public static String makeChecksum(String data) {
 if (data == null || data.equals("")) {
  return "";
 }
 int total = 0;
 int len = data.length();
 int num = 0;
 while (num < len) {
  String s = data.substring(num, num + 2);
  System.out.println(s);
  total += Integer.parseInt(s, 16);
  num = num + 2;
 }
 /**
  * 用256求余最大是255，即16进制的FF
  */
 int mod = total % 256;
 String hex = Integer.toHexString(mod);
 len = hex.length();
 // 如果不够校验位的长度，补0,这里用的是两位校验
 if (len < 2) {
  hex = "0" + hex;
 }
 return hex;
 }