,N,8,1"


            Form1.MSComm1.PortOpen = True
            Form1.MSComm1.Output = pack
            Exit Function
uarterr:
     msg$ = "err号"          '错误显示
    Title$ = "串口调试助手"
    X = MsgBox(msg$, 48, Title$)  '48标示显示警告图标

End Function

Private Sub MSComm1_OnComm()

Dim BytReceived() As Byte
   Dim RecDatas() As Byte
   If (MSComm1.CommEvent = comEvReceive) Then

  Ilens = MSComm1.InBufferCount
  MSComm1.InputLen = Ilens
  RecDatas = MSComm1.Input
  For i = 0 To UBound(RecDatas)
  text2.Text = text2.Text & Hex(RecDatas(i)) & " "

VB comm接收数据首字节丢掉疑难

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