-       // if we received an I- or R(ACK)-Block with a block number equal to the
-       // current block number, toggle the current block number
-       else if (len >= 4 // PCB+CID+CRC = 4 bytes
-                && ((data_bytes[0] & 0xC0) == 0 // I-Block
-                    || (data_bytes[0] & 0xD0) == 0x80) // R-Block with ACK bit set to 0
-                && (data_bytes[0] & 0x01) == iso14_pcb_blocknum) // equal block numbers
-       {
-               iso14_pcb_blocknum ^= 1;
+       } else {
+               // S-Block WTX
+               while (len && ((data_bytes[0] & 0xF2) == 0xF2)) {
+                       uint32_t save_iso14a_timeout = iso14a_get_timeout();
+                       // temporarily increase timeout
+                       iso14a_set_timeout(MAX((data_bytes[1] & 0x3f) * save_iso14a_timeout, MAX_ISO14A_TIMEOUT));
+                       // Transmit WTX back
+                       // byte1 - WTXM [1..59]. command FWT=FWT*WTXM
+                       data_bytes[1] = data_bytes[1] & 0x3f; // 2 high bits mandatory set to 0b
+                       // now need to fix CRC.
+                       AppendCrc14443a(data_bytes, len - 2);
+                       // transmit S-Block
+                       ReaderTransmit(data_bytes, len, NULL);
+                       // retrieve the result again (with increased timeout)
+                       len = ReaderReceive(data, parity);
+                       data_bytes = data;
+                       // restore timeout
+                       iso14a_set_timeout(save_iso14a_timeout);
+               }
+
+               // if we received an I- or R(ACK)-Block with a block number equal to the
+               // current block number, toggle the current block number
+               if (len >= 3 // PCB+CRC = 3 bytes
+                        && ((data_bytes[0] & 0xC0) == 0 // I-Block
+                                || (data_bytes[0] & 0xD0) == 0x80) // R-Block with ACK bit set to 0
+                        && (data_bytes[0] & 0x01) == iso14_pcb_blocknum) // equal block numbers
+               {
+                       iso14_pcb_blocknum ^= 1;
+               }
+
+               // if we received I-block with chaining we need to send ACK and receive another block of data
+               if (res)
+                       *res = data_bytes[0];
+
+               // crc check
+               if (len >= 3 && !CheckCrc14443(CRC_14443_A, data_bytes, len)) {
+                       return -1;
+               }
+
+       }
+
+       if (len) {
+               // cut frame byte
+               len -= 1;
+               // memmove(data_bytes, data_bytes + 1, len);
+               for (int i = 0; i < len; i++)
+                       data_bytes[i] = data_bytes[i + 1];