flightgear/docs-mini/README.protocol

The generic communication protocol for FlightGear provides a powerful way
of adding a simple ASCII based or binary protocol, just by defining an
XML encoded configuration file.

The definition of the protocol consists of variable separators, line separators,
and chunks of text.

Each chunk defines:

<name>		for ease of use
<node>		the property tree node which provides the data
<type>		the value type (needed for formatting)
<format>	defines the actual piece of text which should be sent.
		it can include formatting options like:
				<type>
			%s	string
			%i	integer	(default)
			%f	float
		(not used or needed in binary mode)

<factor>	an optional multiplication factor which can be used for
		unit conversion. (for example, radians to degrees).
<offset>	an optional offset which can be used for unit conversion.
		(for example, degrees Celsius to degrees Fahrenheit).


The output section also could define the variable separator and line separator.

The separators can be either a control character such as a tab or newline, or a
user specified string or other single character. The currently supported
control characters are:

<var_separator>:
<line_separator>:
Name		Character

newline		'\n'
tab		'\t'
formfeed	'\f'
carriagereturn	'\r'
verticaltab	'\v'

any other characters just need to be added to "Network/generic.cxx"

The var_separator is placed between each variable, while the line_separator is
placed at the end of each lot of variables.

To enable binary mode, simply include a <binary_mode>true</binary_mode> tag in
your XML file. The format of the binary output is tightly packed, with 1 byte
for bool, 4 bytes for int, and 8 bytes for double. At this time, strings are not
supported. A configurable footer at the end of each "line" or packet of binary
output can be added using the <binary_footer> tag. Options include the length
of the packet, a magic number to simplify decoding. Examples:

	<binary_footer>magic,0x12345678</binary_footer>
	<binary_footer>length</binary_footer>
	<binary_footer>none</binary_footer>			<!-- default -->

A simple protocol configuration file then could look something like the
following:

<?xml version="1.0"?>

<PropertyList>

 <generic>

  <output>
   <line_separator>newline</line_separator>
   <var_separator>newline</var_separator>
   <binary_mode>false</binary_mode>

   <chunk>
    <name>speed</name>
    <format>V=%d</format>
    <node>/velocities/airspeed-kt</node>
   </chunk>

   <chunk>
    <name>heading</name>
    <format>H=%02d</format>
    <node>/orientation/heading-deg</node>
    <factor>57.29578</factor>  <!-- radians to degrees -->
   </chunk>

   <chunk>
    <name>pitch angle</name>
    <format>P=%05.1f</format>
    <type>float</type>
    <node>/orientation/pitch-deg</node>
   </chunk>

 </generic>

</PropertyList>