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294 lines
8.7 KiB
294 lines
8.7 KiB
The generic communication protocol for FlightGear provides a powerful way
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of adding a simple ASCII based or binary input/output protocol, just by
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defining an XML encoded configuration file and placing it in the
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$FG_ROOT/Protocol/ directory.
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== file layout ================================================================
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A protocol file can contain either or both of <input> and <output>
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definition blocks. Which one is used depends on how the protocol
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is called (e.g. --generic=file,out,1,/tmp/data.xml,myproto would
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only use the <output> definitions block).
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<?xml version="1.0"?>
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<PropertyList>
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<generic>
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<output>
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<binary_mode>false</binary_mode>
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<line_separator></line_separator>
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<var_separator></var_separator>
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<preamble></preamble>
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<postamble></postamble>
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<chunk>
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... first chunk spec ...
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</chunk>
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<chunk>
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... another chunk etc. ...
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</chunk>
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</output>
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<input>
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<line_separator></line_separator>
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<var_separator></var_separator>
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<chunk>
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... chunk spec ...
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</chunk>
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</input>
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</generic>
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</PropertyList>
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== input/output parameters ====================================================
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Both <output> and <input> blocks can contain information about
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the data mode (ascii/binary) and about separators between fields
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and data sets, as well as a list of <chunk>s. Each <chunk> defines
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a property that should be written (and how), or a variable and which
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property it should be written to.
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--- ASCII protocol parameters ---
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output only:
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<preamble> STRING default: "" file header put on top of the file
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<postamble> STRING default: "" file footer put at the end of the file
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input & output:
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<binary_mode> BOOL default: false (= ASCII mode)
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<var_separator> STRING default: "" field separator
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<line_separator> STRING default: "" separator between data sets
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<var_separator> are put between every two output properties, while
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<line_separator> is put at the end of each data set. Both can contain
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arbitrary strings or one of the following keywords:
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Name Character
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newline '\n'
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tab '\t'
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formfeed '\f'
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carriagereturn '\r'
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verticaltab '\v'
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Typical use could be:
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<var_separator>tab</var_separator>
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<line_separator>newline</var_separator>
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or
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<var_separator>\t</var_separator>
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<line_separator>\r\n</line_separator>
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--- Binary protocol parameters ---
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To enable binary mode, simply include a <binary_mode>true</binary_mode> tag in
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your XML file. The format of the binary output is tightly packed, with 1 byte
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for bool, 4 bytes for int, and 8 bytes for double. At this time, strings are not
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supported. A configurable footer at the end of each "line" or packet of binary
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output can be added using the <binary_footer> tag. Options include the length
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of the packet, a magic number to simplify decoding. Examples:
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<binary_footer>magic,0x12345678</binary_footer>
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<binary_footer>length</binary_footer>
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<binary_footer>none</binary_footer> <!-- default -->
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== variable parameters (chunk spec) ===========================================
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Both <input> and <output> block can contain a list of <chunk> specs,
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each of which describes the properties of on variable to write/read.
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<name> for ease of use (not tranferred)
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<node> the property tree node which provides the data
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<type> the value type (needed for formatting)
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one of string, float, bool, int (default: int)
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<format> (ASCII protocol only, not used or needed in binary mode)
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defines the actual piece of text which should be sent.
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it can include "printf" style formatting options like:
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<type>
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%s string
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%d integer (default)
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%f float
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<factor> an optional multiplication factor which can be used for
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unit conversion. (for example, radians to degrees).
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<offset> an optional offset which can be used for unit conversion.
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(for example, degrees Celcius to degrees Fahrenheit).
