Serial Call and Response (handshaking)

This example demonstrates multi-byte communication from the Arduino or Genuino board to the computer using a call-and-response (handshaking) method.

This sketch sends an ASCII A (byte of value 65) on startup and repeats that until it gets a serial response from the computer. Then it sends three sensor values as single bytes, and waits for another response from the computer.

You can use the Arduino Software (IDE) serial monitor to view the sent data, or it can be read by Processing (see code below), Flash, PD, Max/MSP (see example below), etc.

Hardware Required
  • Arduino
  • 2 analog sensors (potentiometer, photocell, FSR, etc.)
  • pushbutton
  • 3 10K ohm resistors
  • hook-up wires
  • breadboard

Software Required


Connect analog sensors to analog input pin 0 and 1 with 10K ohm resistors used as voltage dividers. Connect a pushbutton or switch to digital I/O pin 2 with a 10K ohm resistor as a reference to ground.

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image developed using Fritzing. For more circuit examples, see the Fritzing project page


click the image to enlarge


  Serial Call and Response
 Language: Wiring/Arduino

 This program sends an ASCII A (byte of value 65) on startup
 and repeats that until it gets some data in.
 Then it waits for a byte in the serial port, and
 sends three sensor values whenever it gets a byte in.

 Thanks to Greg Shakar and Scott Fitzgerald for the improvements

   The circuit:
 * potentiometers attached to analog inputs 0 and 1
 * pushbutton attached to digital I/O 2

 Created 26 Sept. 2005
 by Tom Igoe
 modified 24 April 2012
 by Tom Igoe and Scott Fitzgerald

 This example code is in the public domain.


int firstSensor = 0;    // first analog sensor
int secondSensor = 0;   // second analog sensor
int thirdSensor = 0;    // digital sensor
int inByte = 0;         // incoming serial byte

void setup()
    // start serial port at 9600 bps:
    while (!Serial)
        ; // wait for serial port to connect. Needed for native USB port only

    pinMode(2, INPUT);   // digital sensor is on digital pin 2
    establishContact();  // send a byte to establish contact until receiver responds

void loop()
    // if we get a valid byte, read analog ins:
    if (Serial.available() > 0)
        // get incoming byte:
        inByte =;
        // read first analog input, divide by 4 to make the range 0-255:
        firstSensor = analogRead(A0) / 4;
        // delay 10ms to let the ADC recover:
        // read second analog input, divide by 4 to make the range 0-255:
        secondSensor = analogRead(1) / 4;
        // read  switch, map it to 0 or 255L
        thirdSensor = map(digitalRead(2), 0, 1, 0, 255);
        // send sensor values:

void establishContact()
    while (Serial.available() <= 0)
        Serial.print('A');   // send a capital A

Processing sketch to run with this example:

// This example code is in the public domain.

import processing.serial.*;

int bgcolor;                 // Background color
int fgcolor;                 // Fill color
Serial myPort;                       // The serial port
int[] serialInArray = new int[3];    // Where we'll put what we receive
int serialCount = 0;                 // A count of how many bytes we receive
int xpos, ypos;                  // Starting position of the ball
boolean firstContact = false;        // Whether we've heard from the microcontroller

void setup() {
  size(256, 256);  // Stage size
  noStroke();      // No border on the next thing drawn

  // Set the starting position of the ball (middle of the stage)
  xpos = width/2;
  ypos = height/2;

  // Print a list of the serial ports for debugging purposes
  // if using Processing 2.1 or later, use Serial.printArray()

  // I know that the first port in the serial list on my mac
  // is always my  FTDI adaptor, so I open Serial.list()[0].
  // On Windows machines, this generally opens COM1.
  // Open whatever port is the one you're using.
  String portName = Serial.list()[0];
  myPort = new Serial(this, portName, 9600);

void draw() {
  // Draw the shape
  ellipse(xpos, ypos, 20, 20);

void serialEvent(Serial myPort) {
  // read a byte from the serial port:
  int inByte =;
  // if this is the first byte received, and it's an A,
  // clear the serial buffer and note that you've
  // had first contact from the microcontroller.
  // Otherwise, add the incoming byte to the array:
  if (firstContact == false) {
    if (inByte == 'A') {
      myPort.clear();          // clear the serial port buffer
      firstContact = true;     // you've had first contact from the microcontroller
      myPort.write('A');       // ask for more
  else {
    // Add the latest byte from the serial port to array:
    serialInArray[serialCount] = inByte;

    // If we have 3 bytes:
    if (serialCount > 2 ) {
      xpos = serialInArray[0];
      ypos = serialInArray[1];
      fgcolor = serialInArray[2];

      // print the values (for debugging purposes only):
      println(xpos + "\t" + ypos + "\t" + fgcolor);

      // Send a capital A to request new sensor readings:
      // Reset serialCount:
      serialCount = 0;

Processing Code

Copy the Processing sketch from the code sample above. As you change the value of the analog sensor, you'll get a ball moving onscreen something like this. The ball will appear only when you push the button:

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