i2c_backpack_master.ino

#include <Wire.h>

byte i2cdata[4];

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

void loop() {
  
  Wire.requestFrom(4, 4);
  
  int a = Wire.available();
  
  Serial.print("Available: ");
  Serial.println(a);
  
  // Both of these produce the same result

//  for (int i = 0; i < a; i++) {
//    Serial.print(Wire.read());
//    Serial.print(" ");
//  }  

  while (Wire.available()) {
    Serial.print(Wire.read());
    Serial.print(" ");
  }  

  Serial.println();

  memset(&i2cdata, 0, 4);

  delay(500);
}

i2c_backpack_slave_buffer_write.ino

#include <Wire.h>

int address = 4;

void setup() {
  Wire.begin(address);
  Wire.onRequest(onRequest);
}

void loop() {}

void onRequest() {
  byte buffer[4] = {1, 2, 3, 4};
  Wire.write(buffer, 4);
}


/*

When this sketch is running on the slave, the output of the master is: 

Available: 4
1 2 3 4 
...

*/

i2c_backpack_slave_single_byte_write.ino

#include <Wire.h>

int address = 4;

void setup() {
  Wire.begin(address);
  Wire.onRequest(onRequest);
}

void loop() {}

void onRequest() {
  for (int i = 1; i < 5; i++) {
    Wire.write(i);
  }
}


/*

When this sketch is running on the slave, the output of the master is: 

Available: 4
4 255 255 255 
...

*/