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Garmin 496 with Trutrak Autopilot, Dynon EFIS D100 and Gar

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jul 21, 2013 5:01 pm Post subject: Garmin 496 with Trutrak Autopilot, Dynon EFIS D100 and Gar

At 03:23 PM 7/21/2013, you wrote:

  	  Quote: 	
		  Folks,

 Someone must have done this, but I canâ€™t find any references.  The Garmin 496 can provide directional reference to the Pictorial Pilot Directional AP, position and direction to the EFIS HSI display, and VOR and COM frequencies to the SL30 NAV/COMM.    It seems all devices can work with 9600 baud.  
The SL30 and D100 are OK with the 496 â€œData outâ€ setting â€œNMEA & VHF out.â€ The AP documentation doesnâ€™t mention the VHF angle.  
The EFIS and AP can live with the 496 â€œAdvancedâ€ setting â€œNormal.â€  I donâ€™t know about the SL30 but doubt it matters.  

 Do I simply split the 496 â€œdata outâ€ wire three ways and run to each device and hope the AP isnâ€™t confused by the extraneous commands from the GPS intended for the SL30?

 Any recommendations on running grounds between the devices, using screened cable etc.? 	


   NEMA data streams are fixed format, labeled sentences

   of ordered data words. There are few devices that read

   all the data from any given device. Listeners watch all

   data strings starting with the right label, and then they

   disassemble the sentence for the data of interest.



   For example, if your listener wants to know lat,lon it looks

   for a sentence formatted to carry that data. Like "GGA"



 

 GGA - essential fix data which provide 3D location and accuracy data. 



  	  Quote: 	
		  

 $GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47



Where:

    

GGA          Global

Positioning System Fix Data

     123519       Fix

taken at 12:35:19 UTC

     4807.038,N   Latitude 48 deg 07.038'

N

     01131.000,E  Longitude 11 deg 31.000' E

    

1            Fix

quality: 0 = invalid

                             

1 = GPS fix (SPS)

                             

2 = DGPS fix

                             

3 = PPS fix

        

        

        

       4 = Real Time Kinematic

        

        

        

       5 = Float RTK

                             

6 = estimated (dead reckoning) (2.3 feature)

        

        

        

       7 = Manual input mode

        

        

        

       8 = Simulation mode

    

08           Number of

satellites being tracked

    

0.9          Horizontal

dilution of position

     545.4,M      Altitude,

Meters, above mean sea level

     46.9,M      

Height of geoid (mean sea level) above WGS84

                     

ellipsoid

     (empty field) time in seconds since last DGPS

update

     (empty field) DGPS station ID number

    

*47          the checksum

data, always begins with *

	
  In this case, the listener watches for $GPGGA to come along

   then it parses the sentence for the second and third words

   after the header.



   There are proprietary sentences too. For example, Garmin

   may find it useful to output a $PGRME sentence where word

   pairs offer position error distances and units in which

   those data are given.



 [b]GarminThe following are Garmin proprietary sentences. "P" denotes proprietary, "GRM" is Garmin's manufacturer code, and "M" or "Z" indicates the specific sentence type. Note that the PGRME sentence is not set if the output is set to NMEA 1.5 mode. 



  	  Quote: 	
		    $PGRME,15.0,M,45.0,M,25.0,M*1C



where:

     15.0,M      

Estimated horizontal position error in meters (HPE)

     45.0,M      

Estimated vertical error (VPE) in meters

     25.0,M      

Overall spherical equivalent position error



	
   So it's conceivable that your constellation of listeners

    are interested in only a small port of all data offered by

    the talker.



    So as long as your 'talker' offers all the data needed

    by all of your 'listeners', then each listener will sift

    the good stuff out of the noise. Hook them all together.



    Twisted pairs are good data lines. 9600 Baud is not

    a bit EMC risk but shielding the wires is not a bad

    idea. Ground the shield at listener end only.



 



 

   Bob . . .    [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jul 21, 2013 6:32 pm Post subject: Garmin 496 with Trutrak Autopilot, Dynon EFIS D100 and Gar

 	  Quote: 	
		  

Bob and I differ on where to terminate the screen, but that drills 

into some serious minutiae on this project. It's probably just not 

necessary at all. (Bob -- I wouldn't mind hearing your thoughts on 

the termination location rationale though)


	


   Sure . . . easy . . . flip a coin.



   Shielding is a prophylactic against the weakest of all

   EMI propagation modes - electrostatic.



   Modern serial data signals will deliberately soften

   the leading and trailing edges of the data to minimize

   the strength of the propagation mode in bundles.



   Twisting pairs of a balanced data stream (Like RS422)

   drops the coupling mode to a small fraction of and

   already weak coupling mode.



   Twisting an unbalanced data stream can't exploit

   the common mode rejection capability of a balanced

   pair receiver . . . but it still reduces electrostatic

   coupling by a substantial proportion.



   Shielding breaks the electrostatic propagation mode.

   The currents flowing in the shield are so tiny as to be

   difficult to measure . . . one can terminate the shield

   at ANY low impedance node tied to signal or power ground and it's

   got all the conductivity needed to do its nearly

   insignificant job.



   Grounding shields at the listener-end is a convention

   we used most places I worked . . . a convention not

   driven by significant physics.



   Bob . . .

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