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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.
== 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 (e.g. --generic=file,out,1,/tmp/data.xml,myproto would
only use the <output> definitions block).
<?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 <output> 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> are put between every two output properties, while
<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> <!-- default -->
== 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 (not tranferred)
<node> the property tree node which provides the data
<type> the value type (needed for formatting)
one of string, float, bool, int (default: int)
<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).
For input chunks there exist some more options:
<rel> optional boolean parameter to enable handling of incoming values
as relative changes (default: false)
(Can be eg. used to realise up/down buttons by just sending 1 or
-1 respectively)
<min> an optional minimum limit for the value to be clamped to. This
limit is always specified as absolute value, also with relative
changes enabled. (default: 0)
<max> an optional upper limit for the input value to be clamped to. If
<min> equals <max> no limit is applied. (default: 0)
<wrap> instead of clamping to minimum and maximum limits, wrap values
around. Values will be in [min, max[ (default: false)
(Usefull for eg. heading selector to start again with 1 for
values higher than 360)
<rel>, <min>, <max> and <wrap> are only used for numeric data types. <rel> can
additionally be used with type 'bool', where it toggles the value, if the received
value evaluates to 'true', otherwise the value is left unchanged.
Chunks can also consist of a single constant <format>, like in:
<format>Data Section</format>
== examples ===================================================================
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> <!-- degrees to radians -->
</chunk>
<chunk>
<name>pitch angle (deg)</name>
<format>P=%03.2f</format>
<node>/orientation/pitch-deg</node>
</chunk>
</output>
</generic>
</PropertyList>
Control the heading bug by sending relative changes separated by newlines:
<?xml version="1.0"?>
<PropertyList>
<generic>
<input>
<line_separator>newline</line_separator>
<chunk>
<name>heading bug</name>
<type>int</type>
<node>/autopilot/settings/heading-bug-deg</node>
<relative>true</relative>
<min>1</min>
<max>360</max>
<wrap>true</wrap>
</chunk>
</input>
</generic>
</PropertyList>
-- writing data in XML syntax -------------------------------------------------
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>&lt;?xml version="1.0"?&gt;\n\n&lt;data&gt;\n</preamble>
<postamble>&lt;/data&gt;\n</postamble>
<chunk>
<format>\t&lt;set&gt;</format>
</chunk>
<chunk>
<node>/position/altitude-ft</node>
<type>float</type>
<format>\t\t&lt;altitude-ft&gt;%.8f&lt;/altitude-ft&gt;</format>
</chunk>
<chunk>
<node>/velocities/airspeed-kt</node>
<type>float</type>
<format>\t\t&lt;airspeed-kt&gt;%.8f&lt;/airspeed-kt&gt;</format>
</chunk>
<chunk>
<format>\t&lt;/set&gt;</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