#include <Arduino.h>

#define EXTERNAL_RX_PIN 16 // GPIO pin for the external device's TX
#define EXTERNAL_TX_PIN 17 // GPIO pin for the external device's RX
#define BAUD_RATE 115200

void setup() {
  // Initialize communication with the computer (via USB-to-UART chip)
  Serial.begin(BAUD_RATE); 
  
  // Initialize communication with the external device (on designated GPIO pins)
  Serial2.begin(BAUD_RATE, SERIAL_8N1, EXTERNAL_RX_PIN, EXTERNAL_TX_PIN);
}

void loop() {
  // Read from computer (Serial) and send to external device (Serial2)
  if (Serial.available()) {
    Serial2.write(Serial.read());
  }

  // Read from external device (Serial2) and send to computer (Serial)
  if (Serial2.available()) {
    Serial.write(Serial2.read());
  }
}