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Garmin Wiring and Grounding

CharlieWaffles

Well Known Member
As I look through the wiring plans for the Garmin GTN series it shows the cable shield is being grounded in two places. For instance in the picture below, it shows the shield being tied to the Shield Block (Triangle with S) at the GTN and then on the other end the shield is grounded to airframe ground. Since the entire GTN cage and shield blocks are metal and the cage is grounded, isn't this in essential another airframe ground and thus there are two grounding points?

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Not a problem

I am used to seeing a single ground to 'keep noise out' (audio) and double grounds to 'keep noise in' (transmitter). That being said, about the only time this would bite you is a possible ground current induced into an audio line.
Given their in the same cage and digital signals this is more a plus than a minus by keeping digital signals contained in the cable.
 
Answer please

As I look through the wiring plans for the Garmin GTN series it shows the cable shield is being grounded in two places. For instance in the picture below, it shows the shield being tied to the Shield Block (Triangle with S) at the GTN and then on the other end the shield is grounded to airframe ground. Since the entire GTN cage and shield blocks are metal and the cage is grounded, isn't this in essential another airframe ground and thus there are two grounding points?

Capture-7.png

I am hoping with this bump someone that is a near expert, I know we have them, can explain this. I just want to know what to do with the wires. My original panel harness was made up by an avionics shop so as far as I can tell all of the shields got attached to the shield ground block which is what one would expect with the Trangal "S" symbol in the diagram.

The question is for the air frame ground symbol, is the intent to tie those shield pigtails to the air frame ground wire that goes to the D-sub ground pin? I certainly don't see those wires getting connected to a ground wire that goes all the way back to attach to the air frame.

Please educate some of us on how in practical terms these symbol reflect in actual wires. Both get a pig tail attached to the shield. The Triangle "S" goes to the shield ground block integral to all Garmin products, the pig tail that attaches to the shield and is depicted to go to an air frame ground should go where?

Thank you, I am sure there are many that have this question, I have found recommendations on on end and both end grounding of the shield but that is not the question I have. An extensive search of VAF has been fruitless to date.

Another question as well, in metal airplane, is there a difference between a ground that attaches the shield to the metal shield block that is part of the connector and a shield connected to an air frame ground.

Cheers
 
Fundamental rule of thumb:

A. Audio Circuits: Ground shielding only at one end with audio circuits. This elimiates ground loop noise in ICS and speaker feeds. As a rule ground the shielding at the intercom / audio panel / radio end (as applicable) while leaving the other end of a node or lead with the sheild "floating" or not electrically attached to anything on the remote end. That's why headset jacks are installed with insulating washers while their sheilding is terminated unto themselves with a spot of heat shrink.

B. Data Circuits: Ground shielding at both ends with data lines. That's RS232, ARINC 429, High Speed Ethernet or CANBUS.

Shield grounding can be made with enviromental splices to a backshell or to an avionic case as a rule as these components are usually internally and externally grounded and shielded by design. So they are at the same potential as airframe ground. I think the reason the Garmin diagram does not show the "S" in a triangle to designate backshell on the EFIS is because they are showing a generic ARINC 429 EFIS display and not necessarily a Garmin unit. As if to imply follow the grounding instructions of that third party equipment provider. If it were a Garmin specific EFIS you can bet they would put the "S" in the triangle unless it were an unswitched audio circuit for aural warnings.

If you want to have a great training experience in the nuances of shielding, grounding and data lines I highly recommend attending the experimental avionics installation class taught at the AEA headquarters in Lees Summit MO and taught by fellow RV owner Pahan Ranasingha. Very good stuff.

Jim Ivey
 
Need more info

Fundamental rule of thumb:

A. Audio Circuits: Ground shielding only at one end with audio circuits. This elimiates ground loop noise in ICS and speaker feeds. As a rule ground the shielding at the intercom / audio panel / radio end (as applicable) while leaving the other end of a node or lead with the sheild "floating" or not electrically attached to anything on the remote end. That's why headset jacks are installed with insulating washers while their sheilding is terminated unto themselves with a spot of heat shrink.

B. Data Circuits: Ground shielding at both ends with data lines. That's RS232, ARINC 429, High Speed Ethernet or CANBUS.

Shield grounding can be made with enviromental splices to a backshell or to an avionic case as a rule as these components are usually internally and externally grounded and shielded by design. So they are at the same potential as airframe ground. I think the reason the Garmin diagram does not show the "S" in a triangle to designate backshell on the EFIS is because they are showing a generic ARINC 429 EFIS display and not necessarily a Garmin unit. As if to imply follow the grounding instructions of that third party equipment provider. If it were a Garmin specific EFIS you can bet they would put the "S" in the triangle unless it were an unswitched audio circuit for aural warnings.

If you want to have a great training experience in the nuances of shielding, grounding and data lines I highly recommend attending the experimental avionics installation class taught at the AEA headquarters in Lees Summit MO and taught by fellow RV owner Pahan Ranasingha. Very good stuff.

