GNC 255A only has one RS-232

[PT20J]

I spent hours reading manuals and planning my G3X / GFC 500 / GTN 650Xi panel. This included scamming copies of Garmin’s top secret installation manuals and emailing Garmin tech support to ask about unclear passages or mistakes I uncovered in the manuals. I knew I’d miss something. Turns out the GNC 255A Nav/Com only has one RS-232 connection. So, if I want it to show up on the G3X (I do) then I can’t connect it to the GTN. This kills the navigation database frequency lookup feature since the GNC has no position information. It’s not a big deal, but it’s another indication that trying to build a coherent system from Garmin bits, designed by different teams at different times for different markets, can be a challenge. So far, this is the only surprise.

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[Mooney Dog]

So wait if I'm understanding this right, the 255A and 375 in my airplane should be able to talk to each other for frequencies? 

[Warren]

The 255 has a frequency database.  It needs a GPS location for reference to look up nearest frequencies.

Connecting the 255 to a G3X allows the radio frequencies to be programmed directly from the touch screen.

[ArtVandelay]

I think 255/G3X control connection is an ARINC 429 , RS232 IN is GPS data, navigation data is separate VOR connections.
So what are you missing?
There is a RS232 OUT as well but that might be backwards compatible to SL30 which is what the 255 used to be, the 255 manual shows it in the wiring but does elaborate on its use.

Why do people feel the need to try to slam Garmin all the time?

[PT20J]

2 hours ago, ArtVandelay said:

I think 255/G3X control connection is an ARINC 429 , RS232 IN is GPS data, navigation data is separate VOR connections.
So what are you missing?
There is a RS232 OUT as well but that might be backwards compatible to SL30 which is what the 255 used to be, the 255 manual shows it in the wiring but does elaborate on its use.

What do people feel the need to try to slam Garmin all the time?

Well, I thought the same thing which is how I missed it. The GNC 255A connection to G500 is ARINC 429, but it’s RS-232 for a G3X.

Skip

[ArtVandelay]

Well, I thought the same thing which is how I missed it. The GNC 255A connection to G500 is ARINC 429, but it’s RS-232 for a G3X.
Skip

I believe the G3X uses the GAD 29B to interface with the 429 bus, at least it does with the GTN/GNS units.

[Warren]

6 hours ago, ArtVandelay said:

I believe the G3X uses the GAD 29B to interface with the 429 bus, at least it does with the GTN/GNS units

The G3X only has CAN bus communication and RS232.  The GAD29B acts as the interface to ARINC (429) communications.

[PT20J]

The GAD 29B is used for connecting GPS/COMs to the CAN bus and hence the GDU, but for some reason (probably software) the GNC 255A ARINC 429 isn’t similarly supported according to the STC documentation. For kicks though, I’ll ask Garmin if there is any way to make it work.

[Warren]

10 hours ago, PT20J said:

The GAD 29B is used for connecting GPS/COMs to the CAN bus and hence the GDU, but for some reason (probably software) the GNC 255A ARINC 429 isn’t similarly supported according to the STC documentation. For kicks though, I’ll ask Garmin if there is any way to make it work.

The GNC255 only has a single ARINC output for the VOR/ILS signal.  There are no ARINC inputs so it will not be possible.  If you note on the GTN connections there are also RS232 connections which provide the remote control for frequency tuning on these as well.  ARINC is used for NAV, GPS and Air Data.

[PT20J]

14 hours ago, Warren said:

The GNC255 only has a single ARINC output for the VOR/ILS signal.  There are no ARINC inputs so it will not be possible.  If you note on the GTN connections there are also RS232 connections which provide the remote control for frequency tuning on these as well.  ARINC is used for NAV, GPS and Air Data.

That seems to be the case. The real problem is that Garmin should have put two RS-232 interfaces on the GNC 255 to ensure availability of full functionality in all installations.

This goes to my point that if you just read the glossy brochures for each piece of equipment, you cannot assume that all features are available in every configuration. I'm not really faulting Garmin: it is difficult to design a bunch of bits that can be configured in a large number of configurations while supporting legacy products and create a coherent system. My biggest gripe is that the documentation available to owners (the people paying the bills) does not adequately explain this stuff and even Garmin dealers and Garmin aviation support people don't always know. 

One thing I always enjoyed about flying the G1000 is that it was designed as an integrated system.

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[alexz]

Just verified on the plane that has G3X, gtn 750 and gnc255 connected to G3X. All database frequency lookup works with G3X provided information. No functionality seems to be lost. Both G3X shows COM frequency with explanation and GNC radio shows frequency with alphabetic explanation. Cheers!

[PT20J]

3 hours ago, alexz said:

Just verified on the plane that has G3X, gtn 750 and gnc255 connected to G3X. All database frequency lookup works with G3X provided information. No functionality seems to be lost. Both G3X shows COM frequency with explanation and GNC radio shows frequency with alphabetic explanation. Cheers!

