boom3

boom3

Well Known Member
I've been reading about ARINC 429 and it says to use a twisted shielded pair with the shield grounded on both ends. To what? The Dynon HS34 for example really doesn't have a great place to ground something. Do you make your own local ground? Do you intercept the primary ground wire?

Thanks!
 
I believe what they mean is to use the ground point for the radios. On most avionics wiring diagrams, you'll see that all the grounds for the radio are tied together and gounded in one location. That's to prevent ground loops within the radio circuits.

I've always thought that you ground data circuits on the end that is receiving the signal, not the end sending the signal. Grounding both ends can create a ground loop unless the two devices sharing the data line are grounded at the same location.

I'm not an electical expert so take my thoughts with a grain of salt. Maybe other experts who do this for a living can comment further.
 
I agree Randy, in most of my wiring experience with shielded pairs, only one end of the shield is grounded to prevent a ground loop.

Since I've never wired an ARINC 429 circuit, I thought it be worth asking about.
 
boom3 said:
I agree Randy, in most of my wiring experience with shielded pairs, only one end of the shield is grounded to prevent a ground loop.

Since I've never wired an ARINC 429 circuit, I thought it be worth asking about.
Click to expand...

ARINC specification 429 makes specific mention on page 76 (part 1) that "Shields to be grounded on both ends of all breaks". Breaks being nodes in a multi-drop environment (one TX to multiple RX).

I'm going to stick my neck out and recommed that your assumption above is preferable IN THE ENVIRONMENT OF A VERY SMALL AIRCRAFT.
That is bold because it means there are no long cable runs for the ARINC signal. This changes the playing field a bit.

Avoidance of unnecessary ground loops is of prime importance in my experience if you want a complex avionics panel that works without trouble.
Even more so since the ARINC signal is a differential signal and thus is not referenced to ground.

Rainier
CEO MGL Avionics
 
On my previous Rocket, I had an ARINC 429 interface between the autopilot and the EFIS. If I recall correctly, I ran unshielded 22 AWG wires and they worked fine. On my current build, I plan to run shielded twisted pairs grounded at the AP head FWIW.
 
Thanks for your replies guys!

Since we're definately talking short runs here, maybe 2 or 3ft, I'm comfortable in just grounding the shield on 1 end.

On my next project, a slow build C-17, I'll probably have to reconsider.:D
 
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