【求助】SerialPort的DataReceived不触发

Private Sub ReceiveData(sender As Object, e As EventArgs) Handles SerialPort1.DataReceived
TextBox1.Text = TextBox1.Text & SerialPort1.ReadExisting()
End Sub
读写缓存均为1字节，为何该事件不触发，串口可以肯定接收到数据了，因为将上一句放到发送函数的后面紧跟着就可以正常在TextBox里显示数据了(直接将串口的收发引脚接一块了，所以发送完也就接收完了)