校驗位,從SOM到EXT累加值的補碼

        private byte[] cksum(byte[] buff) {

            int byCksum = 0;
            int i = 0;
            for (i = 0; i < buff.Length - 1; i++) {
                byCksum += buff[i];
            }

            byCksum = (~byCksum & 0xff) + 1;

            buff[buff.Length - 1] = (byte) byCksum;

            return buff;
        }