Generic protocol

The generic communication protocol for FlightGear provides a powerful way of adding a simple ASCII based or binary input/output protocol, just by defining an XML encoded configuration file and placing it in the $FG ROOT/Protocol/ directory.

XML File Layout

A protocol file can contain either or both of <input> and <output> definition blocks. Which one is used depends on how the protocol is called. The following example would only use the <output> definitions block.

--generic=file,out,1,/tmp/data.xml,myproto

Overview of the XML file

<?xml version="1.0"?>
<PropertyList>
   <generic>

       <output>
           <binary_mode>false</binary_mode>
           <line_separator></line_separator>
           <var_separator></var_separator>
           <preamble></preamble>
           <postamble></postamble>

           <chunk>
               ... first chunk spec ...
           </chunk>

           <chunk>
               ... another chunk etc. ...
           </chunk>
       </output>

       <input>
           <line_separator></line_separator>
           <var_separator></var_separator>

           <chunk>
               ... chunk spec ...
           </chunk>
       </input>

   </generic>
</PropertyList>

Input/Output Parameters

Both <input> and <input> blocks can contain information about the data mode (ascii/binary) and about separators between fields and data sets, as well as a list of <chunk>s. Each <chunk> defines a property that should be written (and how), or a variable and which property it should be written to.

ASCII protocol parameters

Output only:

<preamble>        STRING  default: ""    file header put on top of the file
<postamble>       STRING  default: ""    file footer put at the end of the file

Input & Output:

<binary_mode>	    BOOL    default: false (= ASCII mode)
<var_separator>   STRING  default: ""    field separator
<line_separator>  STRING  default: ""    separator between data sets

• <var_separator> is put between every two output properties
• <line_separator> is put at the end of each data set.

Both can contain arbitrary strings or one of the following keywords:

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

Typical use could be:

<var_separator>tab</var_separator>
<line_separator>newline</var_separator>

or

<var_separator>\t</var_separator>
<line_separator>\r\n</line_separator>

Binary protocol parameters

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>                 

Variable Parameters - <chunk> spec

Both <input> and <output> block can contain a list of <chunk> specs, each of which describes the properties of on variable to write/read.

<name>

For ease of use and not tranferred (like a notes tag)

<node>

The property tree node which provides the data

<type>

The value type which is needed for formatting, one of string, float, bool, int (default: int). Its recommended that this tag is present otherwise spurious results can occur.

<format>

ASCII protocol only, not used or needed in binary mode. Defines the actual piece of text which should be sent. it can include "printf" style formatting options like:

           <type>
   %s      string
   %d      integer (default)
   %f      float

<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 Celcius to degrees Fahrenheit.

<format>

Chunks can also consist of a single constant <format>, like in:

<format>Data Section</format>

Examples

Example 1

Writes log of this form:

V=16
H=3.590505
P=3.59
V=12
H=3.589020
P=3.59

<?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 (rad)</name>
       <format>H=%.6f</format>
       <type>float</type>
       <node>/orientation/heading-deg</node>
       <factor>0.0174532925199433</factor>  
     </chunk>

     <chunk>
       <name>pitch angle (deg)</name>
       <format>P=%03.2f</format>
       <node>/orientation/pitch-deg</node>
     </chunk>
  </output>

</generic>
</PropertyList>

Writing data in XML format=

Assuming the file is called $FG ROOT/Protocol/xmltest.xml, then it could be used as

fgfs --generic=file,out,1,/tmp/data.xml,xmltest

<?xml version="1.0"?>
<PropertyList>
 <generic>
   <output>
     <binary_mode>false</binary_mode>
     <var_separator>\n</var_separator>
     <line_separator>\n</line_separator>
     <preamble><?xml version="1.0"?>\n\n<data>\n</preamble>
     <postamble></data>\n</postamble>

     <chunk>
       <format>\t<set></format>
     </chunk>

     <chunk>
       <node>/position/altitude-ft</node>
       <type>float</type>
       <format>\t\t<altitude-ft>%.8f</altitude-ft></format>
     </chunk>

     <chunk>
       <node>/velocities/airspeed-kt</node>
       <type>float</type>
       <format>\t\t<airspeed-kt>%.8f</airspeed-kt></format>
     </chunk>

     <chunk>
       <format>\t</set></format>
     </chunk>
   </output>
</generic>
</PropertyList>

Analyzing the resulting binary packet format

A utility called generic-protocol-analyse can be found under FlightGear/utils/xmlgrep which can be used to analyze the resulting data packet for the binary protocol. The output would be something like:

bintest.xml
Generic binary output protocol packet description:

 pos | size |  type  | factor     | description
-----|------|--------|------------|------------------------
   0 |    4 |    int |            | indicated speed (kt)
   4 |    4 |  float |            | pitch att (deg)
   8 |    4 |  float |            | magnetic heading (deg)
  12 |    4 |    int |            | outside air temperarure (degF)
  16 |    1 |   bool |            | autocoord

total package size: 17 bytes

Links

• Sourced from source/docs-mini/README.protocol