SerialPort串口获得返回值

Question

代码：

 SerialPort port = new SerialPort("COM1", 115200, Parity.None, 8, StopBits.One);

port.Encoding = Encoding.ASCII;
            port.Write("02 00 04 A2 31 00 00 93 03");
            string str = port.ReadExisting();
            port.Close();
            this.tb_Msg.Text =this.tb_Msg.Text+ DateTime.Now.ToString() + "返回消息：" + str.ToString() + "\r\n";

接收和发送都是16进制，返回值怎么接收呢？