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  #1  
Old 02-17-2018, 09:56 AM
sblack sblack is offline
 
Join Date: Dec 2009
Location: Montreal
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Default Canbus wiring

I am installing an mgl iefis lite and being os Scots ancestry I am making my own harnesses. It uses a canbus in a daisychain to hook up the engine module, ap servos and any other goodies. The canbus is simply a 2 conductor shielded cable. It goes around the airplane past the various gadgets and you are supposed to tap off of it and run the signal wire to what ever gadget you want, then put a resistor at the end of the canbus. It is all well and good on a wiring diagram, but how does one tap in to a shielded 2 connector cable? Or is that just how they draw the schematic and in real life you do it differently, like hooking up the ingoing and outgoing signal wires at each device?
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  #2  
Old 02-17-2018, 11:06 AM
leok leok is offline
 
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Location: Clarkston, MI
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Default CAN Buss terminations

Scott,

To daisy chain 2 conductor shielded wire for the CAN Buss run a wire from item A to item B, and a separate wire from item B to item C etc..

Now referring specifically to the two CAN wires run to item B; Make a normal shield termination of both, leaving the wires extending a little longer than typical (4" instead of 3"). Taking the two conductors from each shielded can wire, match the white to white and blue to blue.

Splice the two whites together leaving a 2" tail to add a pin or socket. I like to use a window splice where I open about 1/2" of insulation, 2-3" up on one wire. Cut the second wire length to match the window opening on the other wire, wrap around the exposed wire in the window and solder. Add a small piece of heat shrink. Then do the same to the two blue wires.

A final piece of heat shrink over both splices protects and supports the whole thing leaving the white and blue tails to connect to the device.
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Last edited by leok : 02-17-2018 at 11:11 AM.
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  #3  
Old 02-17-2018, 11:44 AM
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9GT 9GT is offline
 
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Garmin has some really nice videos on wiring, including daisy chain method:
https://www.youtube.com/watch?v=KjeTWw4k1nw
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  #4  
Old 02-17-2018, 12:16 PM
sblack sblack is offline
 
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Thank you both! Very helpful
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  #5  
Old 02-17-2018, 04:11 PM
chris mitchell chris mitchell is offline
 
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My understanding is that the CANBUS can just be a twisted pair of 22awg wires, and that they do not need to be shielded. Simpler and cheaper - assuming the information is correct!

Chris
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  #6  
Old 02-17-2018, 04:47 PM
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RV6_flyer RV6_flyer is offline
 
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Default CAN Bus wiring

Quote:
Originally Posted by 9GT View Post
Garmin has some really nice videos on wiring, including daisy chain method:
https://www.youtube.com/watch?v=KjeTWw4k1nw
CAN Bus is generic and used by lots of manufacturers. Garmin does an excellent job of explaining how to do it. It does not matter if it is a Garmin product or another manufacturer's CAN Bus. It is all the same.

I recommend doing a continuity check with an ohm meter after each solder connection. Also check to make sure that it is isolated from all the other conductors after each solder connection.
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  #7  
Old 02-17-2018, 04:53 PM
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RV6_flyer RV6_flyer is offline
 
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Quote:
Originally Posted by chris mitchell View Post
My understanding is that the CANBUS can just be a twisted pair of 22awg wires, and that they do not need to be shielded. Simpler and cheaper - assuming the information is correct!

Chris
The CAN Bus specification is a twisted pair. In aircraft use, one wants a system that will have NO interference. The shield twisted pair wire is needed to help insure that there is no interference. IF you want a reliable CAN Bus in your airplane, follow the recommendation of the manufacturers like Garmin and use the shielded twisted pair.
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  #8  
Old 02-18-2018, 02:53 PM
chris mitchell chris mitchell is offline
 
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Not that I would ever advocate doing something contrary to manufacturerís instructions; if thatís what Garmin says, fine. However MGL suggest just a twisted pair. Shielding is just one aspect of reducing noise in a circuit and unless itís properly connected - including all the places where it needs to be broken into in order to wire other devices (ADHRS, magnetometer etc) - it will add nothing to reducing noise. And of course there are other aspects of wiring practise that can also influence noise. So use of a shielded wire, unfortunately, does not guarantee a noise free circuit.

