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G3X Default HSI Source

ShortSnorter

Well Known Member
Does anyone know of a way to set the default CDI source as GPS vs NAV when booting up?

Its a minor inconvenience that I have to swap the CDI source from NAV to GPS.

Setup:

G3X
GNX 375
GNC 255
 
From page 2-7 of the G3X Installation Manual, revision AN:

When two external navigation sources are connected to the G3X system, the determination of which
source is #1 and which is #2 is determined by the ordering of the RS-232 connections to the GDU displays.
The #1 navigation source should be connected to a lower-numbered RS-232 port on the PFD1 GDU, and
the #2 navigation source should be connected either to a higher-numbered RS-232 port on the PFD1 GDU,
or to any RS-232 port on the MFD1 GDU. For additional information, refer to Section 2.5.1.


This sounds like the G3X is listening to the GNC 255 at startup. Check configuration on G3X, put GNX 375 on RS-232 #1 and GNC 255 on RS-232 #2, etc.
 
Last edited:
From page 2-7 of the G3X Installation Manual, revision AN:

When two external navigation sources are connected to the G3X system, the determination of which
source is #1 and which is #2 is determined by the ordering of the RS-232 connections to the GDU displays.
The #1 navigation source should be connected to a lower-numbered RS-232 port on the PFD1 GDU, and
the #2 navigation source should be connected either to a higher-numbered RS-232 port on the PFD1 GDU,
or to any RS-232 port on the MFD1 GDU. For additional information, refer to Section 2.5.1.


This sounds like the G3X is listening to the GNC 255 at startup. Check configuration on G3X, put GNX 375 on RS-232 #1 and GNC 255 on RS-232 #2, etc.

Seems logical - I'll have to look at that.

It seems the pinout would also have to agree as well though?

FWIW - I did spend ~.5 hr on the phone with the G3X team and it seemed they had never had encountered this question before and since they were working remotely couldn't answer the question.
 
Seems logical - I'll have to look at that.

It seems the pinout would also have to agree as well though?

FWIW - I did spend ~.5 hr on the phone with the G3X team and it seemed they had never had encountered this question before and since they were working remotely couldn't answer the question.

Correct - you would need to move the pins on the G3X to match the serial configuration.
 
Similar issue

Just had a similar type of issue integrating a GNX 375 with the Aera 760, seems that if you have the the 375/MapMx on port 1 you cannot upload frequencies to the SL30 on port 2, MapMx traps the frequencies for some reason. Swapped the order of the ports and it all works. Took a while to troubleshoot :(
These idiosyncrasies I am sure are for a good reason but it would be nice to have a note or two in the installation manual to stop a lot of head scratching, rewiring, and frustration.
Figs
 
I cross checked my interconnect diagrams with the plane, and what was configured:

GNX RS-232 OUT 1 --> GDU RS-232 IN 1
GNC RS-232 OUT --> GDU RS-232 IN 2


Everything functions fine from a navigation, crossfill, etc. standpoint. It would be interesting to hear from others with a similar setup on what their default CDI source is upon startup.
 
Wondering out loud...

What happens when you power everything up EXCEPT for the GNC255? Does the G3X immediately show a magenta CDI or does it wait for the GNX375 to get a 3D position solution? What happens when you turn the GNC255 after? Does the CDI flip to green or does it stay Magenta?
 
Wondering out loud...

What happens when you power everything up EXCEPT for the GNC255? Does the G3X immediately show a magenta CDI or does it wait for the GNX375 to get a 3D position solution? What happens when you turn the GNC255 after? Does the CDI flip to green or does it stay Magenta?

I tried powering up with GNC255 off and instead of defaulting to magenta which is online, the CDI stays green & displays the VLOC unavailable (or whatever error it gives you) followed by the RS232 unavailable errors.

I feel like either I'm missing something completely simple or this is just the way it was designed. Maybe one of the other G3X folks on this forum will see this be able to chime in.
 
The G3X automatically assigns the hierarchy of any connected external navigation devices. The first device found (by software) with GPS capability becomes the #1 external navigator. If two equal devices are found (as in dual GTN/GNS installations), the RS-232 serial port number of the GDU 4XX/37X
determines the hierarchy. GTN #1 should always be connected to a lower numbered port on the PFD. GTN #2 can be connected to a higher numbered port on the PFD or to any port in the MFD. In the case of an SL 30 or GNC 255 being added to a system with a GTN, the SL 30 or GNC 255 gets assigned as the #2 external navigator. The hierarchy applies to the entire navigation radio even if only the GPS source is in use. So, in a system with a GTN 625 and a GNC 255, GPS1 and NAV2 (respectively) would be available
as external navigation sources.
G3X rev AN. Page 2-32.
 
Does anyone know of a way to set the default CDI source as GPS vs NAV when booting up?

Its a minor inconvenience that I have to swap the CDI source from NAV to GPS.

Setup:

G3X
GNX 375
GNC 255

What CDI source was selected when you powered down?

I only have a GNX 375 external navigator, but I added a VHF NAV/COM (on a higher number RS-232) as NAV2 just for testing this.

No matter how many times I restart the system, it always powers up with the last selected CDI source (either GPS1 or VOR2). In other words, it retains the selected CDI source through the power cycle.

Steve
 
What CDI source was selected when you powered down?

I only have a GNX 375 external navigator, but I added a VHF NAV/COM (on a higher number RS-232) as NAV2 just for testing this.

No matter how many times I restart the system, it always powers up with the last selected CDI source (either GPS1 or VOR2). In other words, it retains the selected CDI source through the power cycle.

Steve

Hi Steve - Thanks for the reply. My CDI is always selected to GPS1. I'll have to double check the actual RS-232 numbers but VOR2 is what displays when I switch to the NAV source.
 
In the case of an SL 30 or GNC 255 being added to a system with a GTN, the SL 30 or GNC 255 gets assigned as the #2 external navigator. The hierarchy applies to the entire navigation radio even if only the GPS source is in use. So, in a system with a GTN 625 and a GNC 255, GPS1 and NAV2 (respectively) would be available
as external navigation sources.
G3X rev AN. Page 2-32.

This is an interesting point - I wonder if this is the issue?

My GNC is NAV radio displays as VOR2 but the radio portion is mapped as COM1.

COM2 is mapped as my COM2.
 
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