Built-in project · Arduino Uno R3

Two Arduino Unos talking over serial

Two Arduino Unos, each with its own sketch, passing "ping" and "pong" lines back and forth over a SoftwareSerial link on pins 2 and 3. The whole build, an Arduino Uno, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Two Arduino Unos talking over serial · Arduino Uno R3live0.000 s 0.00x

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The circuit itself, running here on the simulator. Press what can be pressed; click anything else to open it in the editor.

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · Arduino Uno R3

// Uno A: sends "ping 1", "ping 2", ... to Uno B over a serial link on
// two ordinary pins, and prints the answer that comes back.
//
//   A pin 3 (TX)  ->  B pin 2 (RX)
//   A pin 2 (RX)  <-  B pin 3 (TX)
//   A GND         --  B GND
//
// SoftwareSerial is half duplex: while one side sends, it cannot listen.
// So the two take turns, A asks and B answers, and nothing collides.

#include <SoftwareSerial.h>

SoftwareSerial link(2, 3);  // RX on pin 2, TX on pin 3

unsigned long count = 0;
char reply[32];

void setup() {
  Serial.begin(9600);
  link.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("Uno A: a ping every half second");
}

void loop() {
  count++;
  link.print("ping ");
  link.println(count);
  digitalWrite(LED_BUILTIN, HIGH);

  // Wait up to 300 ms for one line back.
  byte len = 0;
  bool done = false;
  unsigned long start = millis();
  while (!done && millis() - start < 300) {
    while (link.available()) {
      char c = link.read();
      if (c == '\n') {
        done = true;
        break;
      }
      if (c != '\r' && len < sizeof(reply) - 1) reply[len++] = c;
    }
  }
  reply[len] = 0;
  digitalWrite(LED_BUILTIN, LOW);

  if (done) {
    Serial.print("A heard: ");
    Serial.println(reply);
  } else {
    Serial.println("A heard nothing back. Is B running, and are the grounds joined?");
  }
  delay(200);
}

sketch.ino · Arduino Uno R3

// Uno B: listens on pin 2 for a line from Uno A and answers each
// "ping N" with "pong N" on pin 3.

#include <SoftwareSerial.h>

SoftwareSerial link(2, 3);  // RX on pin 2, TX on pin 3

char line[32];
byte len = 0;

void setup() {
  Serial.begin(9600);
  link.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("Uno B: waiting for a ping");
}

void loop() {
  while (link.available()) {
    char c = link.read();
    if (c == '\n') {
      line[len] = 0;
      Serial.print("B heard: ");
      Serial.println(line);
      digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
      // "ping 7" -> "pong 7"
      link.print("pong ");
      link.println(len > 5 ? line + 5 : "?");
      len = 0;
    } else if (c != '\r' && len < sizeof(line) - 1) {
      line[len++] = c;
    }
  }
}