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  #1  
Old 03-24-2021, 12:06 PM
mfleming's Avatar
mfleming mfleming is offline
 
Join Date: Sep 2016
Location: Joseph, Oregon
Posts: 674
Default Can Bus Length Concerns

RV-7, all Garmin avionics. Building my own avionics harness with the support of SteinAir.

This is an observation/question.

In all my reading in VAF about the Garmin Can bus maximum length, I see statements that one can't possibly exceed the maximum length (20M or 66ft) in a RV-7.

This interested me since when doing the basic wiring in my -7 I noticed the amount of wire from point A to point B was always much greater than expected. This of course is due to the serpentine routing.

So when I made a mock avionics harness and laid it out on a table I decided to measure the exact length.

61' and I don't have as many goodies as some.

So I'm here to tell future budding avionics installers like me....it's easy to exceed the Can Bus maximum distance of 66'.

Now for the question:
Is the distance really that important?

I can't imagine how the Garmin Can Bus system is installed in an RV-10 or larger aircraft than out RVs without busting this parameter.
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Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring completed
Avionics harness build in progress
Donated for 2021 and so should you
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  #2  
Old 03-24-2021, 01:02 PM
ColoradoSolar ColoradoSolar is offline
 
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Location: Westcliffe, Colorado
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My guess is that number is pretty conservative, don't have any first hand knowledge though.

I have not worked with can bus very much but in a past life I worked with a differential pair bus (like can bus) that was spec'd for a very short length like 3' that was usable up to 100' or more.
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RV-6A Tip-Up Bought Flying
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  #3  
Old 03-24-2021, 03:02 PM
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mfleming mfleming is offline
 
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Location: Joseph, Oregon
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Someone asked how I got 61' on the harness. Here goes.

So in the post there is a photo of my mock can bus harness on the table.

That represents 45' and it starts jut behind the baggage compartment where a DB15 connects the aft portion of the harness already installed, it then runs up to behind the instrument panel across and then behind the sub panel, back down the firewall to the right wing DB15.

So with harness in the photo, the aft harness and the section in the right wing equals 61'

A lot is taken up with the LRUs and displays. If there is a 12" run to a LRU, that equals 24" because of the out and back. I measure 21.8' just for the LRUs out and back.
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Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring completed
Avionics harness build in progress
Donated for 2021 and so should you
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  #4  
Old 03-24-2021, 04:00 PM
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RV6_flyer RV6_flyer is offline
 
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Location: NC25
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I do not know everything about CAN bus and Garmin may be able to give us the correct answer.

My understanding of the CAN bus length has to do with the data rate over longer distances. As the length gets longer, the data rate will get lower. Longer line means a lower amount of data transmitted reliability. Garmin may have picked a length that will allow a data rate that works with everything all the time. IF length gets longer, some devices may not work correctly or not work all the time. My understanding, see link above, "BIT rates up to 1 Mbit/s are possible at network lengths below 40 m". (131 feet)
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Last edited by RV6_flyer : 03-24-2021 at 04:04 PM. Reason: add another link
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  #5  
Old 03-24-2021, 04:29 PM
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mfleming mfleming is offline
 
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Location: Joseph, Oregon
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Quote:
Originally Posted by RV6_flyer View Post
I do not know everything about CAN bus and Garmin may be able to give us the correct answer.

My understanding of the CAN bus length has to do with the data rate over longer distances. As the length gets longer, the data rate will get lower. Longer line means a lower amount of data transmitted reliability. Garmin may have picked a length that will allow a data rate that works with everything all the time. IF length gets longer, some devices may not work correctly or not work all the time. My understanding, see link above, "BIT rates up to 1 Mbit/s are possible at network lengths below 40 m". (131 feet)
I've seen the reference to 40 meters before. Here's what Gamin say's in there G3X manual version _am.

"2.3.1.3.1 CAN Bus Architecture
The electrical architecture of the CAN bus takes the form of a linear “backbone” consisting of a single
twisted wire pair with an LRU connected (terminated) at each end (Figure 2-5). The overall length of the
CAN bus from end to end should be 20 meters (66 feet) or less. At each of the two extreme ends of the
CAN bus, a 120 Ω resistor is installed to “terminate” the bus. In the G3X system, termination resistors are
provided either within the LRUs themselves, or via termination adapters that plug into an LRUs CAN
connection."
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Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring completed
Avionics harness build in progress
Donated for 2021 and so should you
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  #6  
Old 03-24-2021, 11:51 PM
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BuckWynd BuckWynd is offline
 
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Quote:
Originally Posted by mfleming View Post
If there is a 12" run to a LRU, that equals 24" because of the out and back. I measure 21.8' just for the LRUs out and back.
Just to clarify... There are no out-and-back "spokes" on a CANBUS. It is one continuous run, with very short "nodes" at each unit of perhaps 3 inches. And the units connected to it don't have to be in any particular order.

