private static byte Sum(byte[] cmd, int startIndex, int endIndex) {
    if (startIndex < 0 || startIndex > endIndex)
        return 0;
    int sum = 0;
    for (int i = startIndex; i <= endIndex && i < cmd.length; i++) {
        sum += (cmd[i] & 0xff);
    }
    return (byte) (sum & 0xff);
}