Lab: Two-Way (Duplex) Serial Communication Using An Arduino and the p5.serialport Library

Introduction

In the Introduction to Asynchronous Serial Communication lab, you learned about various methods for managing the communications between computers via asynchronous serial communication. These included formatting your data as ASCII-encoded strings or raw serial bytes and managing the flow of data using handshaking. In the P5.js Serial Input Lab, you sent data from one sensor to a personal computer. In this lab, you’ll send data from multiple sensors to a program in P5.js. You’ll use the data from the sensors to create a pointing-and-selecting device (i.e. a mouse).

What You Should Know

To get the most out of this tutorial, you should know what a microcontroller is and how to program them. You should also understand asynchronous serial communication between microcontrollers and personal computers. You should also understand the basics of P5.js. It would also help to go through the following labs first:

• Lab: Intro to Asynchronous Serial Communication
• Lab: Serial Input to P5.js
• Lab: Serial Output From P5.js

These videos might help in understanding this lab as well:

• Video: Two-way Serial Communication (Call-and-Response)
• These videos are explained in the Processing programming environment, but the concepts still apply:
  • Video: Reading Serial Strings in Processing
  • Video: Sending and Reading Multiple Serial Values in Processing

Things You’ll Need

For this lab, you’ll need the hardware below, and you’ll need the same software setup as the Serial Input to P5.js lab: You’ll create a p5.js sketch. You’ll also use the p5.serialport library and the P5.serialcontrol app. You can use the p5.js web editor or your favorite text editor for this (the Visual Studio Code editor or the  Atom text editor work well).

Figures 1-5 below are the parts you’ll need for this exercise. Click on any image for a larger view.

Connect the Sensors

For this exercise, you’re going to need two analog inputs to your microcontroller, and one digital input. It doesn’t matter what they are, so use something that’s easy for you to set up. The photos and schematic in this lab show potentiometers and a pushbutton. You don’t have to use these, though. Any three sensor inputs will do the job. If you’re looking for options, consider:

As long as you have three sensors that will output changing readings, you can follow this lab.

Connect the two analog sensors to analog pins 0 and 1 like you did in the analog input to Arduino lab. Connect a pushbutton to digital pin 2 like you did in the digital input and output with Arduino lab.

(Diagrams made with Fritzing, a circuit design program)

Sending Multiple Serial Data using Punctuation

You’re going to program the microcontroller to read the pushbutton and two analog sensors just like you did in the Intro to Serial Communications Lab. When you have to send multiple data items, you need a way to separate them. If you’re sending them as ASCII-encoded strings, it’s simple: you can just put non-numeric punctuation bytes between them (like a comma or a space) and a unique termination punctuation at the end (like a newline and/or carriage return).

This program will send the two analog sensor values and then the pushbutton. All three will be ASCII-encoded numeric strings, separated by commas. The whole line of sensor values will be terminated by carriage return (\r, ASCII 13) and newline (\n, ASCII 10).

const int buttonPin = 2;      // digital input

void setup() {
  // configure the serial connection:
  Serial.begin(9600);
  // configure the digital input:
  pinMode(buttonPin, INPUT);
}

void loop() {
  // read the first analog sensor:
  int sensorValue = analogRead(A0);
  // print the results:
  Serial.print(sensorValue);
  Serial.print(",");

  // read the second analog sensor:
  sensorValue = analogRead(A1);
  // print the results:
  Serial.print(sensorValue);
  Serial.print(",");

  // read the button:
  sensorValue = digitalRead(buttonPin);
  // print the results:
  Serial.println(sensorValue);
}

When you run this and output it to the Serial Monitor, you should see something like this:

348,363,1
344,362,1
345,363,1
344,375,0
365,374,0
358,369,0
355,369,0
352,373,0
356,373,0

Turn the potentiometers (or tweak the analog sensors) and push the button. Now you’ve got a data format: three sensors, comma-separated, terminated by carriage return and newline. This means that you already have an algorithm for how you’re going to program P5.js to read the serial input:

• Read the incoming serial data into a string until a carriage return and newline appear
• split the string into substrings on the commas
• convert the substrings into numbers
• assign the numbers to variables to change your programNow that you’ve got a plan, put it into action.

