RFID Reader RDM880-T-A seedstudio: How to read Mifare Data Block?

after reading the RFIDs ID sucessfully for some time with the Seedstudio 13.56Mhz Module, I wanted to read out data blocks from the RFID chip now.

based on the protocol’s manual (page 20) you can send out data blocks by sending:

Send Data:AA 00 0A 20 01 01 10 ff ff ff ff ff ff 3A BB(Read the data from the 16th block to 19th
block )

where ff ff ff ff ff ff is supposed to be “the key”.

if I modify the seedstudios example code to this:

#include <SoftwareSerial.h> 

SoftwareSerial mySerial(2, 3);
char txrxbuffer[255];

char get_readID[] = { 0xAA , 0x00, 0x0A, 0x20, 0x01, 0x01, 0x10, 0xFF, 0xFF,0xFF, 0xFF,0xFF, 0xFF, 0x3A, 0xBB };
void setup()
Serial.println("Hit Enter to read the RFID number");

void loop() // run over and over

int counter = 0;
if (mySerial.available())

if (Serial.available()){
    for (counter =0 ; counter < 15 ; counter++){

the serial only returns two empty lines

I think its because i just left the key like in the protocols manual for I could not make out what this key identifies exactly and how to get the proper key.

Maybe its something else in the command or both?

I would appreciate any kind of suggestion.