GNC255 interface

[paul330]

I assume that all the necessary nav data from the GNC255 will interface via the RS232 port. The GNC255 has a COM database that can use GPS position to give frequencies for nearby airfields etc. will the GPS position from Skyview feed back to the radio via RS232?

 

[dynonsupport]

The GNC and GTR radios are brand new and thus we don't have a lot of experience with them, so no promises with this answer.

It does appear they take standard NMEA for position, and we can output that on a serial port, so this should work.

However, they have only one serial port, so if you hook NMEA up to this port, you can't hook the radio to SkyView using the SL-30 protocol in order for us to tune the radio and for it to display on SkyView. So you'll need to choose one feature or the other.

 

[paul330]

I am away from home so don't have access to my units.  If I remember correctly, there is a default setting on the set-up menu to configure a serial port to SL30 which will swap all the required COM/NAV data.  The GNC manual states that it has a legacy SL30 setting which should work (but only at 25khz spacing).

So, is it possible to modify this standard SL30 set up to get 8.33khz spacing and also to include a GPS position in the stream?

Of course, I have absolutely no idea how one would go about that.......... :-/

 

[Dynon]

Probably not. Our guess (but we're not Garmin) is that the GNC can only accept one of NMEA or legacy SL30 commands at once.

 

[paul330]

OK. In that case, as the GNC255 is going to be the de facto replacement for the SL30, how about a software update that will give the option on the Serial setup so that we can use the full functionality of the GNC2555.

I can't imagine that the commands would need much modification to get all the correct data in the stream and so we'd get a worthwhile update for little effort......

Alternatively, is it possible to write custom routines for the serial port? If so, where is the information on how to do that? A copy of the SL30 legacy script would be a good start. Or can it only be done as a firmware update?

 

[dynonsupport]

We'll look into what it takes to support some of the newer features in the GTR or GNC. We won't be able to introduce anything in our next software release though, so it will be a few months minimum before we will have anything, assuming we can even support it since many of Garmin's protocols are proprietary and not documented.

There is no way for users to write code that will change serial port outputs. It would be possible for someone to design an external microcontroller that combined a GPS and an old SL-30 stream in a way that the radio would get a benefit from both.

 

[paul330]

Thanks for all the info and your patience - I'm floundering a bit with this!

Some research in the GNC255 and Skyview installation manuals seem to indicate that NMEA could be used as the GNC outputs all the NAV info needed to drive a CDI and Skyview can output GPS position (I think - I read the parameters that can be output somewhere but now can't find it!).  I need to be at home rather than in a hotel 6000 miles away to see if the SERIAL set-up allows all this.  Since I don't want any of the remote COM functions, this would seem to work - or does Skyview need specific code to decode the standard NMEA serial data?  As I said, I'm floundering a bit.

As for the data formats, the GNC install manual gives full details in its Appendix B for those who know what it is talking about........

One other thought is that the GNC has a Arinc429 output.  It would mean another converter, but could that drive the CDI (as per GNS430) and the serial port used to give GPS position from Skyview?  Another option I will look at is whether I can take the GPS position from the GNS430 but I think that all the serial and Arinc ports are tied up by the Skyview.   :-/

Edit: Nah, doesn't work. Available serial outputs from the GNS430 but the problem is only one serial port on the GNC255 - crazy design - thanks Garmin!

 

[klindell]

You might want to use an external "combiner"/multiplexer, like http://actisense.com/products/nmea-0183/ndc4.html where you could even filter data if needed.
No guarantee given...

 

[paul330]

I have fitted my GNC255 and can report that it appears to work using the SL30 setting in the serial setup. At least, I can see the NAV frequency and it shows NAV 2.

Of course, no GPS input so the advanced database functions are not available.

 

[Janekom]

I have a GNC255 and Arinc429. What connections should I use between the two please?

 

[paul330]

If you connect via the serial and use the SL30 protocols, you will get all the info you need to drive the NAV needles etc.  If you use 25kHz spacing, I assume you will get all the radio tuning stuff as well (SA is still 25kHz, right?)

If you have a GNC430 or similar, then you need the Arinc429 for that.  If not, I don't see why you can't connect it to the GNC255 and configure it to pass GPS position.  That way, you should be able to get the advanced features up and running.

Personally, I think Dynon will probably come up with new protocols via serial for the GNC255 before too long - please...... ;D

 

[Janekom]

Thanks Paul330. I have connected the VOR/ILS out lines from the CNG to the Arinc 429 and can now see the NAV needle on SV as well as a little box with Nav1 and the tuned frequency. Does that sound right?

 

[paul330]

That's what I have via the serial port. If you want to do radio tuning etc via SV then I don't think you can do that via 429 but you should be able to configure it so SV gives the 255 a GPS position for the advanced features.

Otherwise, I don't see any reason to use the expensive 429 module instead of the free serial port