Selecting a serial port using a configuration file; a quick tip

Processing Electronics (Arduino, etc.)
1

I often see Processing code (for communication with e.g. Arduino) where the port is hard-coded. E.g.

myPort = new Serial(this, Serial.list()[0], 9600);

One thing that people don’t seem to realise is that the second parameter for the constructor is a String and one can actually use e.g.

myPort = new Serial(this, "COM3", 9600);

It’s still hardcoded and in both cases, if one needs to change the port for some reason, one has to modify the Processing code to cater for it. Not a big issue if you’re working in the IDE but a little more of a inconvenience if you’re using an exported application.

The below is a way how to prevent to have to change the code (and export again) after a change. The code uses a function to read a specified serial port from a text file; it also provides a function to check if the specified port actually exists. The code is kept simple (always a relative term :wink:)

The configuration file is simple and only contains one line specifying the port; the specified port must be on the first line as the function to read the file only checks the first line.

In setup(), the application attempts to read the configuration file and on success checks if the specified port exists; if so, it attempts to open the specified port. In this example, draw() is used to display the status of the process.

import processing.serial.*;

// serial object
Serial mySerial = null;

// configuration file
final String configFile = "data/PortConfig.txt";
// baudrate
final int baudrate = 115200;

// serial port selected by user; one can set this to a default (e.g. COM8)
String selectedPort = "";

// message to display
String displayMessage= "";

void setup()
{
  size(350, 400);

  // possible Processing bug
  // when the application starts, it does not always react on key presses
  //   and one has to manucally click in the application to give it the focus
  // workaround that might solve it found at https://discourse.processing.org/t/keypressed-only-works-sometimes/22340/
  surface.setVisible(true);

  // if read port from file succeeded
  if (readPortFromFile() == true)
  {
    // if the specified port exists
    if (chkPortExists() == true)
    {
      try
      {
        mySerial = new Serial(this, selectedPort, baudrate);
        displayMessage = "Port " + selectedPort + "' is open";
      }
      catch(RuntimeException rtex)
      {
        displayMessage = rtex.getMessage();
      }
    }
  }
}

void draw()
{
  background(0xFFFFF1C4);
  textSize(16);
  textAlign(CENTER, CENTER);
  if (mySerial == null)
  {
    fill(0xFF800000);
  } //
  else
  {
    fill(0xFF008000);
  }
  text(displayMessage, width/2, height / 2);
}

/*
Retrieve port-to-use from file
 Returns:
 true on successfu read, else false
 */
boolean readPortFromFile()
{
  boolean success = true;
  try
  {
    BufferedReader reader = createReader(configFile);
    // if the file was not opened
    if (reader == null)
    {
      displayMessage = "Could not open file '" + configFile + "'";
      success = false;
    } //
    else
    {
      selectedPort = reader.readLine();
    }
  }
  catch (Exception ex)
  {
    displayMessage = ex.getMessage();
    success = false;
  }

  return success;
}

/*
Check if selected port exists
 Returns:
 true if it exists, else false
 */
boolean chkPortExists()
{
  boolean portExists = false;
  // loop through the list of ports
  for (int pCnt = 0; pCnt < Serial.list().length; pCnt++)
  {
    // if specified port exists in the list
    if (Serial.list()[pCnt].equals(selectedPort) == true)
    {
      portExists = true;
      break;
    }
  }
  // if the specified port does not exists in the list
  if (portExists == false)
  {
    displayMessage = "Serial port '" + selectedPort + "' does not exist";
  }

  return portExists;
}

Configuration file:

COM9

Tested in a Windows world.

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