Multimaster I2C with same address

Question

I use the Wire Library and need to have multi-master setting. I see multi-master examples, where the masters have their own ID's -- which make sense.

Now I have just connected three devices on the bus with the same ID (for example 0x05) and they can communicate! Every node can listen to all traffic, which is not common but this was exactly what I want.

The question is: Will I have trouble in the future? Is my setup something like "multi-slave"?

Answer 1

const byte MY_ADDRESS = x;  // where x is whatever unique address you like  

byte RxByte[32]  //32 byte buffer

void setup() {  
  //insert unique ddress                            
  Wire.begin(MY_ADDRESS);  

// this line allows the arduino to listen for i2c broadcast at address 0

TWAR = (MY_ADDRESS << 1) | 1;  
//set up a receive handler  
 Wire.onReceive(receive_data);

then call 
Wire.beginTransmission (0)  //transmit broadcast so all devices will receive 

Wire.write( address of destination) // make the first byte the destination 

Wire.endTransmission ();

then you need a receive handler 

void receive_data() { 
//use a buffer to temporarily store the data 

for ( int d = 0; Wire.available() > 0 ;d++){ 

RxByte[d] = Wire.read();} // READ WIRE BUFFER UPTO 32BYTES, INTO AN ARRAY
datareceived = 1;

//then test if it was for this device
if (RxByte[0] == MYADDRESS){ do somthing with RxBytes[1-31];}
else {}

I use this extensively its very handy and works well I have a complex transmission table which has a 4 byte header and the rest of the bytes can be used as data send

destination, myaddress, a command, and how many data bytes to follow

---

Addendum to Original answer. below is a complete working example.

//so the basics of the tranmission table are the first 4 bytes (ie Rx or Tx byte 0-3 depending on if we are talking about the sender or recipient) //are used as a header 0= recipient (0 for broadcast. or address) 1= senders address 2= is the command (ie tells the recipient what to do) //and 4= the number of data bytes which follow in the packet //not all of these need to be specified at any time but its usefull to have a structure to follow at each end which never changes //copy and past this upload to 2 arduinos connected by i2c change the 2 addresses and it should work. its a n example of how data can be moved around in a multi master situation that should hopefully give you some ideas. it looks complicated but half of it is just generating some example data and triggering method and the transmit part is repeated 3 times to send out different data.. you should be able to create a more general purpose transmit block for your application

to send data just attach a button or wire to pin 2 directly to ground

watch the 2 serial windows

#include <Wire.h>
//device address  MAKE DIFFERENT FOR EACH

const byte MY_ADDRESS = 20;
const byte PARTNER_DEVICE = 25;

// transmission table stuff
byte RxByte[32]; //setup a receive buffer to hold data
byte TxByte[32]; //setup a transmit buffer to hold data
byte datareceived;

// stuff to generate some data
const int Sensor1 = A0;
byte SensorVal1 = 0;

//stuff to debounce pin 2
const byte digitalpin = 2;
byte pinstate = 0;
byte lastpinstate = 0;
unsigned long checktime = 0;
const byte debouncetime = 50;

// timer
unsigned long timing_loop = 5000;

void setup() {
  Wire.begin (MY_ADDRESS);
  TWAR = (MY_ADDRESS << 1) | 1;                                                      // ENABLE BROADCASTS TO BE RECEIVED
  Wire.onReceive(ReceiveData);     // INTERUPT ON I2C DATA RECEIVE
  Serial.begin(9600);
  pinMode (  digitalpin  , INPUT_PULLUP );// set the pin to input pullup so we can trigger the code by connecting directly to ground
  Serial.println( "READY" );
}

void loop() {
  // generate some random data

  GenRandData();
  //Serial.print(  pinstate);Serial.print( " " );Serial.println( lastpinstate );
  // debounces pin 2 so we can trigger a data broadcast
  pinstate = digitalRead (digitalpin);
  if ( pinstate != lastpinstate && millis() >= checktime) {
    if ( pinstate == 0 && lastpinstate == 1 ) {
      checktime = millis() + debouncetime;
      Serial.println( " button working" );
      REQUEST();
    }
    lastpinstate = pinstate ;
  }


  // this triggers the process data method after receive of data
  if (datareceived == 1  ) {
    datareceived = 0 ;
    ProcessData();
  }

