int i = 0;
String s;
unsigned long previousMillis = 0;
const long interval = 5000; // Send to serial every 5 seconds

void setup() {
  Serial.begin(115200);
}

void loop() {
  while (Serial.available()) {
    s = Serial.readString();
    Serial.print("Received from serial: ");
    Serial.println(s);
  }

  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    Serial.print("from Arduino: ");    
    Serial.println(i++);
  }
}