public static string sendText(byte start, byte cmd, byte mod, String datatekst)
        {
            String hex;

            int lendatatekst = datatekst.Length + 1;

            char[] chardatatekst = new char[lendatatekst];

            hex = start.ToString("X2") + lendatatekst.ToString("X2") + cmd.ToString("X2") + mod.ToString("X2");

            short suma = Convert.ToInt16(lendatatekst);
            suma += cmd;
            suma += mod;


            int i = 0; int a = 0; int b = 0;
            foreach (char s in datatekst)
            {
                chardatatekst[i] = (s);
                i++;

                string ascii = Convert.ToByte(s).ToString("X2");
                hex = hex + ascii;
                suma += Convert.ToInt16(s);
            }

            a = (suma & 0xff);
            b = ((suma >> 8) & 0xff);

            String MlodszyA = a.ToString("X2");
            String StarszyB = b.ToString("X2");

            hex += MlodszyA + StarszyB;
            return hex;
        }

        public static string sendHex(byte start, byte cmd, byte mod, String hexValues)
        {
            try
            {

                string[] hexValuesSplit = hexValues.Split(' ');

                int lenhexValuesSplit = hexValuesSplit.Length + 1;

                Byte[] data = new Byte[lenhexValuesSplit + 3];

                data[0] = start;
                data[1] = Convert.ToByte(lenhexValuesSplit);
                data[2] = cmd;
                data[3] = mod;

                int a = 0; int b = 0;
                int suma = cmd + lenhexValuesSplit + mod;
                int i = 4;

                foreach (String hex in hexValuesSplit)
                {

                    data[i] = byte.Parse(hex, System.Globalization.NumberStyles.AllowHexSpecifier);


                    suma += data[i];
                    i++;

                }

                a = (suma & 0xff);
                b = ((suma >> 8) & 0xff);

                string hexsend = BitConverter.ToString(data).Replace("-", string.Empty);

                String MlodszyA = a.ToString("X2");
                String StarszyB = b.ToString("X2");

                hexsend += MlodszyA + StarszyB;

                return hexsend;
            }
            catch (OverflowException)
            {
                MessageBox.Show("Niepoprawnie wpisane dane \n   Wpisz 'XX XX XX...'", "Błąd typu OverflowException ",
                MessageBoxButtons.OKCancel, MessageBoxIcon.Asterisk);
                return "";
            }
            catch (FormatException)
            {
                MessageBox.Show("Niepoprawne wpisane dane", "Błąd typu FormatException",
                MessageBoxButtons.OKCancel, MessageBoxIcon.Asterisk);
                return "";

            }
        }