private void button1_Click(object sender, EventArgs e)
      {
          byte checksum;
          byte t = 255;     
          byte[] testVal = new byte[]{t};
          CRC8Calc crc = new CRC8Calc(CRC8_POLY.CRC8_CCITT);
          checksum = crc.Checksum(testVal);
          sp.Write(t.ToString());
          sp.Write(checksum.ToString());
      }