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ARINC 429 wiring practices

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ceengland7(at)gmail.com
Guest

Posted: Sun Jul 26, 2015 11:13 am Post subject: ARINC 429 wiring practices

I'm in the process of wiring my panel, which includes an old Garmin 430 (non-WAAS), an AFS AF4500S EFIS, AFS ARINC module, and AFS Pilot autopilot.Â I'm using the AF3000-4000 Installation Guide, v7.4.Â Looking through the AFS docs, theirÂ wiring diagrams are a bit...inconsistent... on how to treat the ARINC and serial port data wiring.Â 



So I did some digging in the InterWebs to find the 'proper' way to wire ARINC 429 stuff. Two different sources say to ground the ARINC 429 Â shield at source, destination, and at every 'break point' where additional receivers are added, in daisy chain fashion. Easy enough; I can handle that.Â 

There are 3 ARINC data runs between the ARINC module and the 430. Should each get its own shielded twisted pair, or would it be kosher to run 3 pairs in one shielded cable (and/or variations on that theme)?

In various places, there are both single and bi-directional serial runs between/among the EFIS, 430, and other devices. In one case, they specify that a ground return *isn't needed* (obviously depending on system ground(s) between the EFIS & the 430). In most cases, they show a separate ground return wire, *within* the shield of the cable. Now, I'd understand that if the serial lines were 'balanced' (floating return), but all the serial ground returns are electrically bonded to the chassis of the AF4500S. Any noise imposed on the shield goes to exactly the same place inside the unit as the separate ground return. Seems silly to run an extra ground wire that's at the same potential as the shield.

Any thoughts on this from those who have either designed or done the actual work on a/c using ARINC 429 and/or serial port technology?

Thanks,

Charlie

  [quote][b]

rv8iator(at)gmail.com
Guest

Posted: Mon Jul 27, 2015 6:18 am Post subject: ARINC 429 wiring practices

Hello Charlie,



My -8 panel is holds a 430W, AF4500, ARINC module, AF3500 and AF-pilot.

I followed the AFS wiring schematic exactly.Â  No shielded cables except for the run to the magnetometer and serial port connections.Â  All remaining connections between the ARINC 429 module, the 430 and the AF4500 are AWG22 single wire bundled into a harness with zip ties. Â 

I attached the ARINC 429 box to the underside of the AF4500 with self adhering velcro.Â  Thus all signal wire runs are short, less than 24 inches.

System works flawlessly. Â 

[img]cid:ii_14ecfd370fb563ec[/img]



[img]cid:ii_14ecfd7d06404a00[/img]



Chris Stone

RV-8

80802

On Sun, Jul 26, 2015 at 12:11 PM, Charlie England <ceengland7(at)gmail.com (ceengland7(at)gmail.com)> wrote:

 	  Quote: 	
		  I'm in the process of wiring my panel, which includes an old Garmin 430 (non-WAAS), an AFS AF4500S EFIS, AFS ARINC module, and AFS Pilot autopilot.Â I'm using the AF3000-4000 Installation Guide, v7.4.Â Looking through the AFS docs, theirÂ wiring diagrams are a bit...inconsistent... on how to treat the ARINC and serial port data wiring.Â 



So I did some digging in the InterWebs to find the 'proper' way to wire ARINC 429 stuff. Two different sources say to ground the ARINC 429 Â shield at source, destination, and at every 'break point' where additional receivers are added, in daisy chain fashion. Easy enough; I can handle that.Â 

There are 3 ARINC data runs between the ARINC module and the 430. Should each get its own shielded twisted pair, or would it be kosher to run 3 pairs in one shielded cable (and/or variations on that theme)?

In various places, there are both single and bi-directional serial runs between/among the EFIS, 430, and other devices. In one case, they specify that a ground return *isn't needed* (obviously depending on system ground(s) between the EFIS & the 430). In most cases, they show a separate ground return wire, *within* the shield of the cable. Now, I'd understand that if the serial lines were 'balanced' (floating return), but all the serial ground returns are electrically bonded to the chassis of the AF4500S. Any noise imposed on the shield goes to exactly the same place inside the unit as the separate ground return. Seems silly to run an extra ground wire that's at the same potential as the shield.

Any thoughts on this from those who have either designed or done the actual work on a/c using ARINC 429 and/or serial port technology?

