String msg;

void setup() {
  Serial.begin(115200); // Start serial communication
  while (!Serial); // Wait until Serial is open
}

void loop() {
  while (Serial.available()) { # Read a line
    delay(3);  //delay to allow buffer to fill 
    if (Serial.available() > 0) {
      char c = Serial.read();  //gets one byte from serial buffer
      msg += c; //makes the string readString
    } 
  }
  if (msg != ""){
    Serial.println("Echo: " + msg); // Print an echo
    msg = ""; # Reset the received message
  }
}