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  #1  
Old 01-25-2021, 12:13 PM
FlyGood FlyGood is offline
 
Join Date: Dec 2009
Location: League City, TX
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Default Right Cable for RS 485

I am preparing to build the cable between my Garmin GDU-700L display and my engine analyzer. There is a twisted shielded two-conductor cable called out, with the notation that it is an RS485 system. Is any twisted shielded cable OK for this run?

The Garmin diagram shows a shield block ground at both the GDU and the GEA. The general rule seems to be to only ground one end of any shielded run. Should I follow the general rule or the Garmin diagram? If only one end of the shield is grounded, should it be at the display or the analyzer?

Thanks, in advance, for your wisdom and help.
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  #2  
Old 01-25-2021, 12:56 PM
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Raymo Raymo is offline
 
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Shielded 22g Tefzel is fine (MIL-C-27500) for the HSDB. Yes, follow the Garmin spec and ground both ends of the shield (same for CAN bus).
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  #3  
Old 01-25-2021, 10:38 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by FlyGood View Post
I am preparing to build the cable between my Garmin GDU-700L display and my engine analyzer. There is a twisted shielded two-conductor cable called out, with the notation that it is an RS485 system. Is any twisted shielded cable OK for this run?

The Garmin diagram shows a shield block ground at both the GDU and the GEA. The general rule seems to be to only ground one end of any shielded run. Should I follow the general rule or the Garmin diagram? If only one end of the shield is grounded, should it be at the display or the analyzer?

Thanks, in advance, for your wisdom and help.
Not sure it is an issue here, but the 485 spec calls for UTP - unshielded twisted pair cable. The std mispec, shielded tefzel cable we use is NOT twisted pair. There is a twist, but all of the wires are twisted in a bundle, with the wires still parallel to one another. There is a special milspec cable that uses true twisted pairs, where pairs of wires are first twisted and THEN bundled inside the sheath. These use solid color wires and not white with color tracers. I use it for arinc runs, as that is spec'ed for twisted pair.

The point of twisted pair cable is to minimize inductance or cross talk for the old phone guys, from one wire to another when they go long lengths while parallel to one another. The cross talk principle still applies in high speed data networks, though instead of hearing stray words, it blurs the sharp difference in distinguishing hi/lo or 0 / 1. Twisting the wires in a way that leaves them parallel does not meet the definition of twisted pair cable. Better to call it twisted cable

LIkely non an issue if Garmin is using simplex or single pair. It could be a problem if they're using duplex or two pairs.

Larry
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Last edited by lr172 : 01-25-2021 at 10:53 PM.
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  #4  
Old 01-26-2021, 08:00 AM
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vlittle vlittle is offline
 
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For short distances, you could use barbed wire, but...

If you are looking for a ready source of twisted pair, use wire from an ethernet cable. Not tefzel, but for this application is o.k.

Back when I was working on the IEEE standards committees for fast ethernet and gigabit ethernet, the conventional thinking was that a shield made everything worse (and more expensive).

This was because of installation problems mostly. Craftsfolk would damage shields, forget to connect them up, or short them to a signal wire. Any kind of mis-installation would not only mess up one signal, but other signals. It would also radiate radio frequency energy.

It gets worse. We recently had an installation at Lionsgate Entertainment World in China using twisted pair. The bundle of wires carrying these signals had a 'shield'. At install time, none of the installers read the instructions, so it turns out all of the equipment in the room had different ground potentials. Predictably, the grounded shields carried the loop currents and promptly fried the $$ equipment.

This does not apply much to small aircraft, but the point is that a shield is not always required, needed, or advisable. A properly implemented unshielded twisted pair can carry data or audio for thousands of feet. I spent most of my career in voice & data communications. The best esoteric principal I learned when I was a baby engineer was 'longitudinal balance'. Also applies to aircraft, but for different reasons.

V

p.s. the second best principal is 'don't touch the phone wires when someone rings the phone'. Back in the day that was 90 volts AC, superimposed on -48 volts DC. Ouch. These days it's 'don't drop your phone on your foot'.
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Last edited by vlittle : 01-26-2021 at 08:06 AM.
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  #5  
Old 01-26-2021, 08:15 AM
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fl-mike fl-mike is offline
 
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or chuck up two (colors of your choosing) #22 wires in your drill and twist 'em up.

Fun fact: If you do dissect some Enet cable, note that the twist rates of each pair are different, to reduce pair-pair x-talk.
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