Mega four serial ports

// Four serial ports: the thing a Mega has that an Uno does not.
//
// An Uno has one USART and it is spoken for: it is the channel to the serial
// monitor. A Mega has four. Serial is still the monitor, and Serial1, Serial2
// and Serial3 come out on header pins:
//
//   Serial1   TX1 pin 18   RX1 pin 19
//   Serial2   TX2 pin 16   RX2 pin 17
//   Serial3   TX3 pin 14   RX3 pin 15
//
// Wire each TX back to the RX beside it with a short jumper and every port
// talks to itself, which is the cheapest way to see all three working at once
// with nothing else on the bench. In the simulator those are real wires and
// the bytes are real 8N1 frames on them: start bit, eight data bits least
// significant first, stop bit, at the baud rate each port was opened with.
// Change one port's baud rate and its line comes back as mojibake, exactly as
// it would on the bench.
//
// The three ports run at different rates on purpose, so the frames on the
// three wires are visibly different lengths.

const unsigned long RATE[3] = { 9600, 19200, 38400 };

HardwareSerial *port(int n) {
  if (n == 0) {
    return &Serial1;
  }
  if (n == 1) {
    return &Serial2;
  }
  return &Serial3;
}

unsigned long lastSend = 0;
int round_ = 0;

void setup() {
  Serial.begin(115200);
  Serial1.begin(RATE[0]);
  Serial2.begin(RATE[1]);
  Serial3.begin(RATE[2]);

  Serial.println("Mokxi Arduino Mega 2560: four serial ports");
  for (int n = 0; n < 3; n++) {
    Serial.print("Serial");
    Serial.print(n + 1);
    Serial.print(" at ");
    Serial.print(RATE[n]);
    Serial.println(" baud, TX jumpered to its own RX");
  }
}

void loop() {
  unsigned long now = millis();

  // Once a second, send one line out of each port.
  if (now - lastSend >= 1000) {
    lastSend = now;
    round_++;
    for (int n = 0; n < 3; n++) {
      port(n)->print("ping ");
      port(n)->println(round_);
    }
  }

  // Anything a port hears, it heard from itself. Report it with the port it
  // arrived on, so three lines a second appear in the monitor.
  for (int n = 0; n < 3; n++) {
    while (port(n)->available() > 0) {
      int c = port(n)->read();
      if (c == '\r') {
        continue;
      }
      if (c == '\n') {
        Serial.println();
        continue;
      }
      if (c == 'p') {
        // The start of a line: label it with the port it came back on.
        Serial.print("Serial");
        Serial.print(n + 1);
        Serial.print(" heard: ");
      }
      Serial.print((char)c);
    }
  }
}