user47810: Can someone help me what is the meaning of this sketch? It always outputs in the serial monitor:
Firmware Version: 0x1C = (unknown)
Scan PICC to see UID, SAK, type, and data blocks...

This is the code of the MFRC522 library I use:
#include <SPI.h>
#include <MFRC522.h>

constexpr uint8_t RST_PIN = 9;          // Configurable, see typical pin layout above
constexpr uint8_t SS_PIN = 10;         // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance

void setup() {
   Serial.begin(9600);   // Initialize serial communications with the PC
   while (!Serial);    // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
   SPI.begin();      // Init SPI bus
   mfrc522.PCD_Init();   // Init MFRC522
   mfrc522.PCD_DumpVersionToSerial();  // Show details of PCD - MFRC522 Card Reader details
   Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
}

void loop() {
   // Look for new cards
   if ( ! mfrc522.PICC_IsNewCardPresent()) {
     return;
   }

   // Select one of the cards
   if ( ! mfrc522.PICC_ReadCardSerial()) {
     return;
   }

   // Dump debug info about the card; PICC_HaltA() is automatically called
   mfrc522.PICC_DumpToSerial(&(mfrc522.uid));
}


user6936: The mention of unknown in the output line:
Firmware Version: 0x1C = (unknown)

isn't a good sign.
Run the firmware_check example to verify that first, before trying to run other code.
The unknown is output by line 1302 of MFRC522.cpp:
    default:   Serial.println(F(" = (unknown)"));

The firmware_check example states on line 49:
  Serial.println(F("Only known versions supported"));

So, your hardware (i.e. the reader) may not be supported. 
The only thing I can suggest is try another reader. Also, read the README, there maybe some thing there, or contact the author and/or raise an issue to see if you can get support for your version of the firmware (0x1C) added.

47810: <p>Can someone help me what is the meaning of this sketch? It always outputs in the serial monitor:</p>

<pre><code>Firmware Version: 0x1C = (unknown)
Scan PICC to see UID, SAK, type, and data blocks...
</code></pre>

<p>This is the code of the <a href="https://github.com/miguelbalboa/rfid" rel="nofollow noreferrer">MFRC522 library</a> I use:</p>

<pre><code>#include &lt;SPI.h&gt;
#include &lt;MFRC522.h&gt;

constexpr uint8_t RST_PIN = 9;          // Configurable, see typical pin layout above
constexpr uint8_t SS_PIN = 10;         // Configurable, see typical pin layout above

MFRC522 mfrc522(SS_PIN, RST_PIN);  // Create MFRC522 instance

void setup() {
   Serial.begin(9600);   // Initialize serial communications with the PC
   while (!Serial);    // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
   SPI.begin();      // Init SPI bus
   mfrc522.PCD_Init();   // Init MFRC522
   mfrc522.PCD_DumpVersionToSerial();  // Show details of PCD - MFRC522 Card Reader details
   Serial.println(F("Scan PICC to see UID, SAK, type, and data blocks..."));
}

void loop() {
   // Look for new cards
   if ( ! mfrc522.PICC_IsNewCardPresent()) {
     return;
   }

   // Select one of the cards
   if ( ! mfrc522.PICC_ReadCardSerial()) {
     return;
   }

   // Dump debug info about the card; PICC_HaltA() is automatically called
   mfrc522.PICC_DumpToSerial(&amp;(mfrc522.uid));
}
</code></pre>
 6936: <p>The mention of <code>unknown</code> in the output line:</p>

<pre><code>Firmware Version: 0x1C = (unknown)
</code></pre>

<p>isn't a good sign.</p>

<p>Run the <code>firmware_check</code> example to verify that first, before trying to run other code.</p>

<p>The <code>unknown</code> is output by <a href="https://github.com/miguelbalboa/rfid/blob/master/src/MFRC522.cpp#L1302" rel="nofollow noreferrer">line 1302</a> of <a href="https://github.com/miguelbalboa/rfid/blob/master/src/MFRC522.cpp" rel="nofollow noreferrer">MFRC522.cpp</a>:</p>

<pre><code>    default:   Serial.println(F(" = (unknown)"));
</code></pre>

<p>The <a href="https://github.com/miguelbalboa/rfid/blob/master/examples/firmware_check/firmware_check.ino" rel="nofollow noreferrer"><code>firmware_check</code></a> example states on <a href="https://github.com/miguelbalboa/rfid/blob/master/examples/firmware_check/firmware_check.ino#L49" rel="nofollow noreferrer">line 49</a>:</p>

<pre><code>  Serial.println(F("Only known versions supported"));
</code></pre>

<p>So, your hardware (i.e. the reader) <em>may</em> not be supported. </p>

<p>The only thing I can suggest is try another reader. Also, read the <a href="https://github.com/miguelbalboa/rfid/blob/master/README.rst" rel="nofollow noreferrer">README</a>, there maybe some thing there, or contact the author and/or <a href="https://github.com/miguelbalboa/rfid/issues" rel="nofollow noreferrer">raise an issue</a> to see if you can get support for your version of the firmware (<code>0x1C</code>) added.</p>