Thanks,

Charlie

   	  Quote: 	
		  



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dlj04(at)josephson.com
Guest

Posted: Mon Jul 27, 2015 7:10 am Post subject: ARINC 429 wiring practices

I built nav processors for survey planes using several ARINC 429 devices 

tied together. Twisted pair wire was sufficient although we used 

shielded. Shielding for differential serial data like that is to reduce 

electromagnetic interference (transmitted and received) among the other 

systems, so tying the shield to chassis at one end should be fine. EMI 

was the only place where we had problems, and in some cases we needed to 

swap which end was shielded to get the quietest results. Some of our 

signals used multiple pairs inside one shield and that worked fine. No 

use for a separate ground wire.



David Josephson

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jul 27, 2015 7:24 am Post subject: ARINC 429 wiring practices

 	  Quote: 	
		  

 Any thoughts on this from those who have either designed or done the actual work on a/c using ARINC 429 and/or serial port technology?

 	


    ARINC 429 is implemented  on aircraft of all 

    sizes wherein local grounds for appliances

    can be yards apart and serial data is conducted

    on wires that traverse the wild and wooly environs

    for all manner of potential antagonist and victim.



    The short story is that for a single-engine,

    light aircraft with all the goodies mounted

    on the panel and sharing local grounds, the

    risks for loss of serial data signal integrity

    are zero.



    There is no value in adding any sort of ground

    external to a shielded-twisted-pair . . . why

    anyone would suggest such at thing is a mystery.

    There is no value in grounding a shield at both

    ends . . . shields on STP data lines are just that,

    stop-gaps for electrostatically coupled noises

    into or out of the lines which are already

    protected by the physics of twisted, balanced

    signal paths. Telephone companies have successfully

    exploited this particular physics in the fabrication

    and operation of phone lines in bundles with

    hundreds of other signals and traversing horrible

    environs over many miles.



    The twisted, balanced pair is about as bullet

    proof as you can get without shielding. Ground

    the shields at either end and only once.  



 

   Bob . . .    [quote][b]

ceengland7(at)gmail.com
Guest

Posted: Mon Jul 27, 2015 11:57 am Post subject: ARINC 429 wiring practices

Thanks for the pics. I suspected that the elaborate shielding described in the ARINC docs was a bit of overkill, but I went ahead with actual shielded twisted pair. I'm about 75% done at this point; should have waited a couple of days. 



Charlie



On Mon, Jul 27, 2015 at 9:16 AM, Christopher Cee Stone <rv8iator(at)gmail.com (rv8iator(at)gmail.com)> wrote:

[quote]Hello Charlie,



My -8 panel is holds a 430W, AF4500, ARINC module, AF3500 and AF-pilot.

I followed the AFS wiring schematic exactly.Â  No shielded cables except for the run to the magnetometer and serial port connections.Â  All remaining connections between the ARINC 429 module, the 430 and the AF4500 are AWG22 single wire bundled into a harness with zip ties. Â 

I attached the ARINC 429 box to the underside of the AF4500 with self adhering velcro.Â  Thus all signal wire runs are short, less than 24 inches.

System works flawlessly. Â 





Chris Stone

RV-8

80802

On Sun, Jul 26, 2015 at 12:11 PM, Charlie England <ceengland7(at)gmail.com (ceengland7(at)gmail.com)> wrote:

 	  Quote: 	
		  I'm in the process of wiring my panel, which includes an old Garmin 430 (non-WAAS), an AFS AF4500S EFIS, AFS ARINC module, and AFS Pilot autopilot.Â I'm using the AF3000-4000 Installation Guide, v7.4.Â Looking through the AFS docs, theirÂ wiring diagrams are a bit...inconsistent.. on how to treat the ARINC and serial port data wiring.Â 



So I did some digging in the InterWebs to find the 'proper' way to wire ARINC 429 stuff. Two different sources say to ground the ARINC 429 Â shield at source, destination, and at every 'break point' where additional receivers are added, in daisy chain fashion. Easy enough; I can handle that.Â 

There are 3 ARINC data runs between the ARINC module and the 430. Should each get its own shielded twisted pair, or would it be kosher to run 3 pairs in one shielded cable (and/or variations on that theme)?

In various places, there are both single and bi-directional serial runs between/among the EFIS, 430, and other devices. In one case, they specify that a ground return *isn't needed* (obviously depending on system ground(s) between the EFIS & the 430). In most cases, they show a separate ground return wire, *within* the shield of the cable. Now, I'd understand that if the serial lines were 'balanced' (floating return), but all the serial ground returns are electrically bonded to the chassis of the AF4500S. Any noise imposed on the shield goes to exactly the same place inside the unit as the separate ground return. Seems silly to run an extra ground wire that's at the same potential as the shield.

