// https://blog.startingelectronics.com/how-to-use-arduino-serial-ports/

#include <SoftwareSerial.h>

#define ARDUINO_TX 5  // Arduino transmit = Connected device receive
#define ARDUINO_RX 6  // Arduino receive  = Connected device transmit

SoftwareSerial gtSerial(ARDUINO_RX, ARDUINO_TX);

void setup() {
  Serial.begin(9600);    // Hardware serial / UART
  gtSerial.begin(9600);  // software serial port
}

byte data_byte = 0;      // to/from each port

void loop() {
  // check if byte available from hardware port
  if (Serial.available()) {
    data_byte = Serial.read();
  // forward to the software serial port
    gtSerial.write(data_byte);
  }

  // check if byte available on the software serial port
  if (gtSerial.available()) {
  // read from the software serial port
    data_byte = gtSerial.read();
    Serial.write(data_byte);
  }
}