#include <Wire.h>

#define EU 4
#define OUTRO 5

void setup()
{
  Wire.begin(EU);                
  Wire.onReceive(recebe); 
  Serial.begin(9600);          
}

byte x = 0;

void loop()
{
  Wire.beginTransmission(OUTRO); 
  Wire.write("x vale ");       
  Wire.write(x);             
  Wire.endTransmission();   

  x++;
  delay(500);
}
void recebe(int quantos)
{
  while(1 < Wire.available()) 
  {
    char c = Wire.read(); 
    Serial.print(c);      
  }
  int x = Wire.read();  
  Serial.println(x);       
}