Receive the data in P5.js

Now write a P5.js sketch that reads the data as formatted by the Arduino program above. Download the latest version of the P5.serialcontrol application if you haven’t already and save it in your Applications folder. When you run it, it will check serial ports on your machine. You don’t need to do anything with the app, just have it open.

You’ll need to know the name of your serial port to get started. If you’re not sure how to get this, see the Serial Input to P5.js lab for how to get a list of ports.

The P5.js Sketch

The sketch you’re going to write will control the microcontroller’s LED from P5.js. Dragging the mouse up and down the canvas will dim or brighten the LED, and typing 0 through 9 will set the LED’s brightness in increments from off (0) through almost full brightness (9). There’s an alternate sketch that will make changing tones if you prefer that instead of a changing LED. The sketch will also receive serial input from the microcontroller just as in the Serial Input to P5.js lab, so that you can see that the microcontroller is getting the same values you’re sending.

As you saw in the Serial Input to P5.js lab,  you’ll need to copy the p5.seriaport.js lbrary into your sketch’s libraries folder. You’ll need to run p5.serialcontrol, or run the p5.serialserver from the command line as you did in the Serial Input to P5.js lab as well.

Make a P5.js sketch. If you’re using the p5.js web editor, make a new sketch. Click the Sketch Files tab, and then choose the index.html file. In the head of the document, look for this line:

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.4.0/p5.js"></script>

Right after that line, add this line:

<script language="javascript" type="text/javascript" src="https://cdn.jsdelivr.net/npm/p5.serialserver@0.0.28/lib/p5.serialport.js"></script>

The setup of your sketch will initialize the P5.serialport library and define your callback functions for serial events. , as you did in other sketches

Then in the setup(), create a canvas, make an instance of the serialport library, and declare your callback functions.

var serial;          // variable to hold an instance of the serialport library

function setup() {
  createCanvas(800, 600);          // make canvas
  smooth();                        // antialias drawing lines
  serial = new p5.SerialPort();    // make a new instance of the serialport library
  serial.on('list', printList);    // set a callback function for the serialport list event
  serial.on('connected', serverConnected); // callback for connecting to the server
  serial.on('open', portOpen);     // callback for the port opening
  serial.on('data', serialEvent);  // callback for when new data arrives
  serial.on('error', serialError); // callback for errors
  serial.on('close', portClose);   // callback for the port closing

  serial.list();                   // list the serial ports
}

This time, you’ll add a serialport select menu as you did in the Serial Input to P5.js  lab.First, declare a global variable to hold the port menu at the top of your sketch like so:

var portSelector; // a select menu for the port list

Then change the printList function like so:

// get the list of ports:
function printList(portList) {
  // make a select menu and position it:
  portSelector = createSelect();
  portSelector.position(10,10);
  
  // portList is an array of serial port names
  for (var i = 0; i < portList.length; i++) {
    // Display the list the console:
    // console.log(i + " " + portList[i]);
    // add item to the select menu:
    portSelector.option(portList[i]);
  }
  // set a handler for when a port is selected from the menu:
  portSelector.changed(mySelectEvent);
}

When the select menu’s value has changed, you can assume a serial port has been selected, so write a handler to open it like so:

function mySelectEvent() {
  let item = portSelector.value();
   // if there's a port open, close it:
  if (serial.serialport != null) {
    serial.close();
  }
  // open the new port:
  serial.open(item);
}

From now on, when you run this sketch, you’ll need to select the serial port to open the port.