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For input chunks there exist some more options:
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<rel> optional boolean parameter to enable handling of incoming values
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as relative changes (default: false)
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(Can be eg. used to realise up/down buttons by just sending 1 or
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-1 respectively)
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<min> an optional minimum limit for the value to be clamped to. This
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limit is always specified as absolute value, also with relative
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changes enabled. (default: 0)
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<max> an optional upper limit for the input value to be clamped to. If
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<min> equals <max> no limit is applied. (default: 0)
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<wrap> instead of clamping to minimum and maximum limits, wrap values
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around. Values will be in [min, max[ (default: false)
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(Usefull for eg. heading selector to start again with 1 for
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values higher than 360)
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<rel>, <min>, <max> and <wrap> are only used for numeric data types. <rel> can
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additionally be used with type 'bool', where it toggles the value, if the received
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value evaluates to 'true', otherwise the value is left unchanged.
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Chunks can also consist of a single constant <format>, like in:
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<format>Data Section</format>
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== examples ===================================================================
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Writes log of this form:
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V=16
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H=3.590505
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P=3.59
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V=12
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H=3.589020
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P=3.59
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<?xml version="1.0"?>
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<PropertyList>
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<generic>
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<output>
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<line_separator>newline</line_separator>
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<var_separator>newline</var_separator>
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<binary_mode>false</binary_mode>
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<chunk>
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<name>speed</name>
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<format>V=%d</format>
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<node>/velocities/airspeed-kt</node>
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</chunk>
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<chunk>
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<name>heading (rad)</name>
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<format>H=%.6f</format>
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<type>float</type>
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<node>/orientation/heading-deg</node>
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<factor>0.0174532925199433</factor> <!-- degrees to radians -->
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</chunk>
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<chunk>
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<name>pitch angle (deg)</name>
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<format>P=%03.2f</format>
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<node>/orientation/pitch-deg</node>
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</chunk>
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</output>
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</generic>
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</PropertyList>
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Control the heading bug by sending relative changes separated by newlines:
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<?xml version="1.0"?>
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<PropertyList>
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<generic>
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<input>
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<line_separator>newline</line_separator>
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<chunk>
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<name>heading bug</name>
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<type>int</type>
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<node>/autopilot/settings/heading-bug-deg</node>
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<relative>true</relative>
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<min>1</min>
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<max>360</max>
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<wrap>true</wrap>
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</chunk>
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</input>
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</generic>
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</PropertyList>
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-- writing data in XML syntax -------------------------------------------------
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Assuming the file is called $FG_ROOT/Protocol/xmltest.xml, then it could be
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used as $ fgfs --generic=file,out,1,/tmp/data.xml,xmltest
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<?xml version="1.0"?>
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<PropertyList>
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<generic>
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<output>
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<binary_mode>false</binary_mode>
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<var_separator>\n</var_separator>
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<line_separator>\n</line_separator>
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<preamble><?xml version="1.0"?>\n\n<data>\n</preamble>
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<postamble></data>\n</postamble>
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<chunk>
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<format>\t<set></format>
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</chunk>
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<chunk>
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<node>/position/altitude-ft</node>
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<type>float</type>
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<format>\t\t<altitude-ft>%.8f</altitude-ft></format>
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</chunk>
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<chunk>
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<node>/velocities/airspeed-kt</node>
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<type>float</type>
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<format>\t\t<airspeed-kt>%.8f</airspeed-kt></format>
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</chunk>
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<chunk>
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<format>\t</set></format>
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</chunk>
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</output>
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</generic>
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</PropertyList>
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-- Analyzing the resulting binary packet format -------------------------------
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A utility called generic-protocol-analyse can be found under
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FlightGear/utils/xmlgrep which can be used to analyze the resulting
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data packet for the binary protocol.
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The output would be something like:
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bintest.xml
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Generic binary output protocol packet description:
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pos | size | type | factor | description
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-----|------|--------|------------|------------------------
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0 | 4 | int | | indicated speed (kt)
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4 | 4 | float | | pitch att (deg)
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8 | 4 | float | | magnetic heading (deg)
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12 | 4 | int | | outside air temperarure (degF)
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16 | 1 | bool | | autocoord
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total package size: 17 bytes
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