Jim Ivey

I can't post the photo but on the install manual for connections from GTX 345 to the GNS 430W, they show a Triangle "S" at the GTX 345 end and and air frame ground at the GNS 430W end.

Where does the pig tail on the shield at the 430W end get attached (diagram shows air frame ground symbol? I don't want to know the theory I want to know the application, as in:

Attach pig tail to shield
Attach pig tail wire to ___________.

There must be a difference but then in application there does not seem to be any. Your speculation on non-Garmin products, though a good guess does not seem to apply as this diagram is two Garmin products.

Thanks
 
I can't post the photo but on the install manual for connections from GTX 345 to the GNS 430W, they show a Triangle "S" at the GTX 345 end and and air frame ground at the GNS 430W end.

Where does the pig tail on the shield at the 430W end get attached (diagram shows air frame ground symbol? I don't want to know the theory I want to know the application, as in:

Attach pig tail to shield
Attach pig tail wire to ___________.

There must be a difference but then in application there does not seem to be any. Your speculation on non-Garmin products, though a good guess does not seem to apply as this diagram is two Garmin products.

Thanks

I recently installed a GTX345 in my RV-8. It has a single GNS 430W.

I attached the (pig tail) ground wires to the black connector block located on the bottom left side of the 430W looking at it from the back. I hope this helps.
 
I recently installed a GTX345 in my RV-8. It has a single GNS 430W.

I attached the (pig tail) ground wires to the black connector block located on the bottom left side of the 430W looking at it from the back. I hope this helps.


This does help. Did you also add pig tail to the shield on the GTX 345 end and attach that to the shield ground block on that connector?

For the life of me, I can't understand the difference between these two connections and why they are depicted differently. It seems to me to be the same thing.

Perhaps it is the difference between a ground block that is part of the back panel, like the 430W, versus the ground block that is part of the D-sub connector body.

Cheers
 
This does help. Did you also add pig tail to the shield on the GTX 345 end and attach that to the shield ground block on that connector?

For the life of me, I can't understand the difference between these two connections and why they are depicted differently. It seems to me to be the same thing.

Perhaps it is the difference between a ground block that is part of the back panel, like the 430W, versus the ground block that is part of the D-sub connector body.

Cheers

I just looked at page 164 in the Garmin GTX 3X5 TSO install manual. I think you may be confused by the symbols for the shielded cables. Are you referring to the little "s" inside the upside down triangle?

If so, the above referenced manual indicates that the RS232 and ARIC 429 cable grounds (pig tails) get terminated on the GNS 430W side. On the 345 side (little "s" in the upside down triangle) they terminate to the shell.
 
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Ask the experts

I would suggest you send the G3Xperts an email. I have asked them several questions and they have always been very helpful. I had a question about some differences in ways to wire RS-232 connections in the plans, and they said it was just different engineer's preferences on how to spec it. That may not apply to your particular question, but they will know.

Mark
 
The "S" means "shell", as in back shell. If it's not then the official training class was wrong....

When it comes to advice sometimes you gets what you pays for. You really should contact Garmin Support.
 
I believe when the drawing designates the Garmin Shield Block (triangle with the "s") the draftsman anticipated that a Garmin backshell with provision for ground wires will be used, and when the drawing designates an aircraft ground the draftsman anticipated the backshell for that device would not have a provision for the ground wires. In this latter situation I have grounded the shield to the devices' tray. Eg - the 430 backshell does not have a provision for ground wires on it and shield grounding goes to the tray.
 
Mark and the others are right. The ground wires on the GTX 345 RS232 and ARINC 429 cables terminate on the 345 connector shell. My applogies for any confusion I may have caused.
 
Chapter 24 of the G3X install manual shows all of the LRU (line replaceable units) interconnect drawings with a symbol chart. (note 4)

Note 5 states : "Unless otherwise noted, all shield grounds must be made to the respective backshells. All other grounds should be terminated to the aircraft ground as close to the respective unit as possible."

Based on that note, I have been adding or using an existing bolt/ring terminal to the structure, that I can access for maintenance, directly next to the component.
 
This does help. Did you also add pig tail to the shield on the GTX 345 end and attach that to the shield ground block on that connector?

For the life of me, I can't understand the difference between these two connections and why they are depicted differently. It seems to me to be the same thing.

Perhaps it is the difference between a ground block that is part of the back panel, like the 430W, versus the ground block that is part of the D-sub connector body.

Cheers

Mike,

I sent you a PM.
 
I recently installed a GTX345 in my RV-8. It has a single GNS 430W.

I attached the (pig tail) ground wires to the black connector block located on the bottom left side of the 430W looking at it from the back. I hope this helps.



Hi Mike,

As Charlie has indicated, the Garmin way of connecting the shield on a 430 is to the black connector block on the rear of the rack. I have found this difficult if you don't have the pins for this.

I have done quite a few of these, I would connect the shields to the connector shield block if they have one(The GTX does), and at the 430 end connect to the black earthing block if you have pins for the connector and there is spare/open position. If not I would put a ring on the shield wire and connect it to the strain relief screw on the 430 connector or anywhere close where you could get a screw installed.

Cheers.

Bruce.
 
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