Re: [Flightgear-devel] Feeding FlightGear data through the Protocol Pipe

[Curtis O. <cur...@fl...>]

Hi Jon,

For what it's worth, the "ExternalPipe" fdm was setup to cover much of the
areas you have touched on.  It's probably not exactly what you want but
here was some of my logic in how/why I setup it up the way I did.

   - Using a network interface adds some indeterminism ... sometimes
   network packets are delayed or stack up depending on what is going on with
   the machine so there isn't a guaranteed lockstep relationship between the
   simulator the the flight dynamics.
   - Usually the network interface is good, but I observed times when it
   had extra delays in packets getting where they needed to go.
   - As an alternative, I setup a pair of "named pipes".  Pipes are a unix
   construct, they look like a file to the program, but what you write into
   them is collected and held for some other application to read out.
    Unfortunately these are not supported in windows. Pipes are single
   direction, thus the need for two of them for bi-directional communication.
    A pipe is really simple: you  open it up like any other file, and one
   process writes into it; the other process reads from it.  In between the OS
   can buffer some amount of information until the reader grabs it.  You can
   use blocking reads (carefully) to ensure FlightGear and your external FDM
   run in exact lockstep.
   - Pipes imply both processes will run on the same machine, a network
   interface would allow the processes to live on separate machines.
   - The ExternalPipe interface supports a flexible property interface, so
   the external FDM process can send any name/value pairs it wishes to send
   and the interface will dutifully copy them into the FlightGear property
   tree.  The FDM side can also send a list of property names it would like to
   receive in reply and the interface will dutifully send them over the
   outgoing pipe every frame.  This was a nice addition, and there is plenty
   of bandwidth through a named pipe to do this in clear text, even with a lot
   of extra property values.

History: I used this as part of a project I did with ATC flight sim.  They
had their own proprietary flight dynamics applications that modeled
specific aircraft in the code itself.  These models had all the necessary
source documentation and flight test data to satisfy FAA certification
testing requirements of the final simulator.  In addition, this external
code modelled many of the aircraft systems for one of their high end sims.
 This required the ability to be flexible in what values were sent back and
forth and enabled me to drive some instrument gauges directly from the
external code.

Regards,

Curt.


On Wed, Jan 29, 2014 at 10:02 AM, Anders Gidenstam <and...@gi...
> wrote:

> On Wed, 29 Jan 2014, Jon S. Berndt wrote:
>
> > Would it be possible to use net_fdm to send named properties over to
> > FlightGear?
>
> IIRC net_fdm is just a fixed format packet. Changing that would be a major
> change and would need measures to preserve backwards compatibility.
>
> The quickest way to your goal with least (if any) source changes would be
> to switch to a generic protocol - with a JSBSim output specification you
> should be able to stream a comma separated list of states (rows) to FG.
> You then set up the generic protocol for FG to take each element in the
> rows and put it in the right FG property.
>
> This way the set of properties to transfer would be defined by XML files
> both on the JSBSim and on the FG side.
>
>
> Cheers,
>
> Anders
> --
> ---------------------------------------------------------------------------
> Anders Gidenstam
> WWW: http://gitorious.org/anders-hangar
>       http://www.gidenstam.org/FlightGear/
>
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-- 
Curtis Olson:
http://www.atiak.com - http://aem.umn.edu/~uav/
http://www.flightgear.org - http://gallinazo.flightgear.org