The rest of the serial event handlers are all the same as you saw in the P5.js Serial Input Lab, except for the serialEvent(). Here are all but the serialEvent():

function serverConnected() {
  console.log('connected to server.');
}

function portOpen() {
  console.log('the serial port opened.')
}

function serialError(err) {
  console.log('Something went wrong with the serial port. ' + err);
}

function portClose() {
 console.log('The serial port closed.');
}

Program the serialEvent() function to read the incoming serial data as a string until it encounters a carriage return and newline (‘\r\n’). Then check to see that the resulting string has a length greater than 0 bytes. If it does, use the split() function to split it in to an array of strings. If the resulting array is at least three elements long, you have your three sensor readings. The first reading is the first analog sensor, and can be mapped to the horizontal movement using the locH variable. The second is the second analog sensor and can be mapped to the locV variable. The third is the button. When it’s 0, set the circleColor variable equal to 255 and when it’s 1, set the variable to 0. Here’s how:

function serialEvent() {
  // read a string from the serial port
  // until you get carriage return and newline:
  var inString = serial.readStringUntil('\r\n');

  //check to see that there's actually a string there:
  if (inString.length > 0 ) {
    var sensors = split(inString, ',');            // split the string on the commas
    if (sensors.length > 2) {                      // if there are three elements
      locH = map(sensors[0], 0, 1023, 0, width);   // element 0 is the locH
      locV = map(sensors[1], 0, 1023, 0, height); // element 1 is the locV
      circleColor = 255 - (sensors[2] * 255);      // element 2 is the button
    }
  }
}

Note the mappings of sensor[0] and sensor[1]. You should use the input mappings for your accelerometer instead of 0 and 1023. If your analog values are greater than the width of the sketch or the height, the circle will be offscreen, which is why you have to map your sensor range to the screen size.

Program the draw() function to draw a circle that’s dependent on three global variables, locH, locV, and circleColor. Add these three globals to the top of the program:

var locH = 0;
var locV = 0;        // location of the circle
var circleColor = 255; // color of the circle

Finally, here is the draw function:

function draw() {
  background(0);               // black background
  fill(circleColor);           // fill depends on the button
  ellipse(locH, locV, 50, 50); // draw the circle
}

If you run this, you should see the circle moving onscreen whenever you tilt the accelerometer. When you press the pushbutton, the circle will disappear. Okay, it’s not exactly a  mouse, but you are controlling an animation from a device that you built.

Flow Control: Call and Response (Handshaking)

You’ve seen now that by coming up with a serial format (called a protocol), you can write the algorithm for receiving it even before you see any data. You can send multiple pieces of data this way, as long as you format it consistently.

Sometimes you can run into a problem when the sender sends faster than the receiver can read. When this happens, the receiver program slows down as the serial buffer fills up. You can manage this by implementing some form of flow control. The simplest way do to this is using a call-and-response method, where the sending program only sends when it’s told to do so, and the receiving program has to request new data every time it finishes reading what it’s got.

You can add handshaking to the code above fairly simply. Modify the Arduino code as follows. First, add a a new block of code in the setup() This block sends out a message until it gets a byte of data from the remote computer:

void setup() {
  Serial.begin(9600);
  while (Serial.available() <= 0) {
    Serial.println("hello"); // send a starting message
    delay(300);              // wait 1/3 second
  }
}

Now, modify the loop() by adding an if() statement to look for incoming serial data and read it.

void loop() {
  if (Serial.available() > 0) {
    // read the incoming byte:
    int inByte = Serial.read();
    // read the sensor:
    sensorValue = analogRead(A0);
    // print the results:
    Serial.print(sensorValue);
    Serial.print(",");

    // read the sensor:
    sensorValue = analogRead(A1);
    // print the results:
    Serial.print(sensorValue);
    Serial.print(",");

    // read the sensor:
    sensorValue = digitalRead(buttonPin);
    // print the results:
    Serial.println(sensorValue);
  }
}

The rest of the sketch remains the same. When you run this and open the serial monitor, you’ll see:

hello
hello
hello
hello

Type any character in the output box and click Send. You’ll get a string of sensor values at the end of your hellos:

