Garmin G3X Wiring of RS232

[VictorV]

I need some help with grounding of the RS-232 signals in a G3X system. I have mostly Garmin components except for
a VPX, Guardian CO Det and Artex ELT. On the G3X there are various serial ports and each one has a TX and RX.
There are also a bunch of "signal grounds" that are not assigned to specific ports.

I'm a bit confused about what to do when, say, connecting the G3X to the GEA-24. There are no "signal" grounds on
the GEA-24. For an individual serial port do I need to connect to a signal ground on the G3X? What do I do with that
ground on the other end of the wire (GEA-24 in this case)? I am using shielded wire for all my RS-232 cables. Do I need
to do anything with the wire shield?

Not sure it's relevant but I have 2 ground busses. One behind the panel and the other behind the seat. They are connected
together. I don't use the chassis for any grounding. Everything ground to one of these 2 locations.

Victor

[Geek]

232 needs a ground on both ends or probably more accurately I should say it 'wants' a signal ground on both ends that isn't the shield. (I've seen 232 work with the ground wire broken but the reason I was there fixing that was there were drop outs in the data stream to the systems I was working on) I know in the pin out listings on the PFD (GAD 460) the signal grounds are called out and the power grounds are called out as two different animals. On the GEA that wasn't the case. BUT if you go back into the schematics farther back in the book, you can see that pins 4 and 5 are the Rx and Tx while 6 is the signal ground. I know when I did my layout, I pretty much got the basic layout using the pin out list and then verified it using the schematics just because of that. Did not want to mix power and signal grounds. I attached a copy of the GAD to GEA schematic here.

G

31811

[VictorV]

Thanks Gary.

The LRU pinout for the GEA-24 just calls pin 6 "Ground" so maybe it can be used as either power or signal ground.
I assume I would use "Signal Ground" if it's available and use "Ground" otherwise.

Victor

[Shadowrider]

I would use a shielded 3 wire and connect as per diagram. Shield connected to backshell.

[VictorV]

I would use a shielded 3 wire and connect as per diagram. Shield connected to backshell.

I wish it was that easy :) I am currently using 3-conductor shielded wire for all RS-232 connections. In some cases (CO detector) there is
no backshell connection and/or it's made of plastic.

The GPS-20 has RX, TX and serial ground which is pretty straightforward but one of the serial links goes to the ELT which just has RS-232 RX and that's it.

Here's my plan:

Garmin-to-Garmin - Signal ground to signal ground unless device does not have signal ground in which case I would just use ground. Connect shield to connector shield if present on both ends.

Garmin-to-Other - Use signal ground if present on Garmin side. Use whatever ground is available on other device. No shield connection on either end.

Victor

[PapuaPilot]

Garmin-to-Garmin - Signal ground to signal ground unless device does not have signal ground in which case I would just use ground. Connect shield to connector shield if present on both ends.

Garmin-to-Other - Use signal ground if present on Garmin side. Use whatever ground is available on other device. No shield connection on either end.

Victor

You could be asking for trouble if you don't hook up shields on either end of non-Garmin components. You might as well not use shielded wiring at all, but this could lead to big problems if the wiring causes or receives interference from anything else.

In the Garmin installation manual the CO detector shows a three pair shielded wire. Per Fig. 27-2.6 the shielding needs to be connected to the backshell of P3701 on the GDU and go to the nearest airframe ground at the CO detector. BTW you really only need a 2 pair shielded wire because the GDU does not communicate with the CO detector. The CO detector is sending data its RS-232 Out to the GDU.

For the ELT it may only need one wire (TX/RX), but there could be two if there is a signal ground pin available at the ELT. The ELT the GDU is sending GPS position data to the ELT. Therefore, for this RS-232 setup the GDU needs the OUT/TX pin and the ELT only needs the IN/RX pin. It is a one-way path of data because the ELT does not communicate to the GDU.

[VictorV]

You could be asking for trouble if you don't hook up shields on either end of non-Garmin components. You might as well not use shielded wiring at all, but this could lead to big problems if the wiring causes or receives interference from anything else.

In the Garmin installation manual the CO detector shows a three pair shielded wire. Per Fig. 27-2.6 the shielding needs to be connected to the backshell of P3701 on the GDU and go to the nearest airframe ground at the CO detector. BTW you really only need a 2 pair shielded wire because the GDU does not communicate with the CO detector. The CO detector is sending data its RS-232 Out to the GDU.

For the ELT it may only need one wire (TX/RX), but there could be two if there is a signal ground pin available at the ELT. The ELT the GDU is sending GPS position data to the ELT. Therefore, for this RS-232 setup the GDU needs the OUT/TX pin and the ELT only needs the IN/RX pin. It is a one-way path of data because the ELT does not communicate to the GDU.

Yes. I was worried that unwanted noise would impact the RS-232 performance. Based on the pin-out of the CO Detector below and there's no "Signal ground"
31824

Twisted Pair Garmin CO Detector
Cable Shield --> connector --> ground
Cable Ground (3rd wire) --> signal ground --> ground
RX --> RX --> TX
TX --> TX --> RX

Power ground is the same as avionics ground since nothing is tied to the chassis.

It would be similar for the ELT but without the TX from the ELT to the G3X.

Victor

[VictorV]

Here's a better version of my table:




Twisted Pair
Garmin
CO Detector


cable shield
connector
ground


cable ground
signal ground
ground


RX
RX
TX


TX
TX
RX

[Victory_Overland]

1) Think Signal Ground is related to current flow and.....
2) Shield/Chassis is a bond for the entire system and chassis to bond/ground all into one.

What you plan to do is not detrimental to safety but surely can relate to a "noisy" aircraft and disrupting the current flow of component signal grounds that run integral to the wire harness from component to component.

Just my two cents.

[G3Xpert]

I need some help with grounding of the RS-232 signals in a G3X system. I have mostly Garmin components except for
a VPX, Guardian CO Det and Artex ELT. On the G3X there are various serial ports and each one has a TX and RX.
There are also a bunch of "signal grounds" that are not assigned to specific ports.

I'm a bit confused about what to do when, say, connecting the G3X to the GEA-24. There are no "signal" grounds on
the GEA-24. For an individual serial port do I need to connect to a signal ground on the G3X? What do I do with that
ground on the other end of the wire (GEA-24 in this case)? I am using shielded wire for all my RS-232 cables. Do I need
to do anything with the wire shield?

Not sure it's relevant but I have 2 ground busses. One behind the panel and the other behind the seat. They are connected
together. I don't use the chassis for any grounding. Everything ground to one of these 2 locations.

Victor

The pinout for the GEA 24 backup data path is provided in drawing Figure 27-1.11 0- "GEA 24/GSU 25 - GDU 37X/4XX Backup Data Path Interconnect Drawing".

This is on page 27-13 of the AQ revision of the Installation Manua (https://static.garmin.com/pumac/190-01115-01_aq.pdf)l.

In general there is a dedicated signal ground for each serial port connection. Some LRU's have unique grounding requirements/characteristics however. Some you will to pay close attention to are Trim motor control paths, and audio paths. If you have any specific questions you can always email us as well! G3Xpert@Garmin.com.

Thanks,

Justin

[VictorV]

Thanks Justin and everyone else who responded!

Victor