Bridging two arduino's

Other Hardware 3rd Party Boards
1

Arduino1-----ControllingDevice-------Arduino2
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I'd like a controlling device to send a 1/0 to one of the arduino's (and a complementary logic level
to the other arduino) which, if 1, will set that Arduino's RX/TX line active so that arduino can
RX/TX with the controlling device.
I am trying to make an I/O expanded (using arduino's only), but I don't know do to swtich on/off Serial.begin(speed).
Can this be done and how?
I should say the ControllingDevice only has one RX/TX line hence I need some switching method to turn one arduino while 'sleeping' the other.
3

I think it can be done. You must be careful with the TX output of the two Arduinos; if the one sends a logical 1 and the other one a logical 0, it's quite unhealthy.

The device that is not sending needs to place its TX in tristate (pinMode INPUT). Next the other device can do a Serial.begin() and transmit.

The device (Arduino) code could look something like below. No idea if it works and use at own risk; it would be something that I would try though if it was my requirement not to add any additional hardware.

void loop()
{
  static bool serialEnabled = false;

  // if we had the serial connection and the request pin is low
  if (requested == false && serialEnabled == true)
  {
    // flush remaining data and stop
    Serial.end();
    // tx pin tristate
    pinMode(1, INPUT);

    // indicate we no longer have the serial connection
    serialEnabled = false;
  }

  // if we did not have the serial connection and the request pin is high
  if(requested == true && serialEnabled == false)
  {
    // setup serial connection
    Serial.begin(9600);

    // indicate that we have the serial connection
    serialEnabled = true;
  }

  // if we have the serial connection
  if(serialEnabled == true)
  {
    // send data
    Serial.println(someData);
  }
}

The better way though would be a multiplexer (as mentioned) or use RS485. The latter is a bus and takes care of the tristate condition (if implemented correctly).

4

There are simpler and more sensible ways to increase the i/o of an Arduino than using multiple Arduinos.

First step: post your schematic. There are often ways to reduce the i/o pins needed by selecting the right components, e.g. i2c backpacks for LCD displays or arranging buttons or LEDs into matrices.

If the number of i/o cannot be easily reduced, there are several types of i/o expansion chips available, depending on what type of i/o are needed.