Howto:Port I/O from Nasal: Difference between revisions

== Introduction ==

You can't really 'access' a device from [[Nasal]] directly. What you'll most likely want to do is define a [[generic protocol]] for the properties you're interested in sending between fgfs and your device. A generic protocol allows you to easily import external data into [[FlightGear]]'s [[property tree]] and export properties, too.

In Nasal, you could use a conventional setprop() call to set a property, which is then picked up by the I/O subsystem and sent to its destination (e.g. serial port/network).

  setprop("/myoutput-property/data", "my data");

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Now, to send a message, you would only need to modify the property - for example, by using setprop:

setprop('/sim/cockpit/eufd', 'Hello World');

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But you could also create a wrapper named "sendMessage" for this purpose:

var sendMessage = func(msg) {

  print("Sending message:", msg);

Now, you could also create additional wrappers to do device-specific things, like clearing the LCD screen, setting the font color, font size, positioning the cursor etc, simply by adding device specific commands to each wrapper:

var setFontColor = func(color) {

  sendMessage("0xFF,0xAC,0xEF....");

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var setHeading = func(heading) {

  sendMessage("0xFF,0xCA,0xFA,$"~heading);

You could then register a listener that is automatically invoked whenever the "input property" is modified, this will automatically invoke your Nasal callback (i.e. a Nasal function) so that you can process the input.

var processMessage = func() {

   print("Info: Message received!");

Obviously, you could now add some header to each message to identify the message type. For example, by checking a substring of the message:

var processMessage = func() {

   var msg = getprop('/sim/cockpit/eufd-input');

Next, you could come up with a list of supported message types:

var processMessage = func() {

   var msg_types = {'A':'SET FONT','B':'CLEAR SCREEN','C':'RESET','D':'SHUTDOWN','E':'NOP'};

You can also directly use hexadecimal values:

   var ESC=0x32;

   var ACK=0x13;

For your data processing needs (formatting, conversion, marshalling, compression etc), you can use a combination of the sprintf() library function, and the [http://gitorious.org/fg/fgdata/blobs/master/Nasal/bits.nas bits.nas module in $FG_ROOT/bits.nas].

  var output_property = '/sim/cockpit/eufd';

  var data = sprintf( "offset:%x", 255 );