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
If you're using a serial port under Windows, you must use a special escape sequence for COM port numbers higher than COM9.
Ex: --generic=\\.\COM10,out,1,/tmp/data.xml,myproto
More detail can be found in this Microsoft KB article: http://msdn.microsoft.com/en-us/library/aa363858%28v=vs.85%29.aspx
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