passthrough.ino

static int BAUD = 115200, RTS = -1, DTR = -1, ESP32_GPIO = 11, ESP32_RESETN = 12;

void setup() {
  Serial.begin(baud);
  Serial1.begin(baud);
  pinMode(ESP32_GPIO0, OUTPUT);
  pinMode(ESP32_RESETN, OUTPUT);

  // Reset ESP32, put into upload mode
  digitalWrite(ESP32_GPIO0, LOW);
  digitalWrite(ESP32_RESETN, LOW);
  delay(100);
  digitalWrite(ESP32_RESETN, HIGH);
  delay(100);
  digitalWrite(ESP32_RESETN, LOW);
}

void loop() {
  if (rts != Serial.rts()) {
    digitalWrite(ESP32_RESETN, Serial.rts() ? LOW : HIGH);
    rts = Serial.rts();
  }
  if (dtr != Serial.dtr()) {
    digitalWrite(ESP32_GPIO0, (Serial.dtr() == 0) ? HIGH : LOW);
    dtr = Serial.dtr();
  }
  if (Serial.available()) Serial1.write(Serial.read());
  if (Serial1.available()) Serial.write(Serial1.read());
}