A single sketch that covers both sides of reading an Arduino pin state from another Arduino device/sketch

arduino-to-arduino-pin-state.ino

/********************************************************** 
* Arduino to Arduino Pin Status
* Demonstration sketch
***********************************************************/

// =============================================================
// comment out the following line to build the receiver version
// of this sketch.
// =============================================================
#define SENDER
// =============================================================

#ifdef SENDER

#define PIN A0
// configure the HIGH/LOW delay time (all at once)
#define DELAY_VAL 2000

void setup() {
  Serial.begin(115200);
  delay(1000);
  Serial.println();
  Serial.println("Configuring PIN for OUTPUT");
  pinMode(PIN, OUTPUT);
}

void loop() {
  Serial.println("Setting PIN state to HIGH");
  digitalWrite(PIN, HIGH);
  delay(DELAY_VAL);
  Serial.println("Setting PIN state to LOW");
  digitalWrite(PIN, LOW);
  delay(DELAY_VAL);
}

#else

#define PIN A0

uint8_t prevState = LOW;

void setup() {
  Serial.begin(115200);
  delay(1000);
  Serial.println();
  Serial.println("Configuring PIN for INPUT");
  pinMode(PIN, INPUT);
}

void loop() {
  uint8_t state = digitalRead(PIN);
  if (prevState != state) {
    prevState = state;
    Serial.print("PIN status: ");
    if (state == HIGH) {
      Serial.println("HIGH");
    } else {
      Serial.println("LOW");
    }
  }

  delay(25);
}
#endif