c#串口接收数据问题

[shan7719515]

        void com_Recived(object sender, SerialDataReceivedEventArgs e) 
        {
            int n = sp.BytesToRead;
            byte[] b = new byte[n];
            sp.Read(b, 0, n);
            builder.Append(Encoding.ASCII.GetString(b));
            string temp = builder.ToString().Substring(7, 4);
            this.Invoke((EventHandler)(delegate
            {                
                this.richTextBox1.AppendText(builder.ToString());
            }));
        }

只想获得其中四位数据
求高手指点