Chris
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  #9  
Old 02-18-2018, 03:36 PM
JDA_BTR JDA_BTR is offline
 
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I'd be careful about just following that video too. They flow the solder without heating the shield very well and then tool the solder to make it look better. That is technique that will lead to cold-solder joints. The solder sleeves with pigtails make an excellent mechanical connection and the shield is at temp when the solder melts under a heat gun. The G3X docs don't show manually soldered connections like this video. Not to say it won't work, but it isn't hard to do a lot better.

The video also doesn't show the shield reflected back over the wire casing as it should be; they just solder it with it sticking out over the other wires. Not best practice? It could easily cause a chafe point between the end of the soldered shield and the wires coming out of it.

The docs also caution against having the solder splice for the free leads inside the back shell due to preloading. It probably isn't a big difference but I made sure my solders were outside the back shell as a result.

What I did was take one side of the CAN bus and strip back 3.75", then take the other side and strip back 2.5". I soldered about 3/8" of the 2.5" side to the 3.75" side about 1" from the casing end, leaving about 2" to go into the backshell/pins. The stripper is used to make a window in the 3.75" wires at the right place. I also used the Nuckoll's method of wrapping a few strands from the 2.5" side around to make a mechanical bond prior to soldering, for the two CAN bus wires. After I splice, I slide some 1/8 heat shrink over the splice, and then again over that with a little longer piece. Once the two splices are made I trim the ends even for placing pins.

For the ground it can be daisy chained; in my case here I have some bigger solder sleeves that fit over two of the two-conductor shielded cable so I made the shield splice between both and a lead to the back shell ground in one step. These cost about a dollar but its worth it given the tens of thousands of electronics involved.
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  #10  
Old 02-20-2018, 07:48 AM
Rainier Lamers Rainier Lamers is offline
 
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The MGL CAN bus is very forgiving. That should not imply that one can be sloppy but based on experience, here is my take:

The MGL CAN bus in a typical aircraft is fairly short - seldom longer than say 10ft, sometimes up to 20-25ft but that's about it.

According to the literature, the bus starts somewhere and ends somewhere and anything you connect onto it goes via shortish "stubs" with the stubs a few inches long.
At each end you place a 120 ohm resistor across the two wires.
That's it.

Now that works well for long (very long) buses and very high data transmission rates.

But we don't do that. Our rates are low (just 250Kb/s) - now everything becomes uncritical. We have seen buses with total wire lengths in excess of 60ft wired in what you would call a "star" fashion - multiple long stubs joined somewhere and a single 100 ohm resistor "somewhere" - and it just works great.

So, by all means - follow the prescribed ways to "do things right" but keep in mind if there is someplace where it is a little tricky perhaps - no harm in deviating a little.

Regarding the shielding. The signals on the twisted pair are low voltage and differential (that means they swing in opposite directions). That neatly cancels out radiation of interference while at the same time the differential system acts only on differences - interference received affects both wires in the same way so is not seen by the CAN bus drivers.
The reason the wire is twisted - so that both wires on average get the same exposure to the environment.
This originates from the old days of telephone systems - the landlines use twisted wires - unshielded.

In addition to the above the signals are slew rate controlled which prevents transmitting harmonics (which your radio might receive).

So - pretty bullet proof. That is why we use it.

Yes you may shield the twisted pair for good measure if you so desire. If you do please connect the shield only on one side (do not be tempted to use it as convenient ground conductor). You do not want your shield to carry any signals (as these could radiate) and you do not want to create ground loops anywhere in your electrical system - ground loops are the main reason aircraft builders get gray hair prematurely. Yep, 'tis true...

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