/*
  Sketch receives a byte and compares it with a hardcoded address.
  If the address matches the byte, an 'a' for ack is sent back and
  the LED is toggled.

  Makes use of software serial library as we need to tristate the
  Tx line between sends.
*/
#include <SoftwareSerial.h>

const byte rxPin = 1;
const byte txPin = 0;

// Set up a new SoftwareSerial object
SoftwareSerial mySerial (rxPin, txPin);

const int ledPin = 3;
const char address = '0';
char input;

int ledState = LOW;

void setup() {
  // Define pin modes for TX and RX
  pinMode(rxPin, INPUT);
  pinMode(txPin, INPUT);

  pinMode(ledPin, OUTPUT);

  // Set the baud rate for the SoftwareSerial object
  mySerial.begin(9600);
  mySerial.listen();
}

void loop() {
  if (mySerial.available() > 0) {
    input = mySerial.read();

    if (input == address) {
      // toggle on command
      ledState = !ledState;

      // send Ack
      pinMode(txPin, OUTPUT);
      mySerial.print('a');
      pinMode(txPin, INPUT);
    }
  }

  digitalWrite(ledPin, ledState);
}