Any thoughts on this from those who have either designed or done the actual work on a/c using ARINC 429 and/or serial port technology?

Thanks,

Charlie

   	  Quote: 	
		  



ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution



	
	


 [b]

ceengland7(at)gmail.com
Guest

Posted: Mon Jul 27, 2015 1:36 pm Post subject: ARINC 429 wiring practices

Thanks to David & Bob.Â 



Bob,

My question about the 'extra' ground wire was related to the serial port(s), not ARINC. IIRC, serial stuff is typically unbalanced, with a signal line and a shield/ground return. In the case of the AFS docs, they show in one case (to external GPS) a pair of signal wires (bi-directional data), and a ground wire, all wrapped in a shield. In another case (magnetometer) they show a pair of RS422 signal wires, a power wire, and a ground wire, all wrapped in a shield. That's the practice of embedding an extra ground wire inside a shield (when the shield can serve as ground) that I was really questioning.

Charlie

On Mon, Jul 27, 2015 at 10:23 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

[quote]   	  Quote: 	
		  

 Any thoughts on this from those who have either designed or done the actual work on a/c using ARINC 429 and/or serial port technology?

 	


 Â Â  ARINC 429 is implementedÂ  on aircraft of all 

 Â Â  sizes wherein local grounds for appliances

 Â Â  can be yards apart and serial data is conducted

 Â Â  on wires that traverse the wild and wooly environs

 Â Â  for all manner of potential antagonist and victim.



 Â Â  The short story is that for a single-engine,

 Â Â  light aircraft with all the goodies mounted

 Â Â  on the panel and sharing local grounds, the

 Â Â  risks for loss of serial data signal integrity

 Â Â  are zero.



 Â Â  There is no value in adding any sort of ground

 Â Â  external to a shielded-twisted-pair . . . why

 Â Â  anyone would suggest such at thing is a mystery.

 Â Â  There is no value in grounding a shield at both

 Â Â  ends . . . shields on STP data lines are just that,

 Â Â  stop-gaps for electrostatically coupled noises

 Â Â  into or out of the lines which are already

 Â Â  protected by the physics of twisted, balanced

 Â Â  signal paths. Telephone companies have successfully

 Â Â  exploited this particular physics in the fabrication

 Â Â  and operation of phone lines in bundles with

 Â Â  hundreds of other signals and traversing horrible

 Â Â  environs over many miles.



 Â Â  The twisted, balanced pair is about as bullet

 Â Â  proof as you can get without shielding. Ground

 Â Â  the shields at either end and only once.  



 

 Â  Bob . . .     	  Quote: 	
		  



ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution



	




[b]

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jul 30, 2015 5:49 am Post subject: ARINC 429 wiring practices

At 04:34 PM 7/27/2015, you wrote:

 Thanks to David & Bob.Â 



 Bob,



 My question about the 'extra' ground wire was related to the serial port(s), not ARINC. IIRC, serial stuff is typically unbalanced, with a signal line and a shield/ground return. In the case of the AFS docs, they show in one case (to external GPS) a pair of signal wires (bi-directional data), and a ground wire, all wrapped in a shield. In another case (magnetometer) they show a pair of RS422 signal wires, a power wire, and a ground wire, all wrapped in a shield. That's the practice of embedding an extra ground wire inside a shield (when the shield can serve as ground) that I was really questioning.



 



   The 'extra' wire you're describing . . . is it bare?

   . . . and what is the nature of the shield material?



   . . . what you are describing is reminiscent

   of a product from Belden Wire called "Beldfoil"

   shielded wire. Instead of braiding strands of wire

   over the bundle, they wrap an aluminum foil shield

   over it. VERY effective shielding but electrical

   connection to thin aluminum is hard to make up.



   The solution was to bundle a bare stranded wire

   in with the other wires. Given that it is not

   insulated, it makes connection with the shielding

   at innumerable places along the length of the cable.



  http://tinyurl.com/nhpjmup



   Given that electro-static shield currents are

   very low, it matters not that the ground connection

   to the foil is not gas-tight. Hence, the shield ground

   has a hi order of integrity to the task and the foil

   shielding is electro-statically superior and much

   less expensive than braid . . . a kind of win-win.

