自己写的一个串口接收发送的小程序，向串口发送命令是应该有返回信息的
可是写的程序运行没有任何返回信息，求解

public partial class Form1 : Form
    {
        SerialPort sp = new SerialPort();
        public Form1()
        {
            InitializeComponent();
            sp.PortName = "COM4";
            sp.BaudRate = 115200;
            sp.DataBits = 8;
            sp.StopBits = StopBits.One;
            sp.Parity = Parity.None;

            sp.DataReceived += new SerialDataReceivedEventHandler(serialPort1_DataReceived);

            sp.Open();
        }

        //数据接收使用的代理
        private delegate void myDelegate(string s);


        private void button1_Click(object sender, EventArgs e)
        {
            string st1 = textBox1.Text;
            sp.WriteLine(st1);
            textBox1.Text = "";
        }

        private void SetText(string s)
        {

            listBox1.Items.Add(s);

        }


        //串口数据到达时的事件
        private void serialPort1_DataReceived(object sender, SerialDataReceivedEventArgs e)
        {
            //关键 代理
            myDelegate md = new myDelegate(SetText);

            try
            {
                if (sp.IsOpen == true)
                {
                    byte[] readBuffer = new byte[sp.ReadBufferSize];
                    sp.Read(readBuffer, 0, readBuffer.Length);
                    string readstr = Encoding.UTF8.GetString(readBuffer);

                    Invoke(md, readstr);
                }
            }
            catch (Exception err)
            {
                throw err;
            }


        }

    }