@robski Not a bad idea but UART is located on pins G43 and G44 which are not broken out (Port B is G1 and G2). I might be able to use the USB C port but that's just a hassle and a half as far as prototyping, debugging, and expansion is concerned.

in theory you can reconfigure any port to work as uart

ESP_NOW

No, mostly because I did not know this protocol existed. This looks like it could be a promising avenue. Thanks for the suggestion Teastain!

here

Your assumption is correct. From my understanding of I2C, it seems like an I2C bus should be able to support multiple masters on the same bus. And it should be possible to have a master receive information as long as it is not busy sending information.

With that said, it was busy with the RFID peripheral so maybe that's the problem or I could be entirely mistaken all together on how I2C works. You've given me some documents to digest as well. So I'll get back to you with a new configuration hopefully soon. Thanks!

ok, now I understand. Well, one of the M5Dial needs to be an I2C master and the other needs to be an I2C slave. Right now, it seem to me, you're setting both M5Dials up as I2C master.

Please check documentation here and here.

Thanks
Felix

// Libraries to include
#include "M5Dial.h"

// Set up and initialize constants and variables
const byte MY_ADDRESS = 0x12;
const byte SLAVE_ADDRESS = 0x13;

String uid = "";
String oldUid = "";


void setup() 
{
    // Initialize dial rfid and screen
    auto cfg = M5.config();
    M5Dial.begin(cfg, false, true);
    M5Dial.Display.setTextColor(GREEN);
    M5Dial.Display.setTextDatum(middle_center);
    M5Dial.Display.setTextFont(&fonts::Orbitron_Light_32);
    M5Dial.Display.setTextSize(1);
    M5Dial.Display.drawString("RFID Card", M5Dial.Display.width() / 2, M5Dial.Display.height() / 2);
    
    // Set up I2C address and recieve transmission event handling
    Wire.begin (MY_ADDRESS);
    Wire.onReceive (receiveEvent);
}

void loop() 
{
  // Find out if a card is available for reading
  if(M5Dial.Rfid.PICC_IsNewCardPresent() && M5Dial.Rfid.PICC_ReadCardSerial()) 
  {
    uint8_t piccType = M5Dial.Rfid.PICC_GetType(M5Dial.Rfid.uid.sak);

    // Check to see if PICC is Classic MIFARE type
    if(piccType != MFRC522::PICC_TYPE_MIFARE_MINI && piccType != MFRC522::PICC_TYPE_MIFARE_1K && piccType != MFRC522::PICC_TYPE_MIFARE_4K) 
    {
      M5Dial.Display.clear();
      M5Dial.Display.drawString("no support", M5Dial.Display.width() / 2, M5Dial.Display.height() / 2);
      return; // bad card - loop done
    }

    // else get UID data
    uid = "";
    for(byte i = 0; i < M5Dial.Rfid.uid.size; i++) 
    { 
        uid += String(M5Dial.Rfid.uid.uidByte[i], HEX);
    }
    
    // And output stored UID data to the screen if its a different card from the card scanned previously.
    if(uid != oldUid)
    {
      M5Dial.Display.clear();
      M5Dial.Speaker.tone(8000, 20);
      M5Dial.Display.drawString(M5Dial.Rfid.PICC_GetTypeName(piccType), M5Dial.Display.width() / 2, M5Dial.Display.height() / 2 - 30);
      M5Dial.Display.drawString("card id:", M5Dial.Display.width() / 2, M5Dial.Display.height() / 2);
      M5Dial.Display.drawString(uid, M5Dial.Display.width() / 2, M5Dial.Display.height() / 2 + 30);
      oldUid = uid;

      // Also send uid data to microcontroller
      Wire.beginTransmission(SLAVE_ADDRESS);
      Wire.write(uid.c_str());
      Wire.endTransmission();
    }
  }
}

// recieve and display UID data from another M5Dial
void receiveEvent(int tSize)
{
  uid = "";
  for(int i = 0; i < tSize; i++)
  {
    uid += Wire.read();
  }
  M5Dial.Display.clear();
  M5Dial.Speaker.tone(8000, 20);
  M5Dial.Display.drawString("card id:", M5Dial.Display.width() / 2, M5Dial.Display.height() / 2);
  M5Dial.Display.drawString(uid, M5Dial.Display.width() / 2, M5Dial.Display.height() / 2 + 30);
  oldUid = uid;
}

The code above can be copied over to another M5Dial and the addressing is just swapped. Theoretically it should work but when one dial receives RFID card data the other does not display this information. I think that it is transmitting this information but on the interior bus not the external bus. To fix this I added a line to my setup code to include the external I2C bus shown below:

Wire.begin (13, 15);

However that seemed to kill RFID functionality. And this is where I currently sit, hence the questions and comments above hoping for some help.

why are you saying that? Did you try to use the external port A for I2C and hit an issue?

Please find an example here which scans the internal and external I2C bus.

Thanks
Felix