   Unfortunately, this technology is not generally

   offered in combination with our favorite insulations

   so you don't see this wire used in TC or military

   aircraft . . . but it's fine for our purposes.



   That 'extra' wire CAN be used as a part of the

   power path as long as it's 22AWG current ratings

   are observed . . . with the caveat that the 

   return path power wire be included in the bundle

   so that you get parallel path cancellation for

   electro-magnetic coupling. But as a general rule,

   you're almost never wrong to use the shield 

   for electro-static decoupling and attach it 

   to ground one time only at either end.



 



 

   Bob . . .    [quote][b]

ceengland7(at)gmail.com
Guest

Posted: Thu Jul 30, 2015 11:48 am Post subject: ARINC 429 wiring practices

Understand about Belfoil shielded wire; I've built my share of cables using it in one of my previous lives as a sound engineer & audio tech. It's unrelated to my question. As a reminder, my question is about the AFS *documents*, not actual wire. I should have attached an image of the diagram with the 1st post. Image attached now.



See the wiring from the 'EFIS Main Cable' connector to the Nav Radio, Altitude Encoder Output to Transponder, GPS, and Magnetometer. All are shown with a separate ground wire, drawn within the shield. None of the devices draw significant current, and all the 'ground' return pins shown in the EFIS Main Connector are electrically common to the chassis of the EFIS itself. My question is about the 'extra' ground wire, drawn within the shield for each of these destinations.

The only connection that does make sense to me is the 'EFIS Audio Output', which logically shows a signal wire and a shield return, without the extra ground wire.

On Thu, Jul 30, 2015 at 8:48 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

 	  Quote: 	
		    At 04:34 PM 7/27/2015, you wrote:

 Thanks to David & Bob.Ã‚ 



 Bob,



 My question about the 'extra' ground wire was related to the serial port(s), not ARINC. IIRC, serial stuff is typically unbalanced, with a signal line and a shield/ground return. In the case of the AFS docs, they show in one case (to external GPS) a pair of signal wires (bi-directional data), and a ground wire, all wrapped in a shield. In another case (magnetometer) they show a pair of RS422 signal wires, a power wire, and a ground wire, all wrapped in a shield. That's the practice of embedding an extra ground wire inside a shield (when the shield can serve as ground) that I was really questioning.



 



 Â  The 'extra' wire you're describing . . . is it bare?

 Â  . . . and what is the nature of the shield material?



 Â  . . . what you are describing is reminiscent

 Â  of a product from Belden Wire called "Beldfoil"

 Â  shielded wire. Instead of braiding strands of wire

 Â  over the bundle, they wrap an aluminum foil shield

 Â  over it. VERY effective shielding but electrical

 Â  connection to thin aluminum is hard to make up.



 Â  The solution was to bundle a bare stranded wire

 Â  in with the other wires. Given that it is not

 Â  insulated, it makes connection with the shielding

 Â  at innumerable places along the length of the cable.



  http://tinyurl.com/nhpjmup



 Â  Given that electro-static shield currents are

 Â  very low, it matters not that the ground connection

 Â  to the foil is not gas-tight. Hence, the shield ground

 Â  has a hi order of integrity to the task and the foil

 Â  shielding is electro-statically superior and much

 Â  less expensive than braid . . . a kind of win-win.

 Â  Unfortunately, this technology is not generally

 Â  offered in combination with our favorite insulations

 Â  so you don't see this wire used in TC or military

 Â  aircraft . . . but it's fine for our purposes.



 Â  That 'extra' wire CAN be used as a part of the

 Â  power path as long as it's 22AWG current ratings

 Â  are observed . . . with the caveat that the 

 Â  return path power wire be included in the bundle

 Â  so that you get parallel path cancellation for

 Â  electro-magnetic coupling. But as a general rule,

 Â  you're almost never wrong to use the shield 

 Â  for electro-static decoupling and attach it 

 Â  to ground one time only at either end.



 



 

 Â  Bob . . .     	  Quote: 	
		  



ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution

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rossmickey(at)comcast.net
Guest

Posted: Thu Jul 30, 2015 12:50 pm Post subject: ARINC 429 wiring practices

I just got done installing a 5600.Â  I wired it as shown.Â  Each Serial Port bundle has a RX, TX, Gnd and shield.Â  The shields for the four ports are all connected together at the main harness connector with a black wire coming out of the connector that gets grounded to the chassis.Â  Â Â I used whatever TX, RX wires I needed for the particular instrument. Â I ran all of the Gnd wires to my ground block since that is where all of my instruments are grounded.Â  Everything works great ever since I got rid of the 18â€ length of unused OAT cable.Â  

 

If your question is why they want us to ground each serial port, I canâ€™t tell you.Â  I just did it.Â  Have you called AFS and asked them?

