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splicing 3-conductor shielded cable to 2-conductor shielded cable

ac_oldman

Active Member
I have a 3-conductor shielded wire that has RS-232 TX, RX, and signal ground.
I have a 2-conductor shielded wire that has RS-232 TX and signal ground.

What's the best way of splicing these two together, leaving the RS-232 TX disconnected on the 3-conductor wire?

Is it bad to splice these two together? instead should I look at making it a single run of wire between two avionics units (which would mean re-terminating one of the two wires at the D connectors I guess)?
 
I have spliced shielded wire numerous times without ill effect. terminating these wires into mating d sub connectors is no different than a proper splice. I would argue it is better. Just crimp or solder the main leads together and do the same for the shield. I am sure some would argue that the one inch of unshielded connection is a concern, but I don't feel that way and highly doubt it is an issue for RS-232 signals, which can typically do fine with un-shielded wire. Remember that all wires are unshielded for a couple of inches at every connector.

Larry
 
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Electrically there’s no problem. My suggestion would be to just use one run of cable. Two or three conductor doesn’t matter. The reason I’d recommend that over a properly made, insulated, and supported splice is simple. Ten years from now you’ll be re-wiring something, and you’ll have a 3-conductor cable on one end, and you’ll drive yourself batty trying to figure out why there’s no corresponding 3-conductor cable on the other end. Eventually you’ll rip the whole thing apart and finally figure out what past-you did, and wonder why you did that instead of just using one run of new cable and saving yourself all the hassle and confusion.





Not that I speak from experience or anything. Nope, not me.
 
I'd prefer the splice to rewiring as there are premade wiring harnesses on either end.

Would something like two heat shrink butt splices for the TX-RX and sig grnd-sig grnd work?

should I try to connect the shields or leaving both grounded at their connectors ends ok? I presume I shouldn't even bother trying to create an extra bit of shield over the splice.

should I put some sort of a cap on the unused tx wire?

presumably I should coat the whole splice in some shrink wrap for a little more mechanical stability.
 
I have a 3-conductor shielded wire that has RS-232 TX, RX, and signal ground.

I have a 2-conductor shielded wire that has RS-232 TX and signal ground.

What's the best way of splicing these two together, leaving the RS-232 TX disconnected on the 3-conductor wire?

It is not clear what you end up with. Are you saying you will splice the RS-232 RX from the 3-conductor wire to the RS-232 TX wire of the 2-conductor wire and then connect signal ground to signal ground? Implying that the signal is originating on the 2-conductor wire and is read by the device connected to the 3-conductor wire. If I understood this correctly, then:

On each shielded wire assembly strip 4 to 6 inches of the outside insulation off and then cut the shielding back 1 to 3 inches. Then solder and heat shrink the desired wires together either end to end or parallel. You could also use the mentioned solder-splices (sometimes called environmental splices) to do a butt splice of the desired signal wires. Then take a length of scrap wire and jumper the outside shield areas, again with either direct soldering or two more solder splices. Sometimes solder splices come with a wire already in them to be used as shielded drains and that would suffice also. For protection, as a first step you could slide a long piece of large diameter heat shrink over the3-conductor wire up and away from the splice so it can be used over the splice when the work is done. Then there's always electrical tape (eek!) or spiral wrap. Try n. t get too bulky.

Parallel pigtail splicing can also be used but it is ugly. That's when two stripped wires are laid in the same direction so their ends are even and then soldered or solder spliced. Since there is now an open end heat shrink is applied after soldering and doesn't have to be put over one of the wires prior to soldering. Think of wire nuts in home wiring. Looks yuky but works. You end up with a kink in the wire run where the wires are brought together out of plane. Pigtailing is more common in situations like the Garmin bare wire cables for portable GPSs are connected to airframe wiring. Those little wires are sometimes 28 gauge or smaller and gnarly to twist together for an inline butt joint. It's easier to pigtail them. In your case pigtailing might allow soldering the shielding to each other since they will lie along side each other.

Kind of like this image except the stranded wires are laid parallel, not 90 degrees:

slide_2.jpg


I would probably go with butt splicing via solder or solder sleeves myself.

With reference to your other questions, do not disconnect the shielding from either connector end of the existing runs because the rule of thumb is data wires should have their shielding grounded to the devices at each end to reduce interference. Your ends are already done properly so leave them alone. The short section you spliced and jumpered shouldn't be a problem unless that specific spot on the run is laying on top of a motor, strobe wire, power supply or similar noisy device.

As far as tying off the unused wire, industry practice is to put a short piece of heat shrink over the cut end and shrink it down so it is obvious the wire is unused and didn't get cut or pulled out of a connector. The next guy can tell it was intentional. After shrinking the heat shrink I usually pinch it with my fingers while it is still warm to cap it off. I then take and bend the wire back the way it came about an inch and zip tie it to the harness.

What devices are the ends connected to? Transponder? GDU? GPS? etc...

Here are some good videos courtesy of SteinAir in case you want a review of the standard handling of shielding:

https://www.youtube.com/results?search_query=steinair+shielded+wire
 
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I use solder sleeves. It keeps the bulk down. Splice the shields unless you know for sure that one end should be left open. It may be acting as the signal ground. Cap off the unused wire with shrink tube. If you have a labeling system be sure to label the wires on both sides of the splices and the one you cap. That will save headaches in the future.
 
THANKS!

Thanks! The connection is altitude encoding from a dynon d10a (2-conductor tx & sig grnd, shielded) to a garmin transponder (3-conductor general rs-242 shielded). I watched quite a few of the stein videos though I didn't see any directly splicing a bundle of wires together.

so sounds like this is the rough plan:
- strip 4-6" of outer insulation off both assemblies
- slide longish piece of large diameter heat shrink over one end.
- butt splice the two signals
- heat some shrink over the end of the unused tx from the 3-conductor, squeeze end.
- on one side, use a solder sleeve w/ preattached pigtail to get a good contact w/ the remaining shield
- strip a small amount of the pigtail
- on the other side, use another solder sleeve but insert the pigtail (from the side away from the signal wire ends, so as not to physically stress it)
- pull back and ziptie the unused tx over the end it came from.
- test the system w/ power
- disconnect power, cover the splice with heat shrink
- do I need additional mechanical support here? it's just behind the panel, not cowling or anything.

This is my first foray into this, I greatly appreciate the advice.
Is it ok to use a standard label-maker to add labels around these wires?
The garmin side has signal names on the insulation (fancy!), I don't believe the dynon bundle had this.
 
Exactly. I wish I could have said it that way simply rather than my long-winded meandering.

That should work great.

Jim
 
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