510,497,0

Type another character and click Send. It doesn’t matter what character you send, but the loop will always wait for an incoming byte before sending a new set of sensor values. When you write a program to receive this format, it just has to behave the same way you did:

• Open the serial port
• Wait for a Hello
• Send a byte to request data
• Begin loop:
• Wait for one set of data
• Send a byte to request new data
• end loop

Next, modify the P5.js sketch.  Most of the changes are in the serialEvent() function. The initial “hello” messages will trigger this function, so when you get a “hello” or any other string, you need to send a byte back so that the Arduino has a byte available to read. Here’s the new serialEvent():

function serialEvent() {
  // read a string from the serial port
  // until you get carriage return and newline:
  var inString = serial.readStringUntil('\r\n');
  //check to see that there's actually a string there:
  if (inString.length > 0) {
    if (inString !== 'hello') { // if you get hello, ignore it
      var sensors = split(inString, ','); // split the string on the commas
      if (sensors.length > 2) { // if there are three elements
        locH = map(sensors[0], 0, 1023, 0, width); // element 0 is the locH
        locV = map(sensors[1], 0, 1023, 0, height); // element 1 is the locV
        circleColor = 255 - (sensors[2] * 255); // element 2 is the button
      }
    }
    serial.write('x'); // send a byte requesting more serial data
  }
}

You also need to add a line to the openPort() function like so:

function portOpen() {
  console.log('the serial port opened.')
  // send a byte to prompt the microcontroller to send:
  serial.write('x');
}

The reason for this is that if your Arduino has already broken out of the loop (let’s say you opened the Serial monitor to check), then it is waiting for a byte from p5.js to send the next block of code. By sending a byte when you know the port has just been opened in p5.js, you force the Arduino to send you new data.

That’s it. Your sketch should still run just as it did before, though the serial communication is managed better now, because Arduino’s only sending when P5.js is ready to receive.

The full code for all the examples in this lab can be found in this gitHub repository.

Advantages of Raw Binary vs. ASCII

All the examples shown here sent the sensor values as ASCII-encoded strings. As mentioned above, that means you sent three bytes to send a three-digit value. If that same value was less than 255, you could send it in one raw binary byte. So ASCII is definitely less efficient. However, it’s more readable for debugging purposes, and if the receiving program is well-suited to convert strings to numbers, then ASCII is a good way to go. If the receiver’s not so good at converting strings to numbers (for example, it’s more challenging to read a multiple byte string in Arduino than in Processing) then you may want to send your data as binary values.

Advantages of Punctuation or Call-and-Response

The punctuation method for sending multiple serial values may seem simpler, but it has its limitations. You can’t easily use it to send binary values, because you need to have a byte with a unique value for the punctuation. In the example above, you’re using the value 10 (ASCII newline) as punctuation, so if you were sending your sensor values as raw bytes, you’d be in trouble when the sensor’s value is 10. The receiver would interpret the 10 as punctuation, not as a sensor value. In contrast, call-and-response can be used whether you’re sending data as raw binary values or as ASCII-encoded values.

Sometimes the receiver reads serial data slower than the sender sends it. For example, if you have a program that does a lot of graphic work, it may only read serial data every few milliseconds. The serial buffer will get full in that case, you’ll notice a lag in response time. This is when it’s good to switch to a call-and-response method.

Build an Application of Your Own

You just duplicated the basic functionality of a mouse; that is, a device with two analog sensors that affect X and Y, and a digital sensor (mouse button). What applications can you think of that could use a better physical interface for a mouse? A video editor that scrubs forward and back when you tilt a wand? An action game that reacts to how hard you hit a punching bag? An instructional presentation that speeds up if you shift in your chair too much? A music program driven by a custom musical instrument that you design?

Create a prototype in Arduino and P5.js, Node.js, Processing, or whatever programming environment you choose. Come up with a physical interface that makes it clear what actions map to what movements and actions. Figure out which actions can and should be possible at the same time. Present a working software and hardware model of your idea.