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Shielded versus twisted pair?

claycookiemonster

Well Known Member
I contacted Dynon recently to confirm their guidance for wiring their bits. I expected then to recommend shielded wire to connect their boxes, but they said that was unnecessary except for audio components. They said twisted pair was better for digital components. Now I'm confused. Also, then there is the decision on WHICH wires to combine into a single twisted pair?

Can anyone gently educate me on this?
 
The first SkyView install I used (2) four #22 conductor shielded wire for the network wiring. Flying 10 years now and has been flawless. The hard part is proper documentation of what wire goes to what 9 pin D connector.

Note - the conductors are have a shield and are twisted.

Carl
 
Here is a website that tries to explain shielded vs unshielded:
https://patchbox.com/utp-vs-stp-cable-advantages-and-disadvantages/
The answer to your question is: it all depends.
It will not hurt to use all shielded cable except that it is usually unnecessary, costs more, and is more difficult to install.
Below is a quote from Bob Nuckolls post on May 10, 2014.
http://www.matronics.com/archive/archive-get.cgi?AeroElectric-Archive.digest.vol-mi
I'd venture an assertion that the value of shielding of any wires in an airplane is minimal if not zero.
The short answer to your question is: Unless called out on the installation drawings for a device,
you don't need to shield the wires.
 
Most shielded wire is twisted along its length. So if the spec is for twisted pair, the shielded wire provides that. It is cost advantageous to buy a long spool of 2 and 3 wire shielded cable and just use it wherever called for. A good deal of the Garmin avionics drawings call for shielded, and a lot of the audio benefits from it too when properly grounded at one end. The CAN bus must be shielded for example. A good example of unshielded twisted pair is common CAT5/6 which have four twisted pairs. But this isn't aircraft suitable. If you need an ethernet in the plane (as is called for between a GTN650 and some transponders) you can use two shielded 2-wire cables.

When twisted is wanted for a power lead (as opposed to signaling), you can take a red and black wire in the proper gauge and twist a pair yourself. This can be handy for pitot heat or high-current lights to avoid creating magnetic fields that would affect a magnetometer. How many twists per inch is debatable and depends on the rate of change of the current flow, but it doesn't take a lot of twist to work fine. A steady current makes no magnetic field; only the change in current. So a controller-fed pitot tube will bang-bang a field and a non-controlled pitot will only have a significant field when turned on/off.

Look at the spec for shielded cable in the LED strobe and nav lighting from some vendors. I suspect that the shielding does very little, but the twist built into the shielded wire provides the desired effect.
 
Also, then there is the decision on WHICH wires to combine into a single twisted pair?

Can anyone gently educate me on this?

Twisted pairs work when: the magnetic field generated by wire ‘A’ is exactly cancelled by the field generated by wire ‘B’. Or, any noise induced on wire A is also induced on wire B, and the electronics only look at the difference between A and B (this is called ‘common mode rejection’). Examples of the latter would be ARINC wiring, where the electronics only looks at the difference between the A and the B wire. Examples of the former would be noisy LED wiring or pitot heat. Note it is important that all of the return current travel on that wire; you cannot ground the LED locally because then some return current goes thru other paths (aluminum airframe, etc). Most of our RS232 circuits are poor candidates for twisted pair shielding, because they use a common aircraft ground. There is no guarantee that the return current all goes down one wire.
 
The Dynon installation manuals specify the wire type, the rate of twist, and which wire goes to each pin position in the connector. It also tells you what color wires to get. At least, this applies to the Skyview HDX manuals.

Dave
 
I am assuming you are rolling your own SV Net cables. Dynon premade network cables are pricey. A 6" cable is $35. You can purchase a long cable, 30 foot, for $80. Cut it to the lengths you want, crimp the pins, add the d-sub, and you have all the cables you need for your installation.
 
It is true that there is a static magnetic field with a constant current. Varying current causes a varying field, and it is the changing ones that cause most trouble. I should have been more thorough in my prior post.
 
Also, then there is the decision on WHICH wires to combine into a single twisted pair?

Basically, twisted pairs are typically "push-pull" serial device connections. A couple of examples are the serial ports for Skyview and the servo connections for the autopilot.

If you read in-depth in the Skyview install manual (and other documentation), it does tell you which circuits are twisted-pair.
 
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As you can tell, I'm just getting started on this. The only Dynon component I have currently is the roll servo with it's 7 leads. So, all I've tried to do is string wires from the serve to the root. After hearing all about shielded wire for delicaate digital boxes, I was expecting Dynon to recomment such for this too, but they didn't. Yes, upon further investigation, the installation documentation does indeed specify twisted pair wiring for certain color leads.
I will read more closely going foreward.
 
I contacted Dynon recently to confirm their guidance for wiring their bits. I expected then to recommend shielded wire to connect their boxes, but they said that was unnecessary except for audio components. They said twisted pair was better for digital components. Now I'm confused. Also, then there is the decision on WHICH wires to combine into a single twisted pair?

Can anyone gently educate me on this?

All the Dynon serial wires are twisted pairs. You can use shielded but it will add a lot of weight because all each cable will need at least 4 twisted pairs. All the Dynon factory wiring looms in its kit are regular twisted pairs and NOT shielded.
 
Most shielded wire is twisted along its length. So if the spec is for twisted pair, the shielded wire provides that.

Specs like the RS-4xx one that dynon and airinc use call for twisted PAIR wire. The spec assumes that each individual PAIR of wire is twisted together. The standard wire we typically use IS twisted, HOWEVER, the entire bundle is twisted together. This does NOT provide what the spec requires. It helps from bleeding magnetic energy to other nearby wires, but the wires inside the bundle are parallel to each other the entire length. An exception would be a 2 conductor wire, that can be considered a twisted pair. There is a mil spec wire that has actual twisted pairs inside the shielding, but it is much harder to find. In that style wire, the individual wires are solid color, not white with colored tracers.

For things like DSAB and AIRINC, you are MUCH better off with unshielded twisted pair than shielded wire that is twisted in whole. The spec requires it to avoid magnetic bleed over from the TX to RX pair at high speeds. Unlike the RS-2xx protocols, the 4xx protocols use a pair of wires for both Tx and Rx to increase possible length.

There is a reason that ALL copper based high speed data transmission moved from coax to unshielded twisted pair 30 years ago.

Larry
 
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