Arduino Due RS485

Hi MartinL thanks

With the following code i can transmit and receive via RS485
But i need interrupt enable reception.
How i can do that? Please check my code

void setup()
{

pinMode(RTS, OUTPUT); //RTS for USART1
digitalWrite(RTS,LOW);
Serial.begin(9600);
MeterSerial.begin(9600,SERIAL_8N1);   //SERIAL_8N1

USART1->US_WPMR = 0x55534100; //Unlock the USART Mode register, just in case. (mine wasn't locked).
  USART1->US_CR |= US_CR_RSTRX | US_CR_RSTTX | US_CR_RXDIS | US_CR_TXDIS;
  USART1->US_MR |= 0X8C1;//USART1 IN RS485 mode
  USART1->US_CR |= US_CR_TXEN|US_CR_RXEN;  //Enable Transmitter and Receiver


void loop()
{
 char mydat = 0; 
while(1)
  {
while(MeterSerial.available())
     {
          
      mydat =  MeterSerial.read();
     if(mydat!=0) 
     {
      Serial.println(mydat);
      
      
     }
     
      if(mydat == 's')  //check if char 's' received if yes then transmit for test
    {
   
      delay(100);
digitalWrite(RTS,HIGH);   //for transmitter RTS HIGH
 
MeterSerial.println("wating for char");
MeterSerial.flush();   //Wait till transmit all char

   digitalWrite(RTS,LOW);  
  delay(100);
  
  mydat = 0;
    }
 }
}

Now I am not getting that how to enable interrupt when receiving data
I have searched and study and found US_IER to enable interrupt for transmitter and receiver
But how to define interrupt function?
I am trying and if success then will upload the code
if any idea then reply