 

Ross

N9PT

 

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Charlie England

Sent: Thursday, July 30, 2015 12:47 PM

To: aeroelectric-list(at)matronics.com

Subject: Re: ARINC 429 wiring practices

 

Understand about Belfoil shielded wire; I've built my share of cables using it in one of my previous lives as a sound engineer & audio tech. It's unrelated to my question. As a reminder, my question is about the AFS *documents*, not actual wire. I should have attached an image of the diagram with the 1st post. Image attached now.

 



See the wiring from the 'EFIS Main Cable' connector to the Nav Radio, Altitude Encoder Output to Transponder, GPS, and Magnetometer. All are shown with a separate ground wire, drawn within the shield. None of the devices draw significant current, and all the 'ground' return pins shown in the EFIS Main Connector are electrically common to the chassis of the EFIS itself. My question is about the 'extra' ground wire, drawn within the shield for each of these destinations.



 



The only connection that does make sense to me is the 'EFIS Audio Output', which logically shows a signal wire and a shield return, without the extra ground wire.

 

On Thu, Jul 30, 2015 at 8:48 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote: 	  Quote: 	
		  

At 04:34 PM 7/27/2015, you wrote:

Thanks to David & Bob.Ã‚ 



Bob,



My question about the 'extra' ground wire was related to the serial port(s), not ARINC. IIRC, serial stuff is typically unbalanced, with a signal line and a shield/ground return. In the case of the AFS docs, they show in one case (to external GPS) a pair of signal wires (bi-directional data), and a ground wire, all wrapped in a shield. In another case (magnetometer) they show a pair of RS422 signal wires, a power wire, and a ground wire, all wrapped in a shield. That's the practice of embedding an extra ground wire inside a shield (when the shield can serve as ground) that I was really questioning.



  The 'extra' wire you're describing . . . is it bare?

  . . . and what is the nature of the shield material?



  . . . what you are describing is reminiscent

  of a product from Belden Wire called "Beldfoil"

  shielded wire. Instead of braiding strands of wire

  over the bundle, they wrap an aluminum foil shield

  over it. VERY effective shielding but electrical

  connection to thin aluminum is hard to make up.



  The solution was to bundle a bare stranded wire

  in with the other wires. Given that it is not

  insulated, it makes connection with the shielding

  at innumerable places along the length of the cable.



http://tinyurl.com/nhpjmup



  Given that electro-static shield currents are

  very low, it matters not that the ground connection

  to the foil is not gas-tight. Hence, the shield ground

  has a hi order of integrity to the task and the foil

  shielding is electro-statically superior and much

  less expensive than braid . . . a kind of win-win.

  Unfortunately, this technology is not generally

  offered in combination with our favorite insulations

  so you don't see this wire used in TC or military

  aircraft . . . but it's fine for our purposes.



  That 'extra' wire CAN be used as a part of the

  power path as long as it's 22AWG current ratings

  are observed . . . with the caveat that the 

  return path power wire be included in the bundle

  so that you get parallel path cancellation for

  electro-magnetic coupling. But as a general rule,

  you're almost never wrong to use the shield 

  for electro-static decoupling and attach it 

  to ground one time only at either end.



  Bob . . . 	  Quote: 	
		    ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-Listtp://forums.matronics.com_blank">http://www.matronics.com/contribution 	


	




 



  [quote][b]

ceengland7(at)gmail.com
Guest

Posted: Thu Jul 30, 2015 1:32 pm Post subject: ARINC 429 wiring practices

I haven't asked them yet, because I already have a strong suspicion of what their answer will be: 'to shield the ground from noise'. Serial data needs a ground return, but since the ground return pins in the connector are all bonded to the case of the EFIS, any noise on the shield is also on those ground return pins. Which begs the question: why have the extra ground wire? My suspicion is that they are following some 'certified aircraft' practice that was started back in mythological times and hasn't been updated with current knowledge. That's why I've been asking here; we've seen other 'stuff' in certified a/c that can be done better or simpler, based on current knowledge.



Glad to hear that you've got yours working noise-free.