A 61-foot length seems very long to me. Is it possible you are going "out and back" too much?

For reference, I have a full G3X system in my RV-8, and the total CANBUS length is 39 feet. My CANBUS terminates at one end in the tail with the magnetometer, and out in the right wing on the other end at the roll servo. In between, it visits the firewall for my GEA 24, and of course about six different units in on or around the panel.
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  #7  
Old 03-25-2021, 09:07 AM
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mfleming mfleming is offline
 
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Quote:
Originally Posted by BuckWynd View Post
Just to clarify... There are no out-and-back "spokes" on a CANBUS. It is one continuous run, with very short "nodes" at each unit of perhaps 3 inches. And the units connected to it don't have to be in any particular order.

A 61-foot length seems very long to me. Is it possible you are going "out and back" too much?

For reference, I have a full G3X system in my RV-8, and the total CANBUS length is 39 feet. My CANBUS terminates at one end in the tail with the magnetometer, and out in the right wing on the other end at the roll servo. In between, it visits the firewall for my GEA 24, and of course about six different units in on or around the panel.
It sounds like your -8 and my -7 are wired the same.

So this is my interpretation of a CAN bus path length and how to measure it. If I'm in error, I'm eager to know.

I've included my avionics harness layout schematic and a sample from the current G3X manual. Notations have been added to the sample to explain my thinking.

-Michael
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Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring completed
Avionics harness build in progress
Donated for 2021 and so should you
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  #8  
Old 03-25-2021, 10:24 AM
ColoradoSolar ColoradoSolar is offline
 
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Location: Westcliffe, Colorado
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I think if you have things like the GEA 24, GSU 25, GAD 29/27 mounted behind the panel that your "out & back" distances will be much shorter than you think.

I don't have any can bus devices in the wings on my 6A, just tail to panel but I figure I have maybe 30' of can bus wiring.
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  #9  
Old 03-25-2021, 10:49 AM
Mark Browning Mark Browning is offline
 
Join Date: Dec 2010
Location: Doncaster England
Posts: 64
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Hello Michael
There are several mixed messages on the subject of Garmin Can bus.
If you check the specs that Garmin gives for certified aircraft they have a max length of 85ft. This is probably because they have to use the more expensive can bus cable Part number CAN24 TST120 (CIT)

I had a similar problem on my RV-14 so i opted to use the more expensive can bus cable, which i Highley recommend! over the standard milspec.

One option is to shorten the can bus by installing a GMU22 magnetometer in place of the GMU11 ( the GMU22 is not on the can bus.)

Garmin's aviation training webinar, that had a section on can bus wiring.
If you send over your email i can send you the power point slides on this, if it helps. It has Some good installation tips & specs and also a piece on can bus testing and trouble shooting

Best Regards
Mark
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  #10  
Old 03-25-2021, 11:08 AM
mfleming's Avatar
mfleming mfleming is offline
 
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Location: Joseph, Oregon
Posts: 674
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Quote:
Originally Posted by ColoradoSolar View Post
I think if you have things like the GEA 24, GSU 25, GAD 29/27 mounted behind the panel that your "out & back" distances will be much shorter than you think.

I don't have any can bus devices in the wings on my 6A, just tail to panel but I figure I have maybe 30' of can bus wiring.
As I mentioned in post #3, the measured distance (not estimated) to the LRUs behind the panel eat up one 21' +.


GSU25-1 - distance is 9.25" = 18.5" total
GSU25-2 - distance is 13.25" = 26.5" total
GDU 460 - distance is 11.5" = 23" total
GAD 27 - distance is 11.5" = 23" total
GDU 470-1 - distance is 12.75" = 25.5" total
GI 260 - distance is 17.25" = 34.5" total
GAD 29 - distance is 8.5" = 17" total
GMC 507 - distance is 12" = 24" total
GDU 470-2 - distance is 12.25" = 24.5" total
GRT 200B - distance is 8" = 16" total
GEA 24 - distance is 11" = 22" total

Total for all branches behind the instrument panel and sub panel is 21' +
Thats just the branches, this does not count the distance between branches.

So my drawing looks a lot like the sample from the manual...unless someone can show me where I'm wrong, I think a lot of CAN bus structures are under counted for length.

Just a reminder the drawing of my CAN bus layout posted only depicts the main fuselage. The right wing and aft fuselage are not depicted.
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Michael Fleming
Joseph, OR
sagriver at icloud dot com

RV-7 Slider #74572
Started 11/2016
Empennage completed 11/2016 (sans fiberglass)
Ailerons and flaps completed 3/2017.
Wings completed 12/2017
Started on QB fuselage 01/2018
Sliding canopy mostly completed 10/2020
Wiring completed
Avionics harness build in progress
Donated for 2021 and so should you
Reply With Quote
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