This code converts Arduino Leonardo into a USB to UART converter

Leo2UART.ino

/*
* This code converts Arduino Leonardo into a USB to UART converter
*/
int incomingByte = 0;

void setup() {
  // initialize serial:
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for Leonardo only
  }
  Serial1.begin(9600);
}

void loop() {
  if (Serial1.available() > 0)
  {
     incomingByte = Serial1.read();
     Serial.write(incomingByte);
  }
  
  if (Serial.available() > 0)
  {
     incomingByte = Serial.read();
     Serial1.write(incomingByte);
  }
}