Charlie

Â 



On Thu, Jul 30, 2015 at 3:48 PM, Ross Home <rossmickey(at)comcast.net (rossmickey(at)comcast.net)> wrote:

[quote]

I just got done installing a 5600.Â  I wired it as shown.Â  Each Serial Port bundle has a RX, TX, Gnd and shield.Â  The shields for the four ports are all connected together at the main harness connector with a black wire coming out of the connector that gets grounded to the chassis.Â  Â Â I used whatever TX, RX wires I needed for the particular instrument.Â  I ran all of the Gnd wires to my ground block since that is where all of my instruments are grounded.Â  Everything works great ever since I got rid of the 18â€ length of unused OAT cable.Â  

Â 

If your question is why they want us to ground each serial port, I canâ€™t tell you.Â  I just did it.Â  Have you called AFS and asked them?

Â 

Ross

N9PT

Â 

From: owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com) [mailto:owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)] On Behalf Of Charlie England

Sent: Thursday, July 30, 2015 12:47 PM

To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)

Subject: Re: ARINC 429 wiring practices

Â 

Understand about Belfoil shielded wire; I've built my share of cables using it in one of my previous lives as a sound engineer & audio tech. It's unrelated to my question. As a reminder, my question is about the AFS *documents*, not actual wire. I should have attached an image of the diagram with the 1st post. Image attached now.

Â 



See the wiring from the 'EFIS Main Cable' connector to the Nav Radio, Altitude Encoder Output to Transponder, GPS, and Magnetometer. All are shown with a separate ground wire, drawn within the shield. None of the devices draw significant current, and all the 'ground' return pins shown in the EFIS Main Connector are electrically common to the chassis of the EFIS itself. My question is about the 'extra' ground wire, drawn within the shield for each of these destinations.



Â 



The only connection that does make sense to me is the 'EFIS Audio Output', which logically shows a signal wire and a shield return, without the extra ground wire.

Â 

On Thu, Jul 30, 2015 at 8:48 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote: 	  Quote: 	
		  

At 04:34 PM 7/27/2015, you wrote:

Thanks to David & Bob.Ã‚ 



Bob,



My question about the 'extra' ground wire was related to the serial port(s), not ARINC. IIRC, serial stuff is typically unbalanced, with a signal line and a shield/ground return. In the case of the AFS docs, they show in one case (to external GPS) a pair of signal wires (bi-directional data), and a ground wire, all wrapped in a shield. In another case (magnetometer) they show a pair of RS422 signal wires, a power wire, and a ground wire, all wrapped in a shield. That's the practice of embedding an extra ground wire inside a shield (when the shield can serve as ground) that I was really questioning.



Â  The 'extra' wire you're describing . . . is it bare?

Â  . . . and what is the nature of the shield material?



Â  . . . what you are describing is reminiscent

Â  of a product from Belden Wire called "Beldfoil"

Â  shielded wire. Instead of braiding strands of wire

Â  over the bundle, they wrap an aluminum foil shield

Â  over it. VERY effective shielding but electrical

Â  connection to thin aluminum is hard to make up.



Â  The solution was to bundle a bare stranded wire

Â  in with the other wires. Given that it is not

Â  insulated, it makes connection with the shielding

Â  at innumerable places along the length of the cable.



http://tinyurl.com/nhpjmup



Â  Given that electro-static shield currents are

Â  very low, it matters not that the ground connection

Â  to the foil is not gas-tight. Hence, the shield ground

Â  has a hi order of integrity to the task and the foil

Â  shielding is electro-statically superior and much

Â  less expensive than braid . . . a kind of win-win.

Â  Unfortunately, this technology is not generally

Â  offered in combination with our favorite insulations

Â  so you don't see this wire used in TC or military

Â  aircraft . . . but it's fine for our purposes.



Â  That 'extra' wire CAN be used as a part of the

Â  power path as long as it's 22AWG current ratings

Â  are observed . . . with the caveat that the 

Â  return path power wire be included in the bundle

Â  so that you get parallel path cancellation for

Â  electro-magnetic coupling. But as a general rule,

Â  you're almost never wrong to use the shield 

Â  for electro-static decoupling and attach it 

Â  to ground one time only at either end.



Â  Bob . . . 	  Quote: 	
		  Â Â ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-Listtp://forums.matronics.com_blank">http://www.matronics.com/contributionÂ  	


	




Â 



   	  Quote: 	
		  



ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution



	


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