  // 10 second timer triggers transmission of some data
  if ( millis() >= timing_loop ) {
    BROADCAST();
  }
}
void GenRandData() {
  // this just reads analog0 and maps it to a byte then sequentially shifts the transmit buffer
  SensorVal1 = random(255);
  TxByte[7] = TxByte[6];
  TxByte[6] = TxByte[5];
  TxByte[5] = TxByte[4];
  TxByte[4] = SensorVal1;
}

void ReceiveData() {
  // READ AVAILABLE BYTES INTO 32BYTE BUFFER (while data is available)
  for ( byte d = 0; Wire.available() > 0 ; d++) {
    RxByte[d] = Wire.read();
  }
  //set a trigger event
  datareceived = 1;
  Serial.println( "receive" );
}

void ProcessData() {
  Serial.println( "process" );
  // check what the data is an who its for then do something with it (RECEIVE TABLE)
  if (RxByte[0] == MY_ADDRESS) { // for this device
    switch (RxByte[2]) {
      case 1:       Serial.print("RECEIVED DATA FOR THIS DEVICE, = "); Serial.println( RxByte[4] ); break;
      case 255:       Serial.println("RECEIVED A REQUEST FOR DATA, SENDING READ FROM  > Sensor1"  ); ActAsMaster();              break;
      default:      Serial.print("D-Type byte = error"); Serial.println(RxByte[2]);                   break;
    }
  }

  else if (RxByte[0] == 0) {       //handle broardcast
    Serial.print( "RECEIVED BROADCAST FROM DEVICE = " ); Serial.println( RxByte[1] );
    Serial.print( "DATASIZE = " ); Serial.print( RxByte[3] ); Serial.println( " Bytes" );
    Serial.print( "DATABYTE 1 = " ); Serial.println( RxByte[4] );
    Serial.print( "DATABYTE 2 = " ); Serial.println( RxByte[5] );
    Serial.print( "DATABYTE 3 = " ); Serial.println( RxByte[6] );
    Serial.print( "DATABYTE 4 = " ); Serial.println( RxByte[7] );
    Serial.println( "DONE" );
  }
  Serial.println(  );
}

void BROADCAST() {
  // this is transmission table to send data to everyone
  Serial.println("Sending broadcast");
  byte count = 0;
  byte errorcode;
Tx:
  Wire.beginTransmission (0);                       // broadcast
  Wire.write(0);                                    // ADDRESS =0 BROARDCAST
  Wire.write(MY_ADDRESS);                           // SEND my address
  Wire.write(0);                                    // DATA TYPE CODE
  Wire.write(4);                                    //number of bytes being sent
  Wire.write(TxByte[4]);                            //data
  Wire.write(TxByte[5]);                            //data
  Wire.write(TxByte[6]);                            //data
  Wire.write(TxByte[7]);                            //data
  errorcode = Wire.endTransmission ();
  if (errorcode != 0 && count <= 2) {
    count++;
    Serial.println( "fail" );
    goto Tx;
  }
  //reset timer to run this again
  timing_loop = millis() + 10000 ;
  Serial.println(  );
}

void ActAsMaster() {
  //this is transmission table to send data to specific device OR return a result of a task
  Serial.println("Sending data");
  byte count = 0;
  byte errorcode;
Tx:
  Wire.beginTransmission (RxByte[1]);               //the return address
  Wire.write(RxByte[1]);                            //recipient address (return)
  Wire.write(MY_ADDRESS);                           // SEND MASTER ADDRESS
  Wire.write(1);                                  // DATA TYPE CODE
  Wire.write(1);
  Wire.write(SensorVal1);
  errorcode = Wire.endTransmission ();
  if (errorcode != 0 && count <= 2) {
    count++;  // if fail and not exceed count try again
    Serial.println( "fail" );
    goto Tx;
  }
  Serial.println(  );

}
void REQUEST() {
  //this is transmission table to send data request of partner device
  Serial.println("Sending request");
  byte count = 0;
  byte errorcode;
Tx:
  Wire.beginTransmission (PARTNER_DEVICE);               //the return address
  Wire.write(PARTNER_DEVICE);                            //recipient address (return)
  Wire.write(MY_ADDRESS);                           // SEND MASTER ADDRESS
  Wire.write(255);                                  // DATA TYPE CODE (MASTER ADDRESS)
  Wire.write(0);
  errorcode = Wire.endTransmission ();
  if (errorcode != 0 && count <= 2) {
    count++;  // if fail and not exceed count try again
    Serial.println( "fail" );
    goto Tx;
  }
  Serial.println(  );
}