https://docs.macchina.cc/projects/mini-project-tutorials/m2-can-loopback

M2_Loopback.ino

// Required libraries
#include "variant.h"
#include <due_can.h>

//Leave defined if you use native port, comment if using programming port
#define Serial SerialUSB

void setup()
{
  Serial.begin(115200);
  
  // Initialize CAN0 and CAN1, Set the proper baud rates here
  Can0.begin(CAN_BPS_500K);
  Can1.begin(CAN_BPS_500K);
  
  Can0.watchFor();
  Can1.watchFor();
  
  pinMode(Button1, INPUT_PULLUP);
  pinMode(Button2, INPUT_PULLUP);
  
  pinMode(14, OUTPUT);
  pinMode(15, OUTPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  
  digitalWrite(LED_BUILTIN, HIGH);
  digitalWrite(14, HIGH);
  digitalWrite(15, HIGH);
}



void loop()
{
  CAN_FRAME outgoing;
  
  outgoing.id = 0x400;
  outgoing.extended = false;
  outgoing.priority = 4; //0-15 lower is higher priority
  
  outgoing.data.s0 = 0xFEED;
  outgoing.data.byte[2] = 0xDD;
  outgoing.data.byte[3] = 0x55;
  outgoing.data.high = 0xDEADBEEF;
  
  if(digitalRead(Button1)==LOW)
  {
    digitalWrite(14, LOW);
    
    if(Can0.sendFrame(outgoing))
    {
      Serial.println("Sent frame on CAN0");
    }
    else
    {
      Serial.println("Failed to send frame on CAN0");
    }
    
    do
    {
      delay(5);
    }
    while(digitalRead(Button1)==LOW);
    
    delay(500);
    
    digitalWrite(14, HIGH);
  }
  
  if(digitalRead(Button2)==LOW)
  {
    digitalWrite(15, LOW);
    
    if(Can1.sendFrame(outgoing))
    {
      Serial.println("Sent frame on CAN1");
    }
    else
    {
      Serial.println("Failed to send frame on CAN1");
    }
    
    do 
    {
      delay(5);
    }
    while(digitalRead(Button2)==LOW);
    
    delay(500);
    
    digitalWrite(15, HIGH);
  }
  
  CAN_FRAME incoming;
  
  if (Can0.available() > 0)
  {
    Can0.read(incoming);
    Serial.println("Received frame on CAN0");
    flashLED();
  }
  if (Can1.available() > 0)
  {
    Can1.read(incoming);
    Serial.println("Received frame on CAN1");
    flashLED();
  }
}

void flashLED()
{
  for (int i=0; i<10; i++)
  {
    digitalWrite(LED_BUILTIN, LOW);
    delay(50);
    digitalWrite(LED_BUILTIN, HIGH);
    delay(50);
  }
}