Using an Android phone to send same string to Arduino via NFC

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Progress! I found an Android app called NDEF Tag Emulator, which can send NDEF messages. And I think it's doing so, but I still get an error when reading them.

I'm using the Elechouse PN532 library:

And using PN532's I2C mode (setting the jumpers to I2C). I'm connecting to the default SDA and SDL pins on my Mega2560 (pins 20 and 21).

And my slightly modified version of this code:

#if 0
#include <SPI.h>
#include <PN532_SPI.h>
#include <PN532.h>
#include <NfcAdapter.h>

PN532_SPI pn532spi(SPI, 10);
NfcAdapter nfc = NfcAdapter(pn532spi);
#else

#include <Wire.h>
#include <PN532_I2C.h>
#include <PN532.h>
#include <NfcAdapter.h>

PN532_I2C pn532_i2c(Wire);
NfcAdapter nfc = NfcAdapter(pn532_i2c);
#endif

void setup(void) {
  Serial.begin(115200); // changed from 9600
  Serial.println("NDEF Reader");

  Serial.println("Turn on PN532 now...");
  delay(3000); // a delay to allow turning the PN532 on
  
  nfc.begin();
}

void loop(void) {
  Serial.println("\nScan a NFC tag\n");

  if (nfc.tagPresent())
  {
    NfcTag tag = nfc.read();
    Serial.println(tag.getTagType());
    Serial.print("UID: ");Serial.println(tag.getUidString());

    if (tag.hasNdefMessage()) // every tag won't have a message
    {

      NdefMessage message = tag.getNdefMessage();
      Serial.print("\nThis NFC Tag contains an NDEF Message with ");
      Serial.print(message.getRecordCount());
      Serial.print(" NDEF Record");
      if (message.getRecordCount() != 1) {
        Serial.print("s");
      }
      Serial.println(".");

      // cycle through the records, printing some info from each
      int recordCount = message.getRecordCount();
      for (int i = 0; i < recordCount; i++)
      {
        Serial.print("\nNDEF Record ");Serial.println(i+1);
        NdefRecord record = message.getRecord(i);
        // NdefRecord record = message[i]; // alternate syntax

        Serial.print("  TNF: ");Serial.println(record.getTnf());
        Serial.print("  Type: ");Serial.println(record.getType()); // will be "" for TNF_EMPTY

        // The TNF and Type should be used to determine how your application processes the payload
        // There's no generic processing for the payload, it's returned as a byte[]
        int payloadLength = record.getPayloadLength();
        byte payload[payloadLength];
        record.getPayload(payload);

        // Print the Hex and Printable Characters
        Serial.print("  Payload (HEX): ");
        PrintHexChar(payload, payloadLength);

        // Force the data into a String (might work depending on the content)
        // Real code should use smarter processing
        String payloadAsString = "";
        for (int c = 0; c < payloadLength; c++) {
          payloadAsString += (char)payload[c];
        }
        Serial.print("  Payload (as String): ");
        Serial.println(payloadAsString);

        // id is probably blank and will return ""
        String uid = record.getId();
        if (uid != "") {
          Serial.print("  ID: ");Serial.println(uid);
        }
      }
    }
  }
  delay(1000); // changed from 3000
}

When I scan an RFID hotel room card I get this output:

But when using the Android NDEF Tag Emulator app I get:

I don't imagine anyone has any ideas?

And note that there appears to be a sometime bug in the PN532 when in I2C mode, where you need to power it on a short time after the Arduino, at least when Serial Monitor is open. Hence the delay in my setup().