GPS connection to D100 / D120

Robski

Hang on! I've got an idea!
Joined
Nov 29, 2015
Messages
97
Here's a quick question that has just occurred to me.

If my Garmin GPSMap 296 is connected to my D120 / D100 (DSAB'ed together) then do they all have to be configured to the same units (knots or mph) for all the wind aloft etc calculations to work properly, or are the NMEA 0183 messages always in standard units and therefore take care of it?
 

Robski

Hang on! I've got an idea!
Joined
Nov 29, 2015
Messages
97
Ignore this, I think.
Just found the answer...

VTG - Velocity made good. The gps receiver may use the LC prefix instead of GP if it is emulating Loran output.

  $GPVTG,054.7,T,034.4,M,005.5,N,010.2,K*48

where:
        VTG          Track made good and ground speed
        054.7,T      True track made good (degrees)
        034.4,M      Magnetic track made good
        005.5,N      Ground speed, knots
        010.2,K      Ground speed, Kilometers per hour
        *48          Checksum
Note that, as of the 2.3 release of NMEA, there is a new field in the VTG sentence at the end just prior to the checksum.
 

Dynon

Dynon Staff
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Jan 14, 2013
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Woodinville, WA
Yeah, we convert so that it doesn't matter what your displayed units are on either products.
 

Robski

Hang on! I've got an idea!
Joined
Nov 29, 2015
Messages
97
I have a funny with my D100 + D120 setup above.
It is fed NMEA 0183 GPS data at 4800 baud from the 296.

I used to feed it NMEA 0183 GPS data from my Garmin GPS III Pilot at 9600 baud.

I used to get the winds aloft appearing on the D100 display. I do have the necessary OAT (connected to the D120) and I believe that is working OK as I get the OAT display on the EMS and the TAS display on the EFIS.
With the 296 at 4800 I don't get the wind displayed: it is still set to Y in CLUTTR.

Another funny I have just discovered is that under the GPS III Pilot I got date and time displayed in the logs.
Now with the 296 I seem only to get the time, all date fields are zero!
EDIT: Just found that I now don't get GPS ground speed logged either! :(

I am guessing these issues are connected.

I seem to not be able to set COM1 on the 296 to 9600 baud for some inexplicable Garmin-esque reason.
COM2 on the 296, however, does seem to be able to run at 9600! I am using this to feed GPS data to my new ADS-B Transponder!

Reading the manual carefully it appears the 296 has optional extra 'NMEA' messages that are enabled by default.
Could it just that the Garmin is chucking so much data out and at such a slow rate that it isn't finished within the 2 second limit?
The Dynon manuals (far superior to Garmin ones) allude to this as being a possible issue.

If this is the case then I would guess that there are two possible fixes:
1) swap the COM ports over and see if the transponder is happy with 4800
2) turn off the Garmin proprietary / extended 'NMEA' sentences that the 296 is sending down the lines in the hope the bare minimum will cover the Dynons' requirements and still finish in the necessary 2 seconds.
3) Both of the above, with 2 applied to both COM 1 and 2!

Apologies for posting before I have exhausted my own suggestions, but I would like to get to the bottom of it as quickly as possible without having to go through umpteen iterations.
Testing the various options is rather slow as it requires actually flying the plane (because there's no winds aloft displayed when on the ground) and then downloading and examining the logs each time (which takes time and thought).
 

Robski

Hang on! I've got an idea!
Joined
Nov 29, 2015
Messages
97
That seems to have sorted it.

Cut down on the number of NMEA sentences the 296 squirts down the serial lines.
 
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