ARINC to GPS 175 shield?

[Taylorbentz]

I have 2 questions: regarding connecting an ARINC 429 to a Garmin GPS 175 using the below pinout.

1. Garmin calls for a shielded wire, with the shield grounded to the backplate on the GPS 175. Neither Dynon nor Garmin mention what to do with the shield on the Dynon ARINC 419 side. What should I do with the Dynon ARINC 429 side of the shield?

2. The Dynon manual shows a Ground connection on Pin 20 of the ARINC-429. Should this be connected to the same ground bus as the HDX? Or should it be grounded to the shield wire? Or not at all?

This is a C182P.

Anybody's advice would be appreciated!

 

[Rhino]

Ideally all avionics connections would be grounded to the same point. The Third Party Connection and Configuration Guide mentions that several times, though it doesn't specifically mention the 175. Most people in the experimental world use the intercom or audio panel. However, I'm not aware if the certified world allows you to deviate from what's in the 175 manual. It shouldn't be necessary to ground the shield at both ends. In rare circumstances, that can even cause problems. But, if you aren't allowed to deviate from the connection the Garmin manual calls for, I would probably ground it at both ends. Maybe you should ask Dynon to shed some light on that.

 

[Taylorbentz]

Ideally all avionics connections would be grounded to the same point. The Third Party Connection and Configuration Guide mentions that several times, though it doesn't specifically mention the 175. Most people in the experimental world use the intercom or audio panel. However, I'm not aware if the certified world allows you to deviate from what's in the 175 manual. It shouldn't be necessary to ground the shield at both ends. In rate circumstances, that can even cause problems. But, if you aren't allowed to deviate from the connection the Garmin manual calls for, I would probably ground it at both ends. Maybe you should ask Dynon to shed some light on that.

Thanks I appreciate the input. Ground loops seem to be more art than science (art backed by science) so honestly just hearing people's opinions is really helpful. To be clear, when you say "ground at both ends" would you connect the shield on the Dynon side to the Ground pin #20?

 

[Rhino]

Yes. I personally would probably ask Dynon first though. Then you would at least have a written record, which I understand may be important in the certified world (my experience is mainly military and experimental). They may well consider the ARINC lines to not be susceptible to noise, and thus tell you to just ground at the 175 end. That would make things simpler, and I'm a big fan of simpler.

 

[Taylorbentz]

From Dynon:
Shields are typically only grounded on one end with the other left floating or unconnected.
Connecting pin 20 is not necessary. It's there in case the two devices are not using the same battery ground and allows you to ground the arinc module to the navigator so the signals will have a ground path. If they share the same power ground, it's not needed.
I have my GPS175 and Dynon all with power grounds on a common ground bus, so no pin 20 for me, and no connecting the shield to the ARINC 429 module.

Thanks!

 

[Rhino]

Outstanding. Like I said, I'm a big fan of simpler.

 

[GKC Aviation]

From the GPS175 install manual

 

[N941WR]

.... Most people in the experimental world use the intercom or audio panel. ...

Most people use a "Forest of Tabs" on the firewall as a common ground. I have never seen the intercom or audio panel used as a ground. Just my $.02.

 

[Rhino]

Most people use a "Forest of Tabs" on the firewall as a common ground. I have never seen the intercom or audio panel used as a ground. Just my $.02.

Not the best idea. There is a reason for using the intercom or audio panel. A common ground is good no matter what, but since most problems show up in your audio, the intercom or audio panel are a better choice.

Ground Loops and Noise in Aircraft Systems

Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those...

forum.flydynon.com

 

[Taylorbentz]

For what it's worth. I left the dynon side of the shield unterminated, with heat shrink over the end to keep it clean. I grounded the shield to the GPS175 backplate. The ground wire on the GPS175 does go to a forest of tabs, which is common to the Dynon ground wire as well. Working well so far.

 

[Raymo]

Most shielded wires are grounded on one end only (keeps noise out) but Garmin requires both ends of CAN bus and serial wire shields to be grounded (keeps noise in). Follow the guidance in the 175 manual. If you purchase the connector kit for the 175, which I recommend, it will provide several points for shield ground(s). The 429 end can be grounded to a mounting screw.