Interesting. It had not occurred to me that the GDU might supply GPS position over the RS-232 to the GNC, but it certainly could. Garmin uses at least four interface types: ARINC 429 (which is a published standard), HSDB (proprietary Ethernet protocol), RS-232 (proprietary except where NMEA protocol is used), and CAN bus (proprietary). As Garmin doesn't publish details of the proprietary protocols, we are left to guess. Even my Garmin dealer wasn't sure. 

Thanks,

Skip

[alexz]

8 hours ago, PT20J said:

Even my Garmin dealer wasn't sure. 

Thanks,

Skip

Yes, a lot of details are missing in the manuals, but one can expect system like g3x to forward location information back to the radio:) and it pays to work with experienced garmin installer, in our case we verified it at Islip Avionics as KISP. Cheers!

[PT20J]

I emailed aviation support at Garmin. They replied that the G3X Touch only outputs frequency tuning and OBS radial selection to the GNC 255.

Edited September 15, 2021 by PT20J
Clarified that GDU is for G3X Touch

[PT20J]

On 9/13/2021 at 5:01 PM, alexz said:

Just verified on the plane that has G3X, gtn 750 and gnc255 connected to G3X. All database frequency lookup works with G3X provided information. No functionality seems to be lost. Both G3X shows COM frequency with explanation and GNC radio shows frequency with alphabetic explanation. Cheers!

So, I'm curious about this since Garmin said (and pointed to documentation) that the G3X does not send GPS position to the GNC 255. However, this would not be the first time that Garmin support was not correct. 

The GNC provides forward and reverse lookup. The "forward" lookup is a one to one correspondence and does not require a position. You enter a location identifier and the database can find the frequencies associated with it. The "reverse" lookup is a one to many correspondence. You enter a frequency and the database supplies the nearest location identifier associated with this frequency. Since frequencies are reused, GPS position is required to determine which one is closest to you.

Did you verify that both forward and reverse lookup work with the GNC 255 connected to a G3X and not connected to a GPS navigator?

Thanks,

Skip

[alexz]

Yes it works both ways. G3X does forward gps info to the gnc radio over rs232. 
It is not connected to gtn750.  gtn750 does not even display secondary nav/com control buttons only it’s own built in radio. 

[thinwing]

On 9/11/2021 at 6:37 PM, PT20J said:

I spent hours reading manuals and planning my G3X / GFC 500 / GTN 650Xi panel. This included scamming copies of Garmin’s top secret installation manuals and emailing Garmin tech support to ask about unclear passages or mistakes I uncovered in the manuals. I knew I’d miss something. Turns out the GNC 255A Nav/Com only has one RS-232 connection. So, if I want it to show up on the G3X (I do) then I can’t connect it to the GTN. This kills the navigation database frequency lookup feature since the GNC has no position information. It’s not a big deal, but it’s another indication that trying to build a coherent system from Garmin bits, designed by different teams at different times for different markets, can be a challenge. So far, this is the only surprise.

Skip

only 1 232 output.....solution,....signal split

[PT20J]

9 minutes ago, thinwing said:

only 1 232 output.....solution,....signal split

Needs two INPUTS .

[thinwing]

i guess i misunderstood......you have 1 working 232 pair correct?Splitting either one or both wont degrade the signal enough to affect function.Thus getting youre second rs232

[ArtVandelay]

only 1 232 output.....solution,....signal split

It’s 1 232 INPUT to the 255, aviation format, so you can’t have the g3x and gtn both connected, I assume the g3x will supply gps data along with com commands.

[PT20J]

5 hours ago, ArtVandelay said:

It’s 1 232 INPUT to the 255, aviation format, so you can’t have the g3x and gtn both connected, I assume the g3x will supply gps data along with com commands.

Well, I'll find out when it gets out of the shop. Garmin support says no GPS position from the G3X, the documentation doesn't say one way of the other, @alexzsays it's there and I have no reason to doubt him. 

[thinwing]

i guess i need to see a diagram of inputs.The formats need to all be the same......

[201Mooniac]

According to the installation manual, it says:

2. GDU 4XX/37X TO SL30 OR GNC 255
A. ON THE GDU 4XX/37X COMM CONFIG MODE PAGE
SET CONNECTED GDU 4XX/37X RS-232 CHANNEL FORMAT TO "GARMIN VHF NAV/COMM"
B. ON THE SL30 NAV SETUP PAGES
SET INDICATOR HEAD TYPE TO "SERIAL"
C. ON THE GNC 255 CONFIGURATION PAGES
SET CDI INDICATOR TYPE TO "SERIAL" ON NAV CONFIGURATION PAGE
SET SERIAL PORT IO MODE TO "NMEA" ON SYS CONFIGURATION PAGE

Since the GNC serial port is configuraed to NMEA I would assume that GPS position will be sent.

[Zach]

Hi I know this is an old thread but I am installing a similar panel and trying to figure out functionality of it. Will your G3X change and control just your comm frequencies or can you get it to control both your comm and nav frequencies on the 255? Due to the layout of our panel the 255 will likely need to go to the far right side of the panel and that makes it kind of inconvenient to use the radio itself to change these. 

[PT20J]

I should have updated this thread. The GNC 255 does in fact get a GPS position from the G3X via NMEA protocol on the RS-232.

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