chrome extension - reading from serial port

Question

This is my first attempt at developing with a chrome app or extension. I have a GPS receiver on the USB port which is emulated as a serial device.

Running this code

var onGetDevices = function(ports) {

for (var i=0; i<ports.length; i++) {

    // show me some output
    console.log(ports[i].path);

    // Connect to the serial port /dev/ttyUSB0
    chrome.serial.connect(ports[i].path, {bitrate: 9600}, onConnect);   
  }
}
chrome.serial.getDevices(onGetDevices);

gets me "/dev/ttyUSB0" in the console, so it appears to be finding the device.

How do I then connect to the device? I've included the serial.connect line above, with the following functions:

var onConnect = function(connectionInfo) {
    // The serial port has been opened. Save its id to use later.
    _this.connectionId = connectionInfo.connectionId;

    // Do whatever you need to do with the opened port.
    chrome.serial.onReceive.addListener(onReceiveCallback);
}

var stringReceived = '';
var onReceiveCallback = function(info) {
  if (info.connectionId == expectedConnectionId && info.data) {
      var str = convertArrayBufferToString(info.data);

      if (str.charAt(str.length-1) === '\n') {
          stringReceived += str.substring(0, str.length-1);
          onLineReceived(stringReceived);
          stringReceived = '';
      } 
      else {
        stringReceived += str;
      }
  }
};

but I get the following error:

Error in response to serial.connect: ReferenceError: _this is not defined at Object.onGetDevices [as callback]

I'm not sure exactly what I'm doing right or wrong here so any pointers appreciated.

Solution

First the example does not work properly. Try this instead:

var connectionId;

$(document).ready(function() {
    chrome.serial.getDevices(function(devices) {

        for (var i = 0; i < devices.length; i++) {
            $('select#portList').append('<option value="' + devices[i].path + '">' + devices[i].path + '</option>');
        }
    });

    // ui hook
    $('button#open').click(function() {
        var clicks = $(this).data('clicks');

        if (!clicks) {
            var port = $('select#portList').val();
            chrome.serial.connect(port, {bitrate: 9600}, function(info) {
                connectionId = info.connectionId;
                $("button#open").html("Close Port");
                console.log('Connection opened with id: ' + connectionId + ', Bitrate: ' + info.bitrate);
            });
        } else {
            chrome.serial.disconnect(connectionId, function(result) {
                $("button#open").html("Open Port");
                console.log('Connection with id: ' + connectionId + ' closed');
            });
        }

        $(this).data("clicks", !clicks);
    });
});

Now as for the actual reading of the input from the serial connection it will work but converting the ArrayBuffer to a string is a bit harder than expected.

Comments

[Admin]

awesome, thanks Joe, this really set me up on the right track. cant believe the chrome tutorial doesnt have proper working code..!

[Joe Kasavage]

Not a problem. Took me a little to figure it out myself.

[Admin]

Joe - im having issues connecting to the port now. Its weird, just doesnt seem to want to play ball. Did you get something up and running your end?

[Joe Kasavage]

Yes I used this exact code to read from the serial device, barcode scanner in this instance. I am assuming it is failing to open even when you set the bitrate?

[Admin]

yes, i've just downloaded a packaged app and it cant connect either, so must be my hardware.its a USB GPS device which emulates as a serial port. It works fine on linux using gpsmon, so must be down to being a usb device and not natively serial?

[Joe Kasavage]

That may be the case. Try thr chrome.usb lib and see if that helps.

[Admin]

Let us continue this discussion in chat.