More twisted pair questions

[Sam_Beale]

I am installing the roll servo in my RV7A wing with the Dynon mounting kit, and have purchased the pitch servo and mounting kit that will install as the fuse is built. Already kicking myself due to another question I saw on the forum, because I have run all 7 wires as #20 Tefzel, didn't realize the control wires could be smaller.

Two questions:
How twisted is twisted? Right now, I have twisted the blue-green pair together by hand, and they make a full turn about every two inches; in other words, when viewed from any side, they appear to cross about every 1 inch. Is this adequate?
Note I mentioned running 7 wires above, I am running another pair of wires to accomodate the "future products", whatever they might be. Should this pair be twisted also?
Just thought of a third question: Any hint available on what the future products might be?

 

[DBRV10]

You can buy shielded twisted pair from most avionics suppliers. Use that, far less trouble than rolling your own!

 

[mmarien]

If you are making your own harnesses Appendix C of the Installation Manual has the specs:

Twisted Pairs
SkyView networks utilize two data wire pairs for communication between devices. This guide refers to these pairs as Data 1 and Data 2 and each have an A and a B wire. These pairs should have 8 to 10 twists per foot over their entire length.

 

[Sam_Beale]

Thanks David and mmarien.  The only manuals I had found were for the servos themselves, since the two servos were all I have purchased so far.  I did not realize that the info I needed was in the manual for installing the electronic components, but I finally found what you were talking about and quoting from.  Also found that the wire leads for future products are a data pair also, and thus should be twisted as well. 

Thanks again

 

[lenene]

I plan on using the twisted wire with shielding. I assume the shield should connect to ground somewhere. should I try to ground each end, or is one end sufficient and which end might be preferable. Easiest thing for me is to connect the shield to the D-180 case and pick up the ground that way. That sound reasonable?
D-180 and pitch and roll servos and direct command from the D180 buttons, no 74 involved.

 

[Dynon]

One end only is the right way to do. Grounding both ends negates some of the properties of the shield. Picking off the ground at the D180 end is fine.

 

[lgabriel]

Manufacturer guidance takes precedence in specific instances, but I wanted to make sure the whole picture is out there:

The 43.13 states that shielded wire is "typically connected to the aircraft's ground at both ends of the wire" (AC 43.13-1B Chg 1 11-89).

Edit: clarified order

 

[jakej]

Manufacturers guidance usually takes precedence however there are certain 'qualifications' here.  It's all about context.

Ignition P lead shields are grounded both ends however audio shields are grounded at the audio panel or intercom if no audio panel used.  Phone/Mic jacks shields are left unconnected at the jacks end.
IMO manufacturers can get it wrong eg the Apollo (Garmin) wiring diagrams showed grounding both ends.
Some radio people use the shield as a ground return - go figure.
I trust the best audio people (PS-Engineering) more than the radio guys in this case  ;D

Jake J

 

[dynonsupport]

There is a simple rule:

Ground both ends to keep noise in, One end to keep noise out.

An antenna coax grounds both ends to keep the "noise" in the coax until it gets to the antenna, since it's a noise source. Audio wires ground one end to keep noise out, since they are sensitive to it.

So what is DSAB? It's a toss up. It's balanced, low voltage, and low current, so it's not much of a noise source. It's designed to be robust against external noise as a balanced signal. Hence the reason we don't shield it either way in the cables we sell.

 

[lgabriel]

Terminating the shield at one point vs. multiple depends on the electromagnetic coupling mode you're trying to defend against (electric or magnetic)[1].  Electric coupling is a function of the capacitance across the insulator between the signal wire and the shield[2], while magnetic coupling is picked up by the loop formed by the cable and the ground plane[1].

Low frequency signals interfere via magnetic coupling, and high frequency signals via electric[1][2].  Therefore, when trying to defend against low frequency interference (like when you're trying to protect a low frequency audio signal), you use single point shield termination to avoid creating a loop/current path for magnetic coupling to occur.  For high frequencies, it's important to reduce the voltage accumulating on the shield, so multiple grounding points are used[2].

So, to sum up, here's what I'm seeing:
[list bull-blackball][*]Ground the shield wherever the manufacturer says to ground it.  After all, Dynon has considerably more experience, knowledge, and engineering skill than me (some guy from the internet)!  Jake, no offense to you--this is assuming the manufacturer gets it right!
[*]If the manufacturer doesn't specify, try to determine what you're most likely trying to defend against.  If high frequency interference, ground on both ends.  If low frequency, ground on one end.
[*]If you have no idea what the interference is likely to be, follow the 43.13 and ground on both ends.
[/list]
For what it's worth, all of the data cables (RS-232, Dynon, or otherwise) in my airplane are shielded twisted pairs, grounded at both ends.  All of the audio runs are shielded twisted pairs or triples, grounded at the source end only (per audio panel manufacturer guidance).

[1] Fröbel, Anke. "Cable Shielding to Minimize Electromagnetic Interference." Cotbus University of Technology, Germany.
[2] Renner, E. NAVSO P-3181 Aircraft and Avionics Cabling E3 Design Manufacturing Guidelines.  U.S. Navy, 1992 para 4.3

 

[jakej]

Louis - as we say "no worries mate" ;D ;D

Jake j