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Garmin G3X Canbus Shield Question

J

jelliott82

Member
Hi There

Hopefully you good people can answer a quick question for me. I am currently installing a G3X system into my RV7 build and am confused about terminating the shield drain on the Canbus.

I have watched the 10 Garmin G3X wiring fundamentals videos on Youtube and they seem to show that both ends of the Canbus shield are terminated to the connector backshells at each terminated end of the Canbus which makes sense.

The question and confusion is about the shield drain at every LRU in between the terminated ends. The videos show the shield drain looped into the next piece of Canbus heading to the following LRU ... BUT the shield drain isn't terminated to the Backshall connectors at every LRU?

So the question is, is it acceptable to just loop the shield drains together at each LRU down the Canbus (except for each end device), or should the shield drains not only loop together but be terminated to every device Backshall connector?

Just want to check before I finish all my can bus wiring and get it wrong!

Thanks,
Jonathan
 
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Each node..

It appears that they want the shields tied together AND connected to the backshell at each node along the network. (ref. diagrams from Garmin G3X Installation manual rev. AL)

The real "trick" to CANbus is eliminating reflections; keep node lengths to a minimum -- ideally two wires into each pin.

Cheers!

B
 
I just recently went through this exercise with my Rocket. I started out with what the videos showed, then noticed on the drawing supplied by Stein that each CAN termination also had a shield drain to each LRU backshell. What I was told was that Garmin has revised their install guidance since the videos were produced. After a few choice words to myself I realized that adding the extra drain wire to the existing assemblies wasn’t much trouble.
 
jelliott82 said:
Hi There

Hopefully you good people can answer a quick question for me. I am currently installing a G3X system into my RV7 build and am confused about terminating the shield drain on the Canbus.

I have watched the 10 Garmin G3X wiring fundamentals videos on Youtube and they seem to show that both ends of the Canbus shield are terminated to the connector backshells at each terminated end of the Canbus which makes sense.

The question and confusion is about the shield drain at every LRU in between the terminated ends. The videos show the shield drain looped into the next piece of Canbus heading to the following LRU ... BUT the shield drain isn't terminated to the Backshall connectors at every LRU?

So the question is, is it acceptable to just loop the shield drains together at each LRU down the Canbus (except for each end device), or should the shield drains not only loop together but be terminated to every device Backshall connector?

Just want to check before I finish all my can bus wiring and get it wrong!

Thanks,
Jonathan
Click to expand...
Hello Jonathan,

Just follow the guidance in the G3X Touch Installation Manual like the core wiring diagrams on pages 27-2 and 27-3 which show the CAN bus shields connected to the connector backshell shield block at every CAN device.

This has been the recommended guidance for many years.

Thanks,
Steve
 
Thanks all, really appreciate your help ... I only need to go back and sort Backshell grounds for the elevator and aileron servos so thats not so bad. Pleased I asked!

And a final ground related question ... Unlike all other grounds/shields in the aircraft that are coming back to a ground block on the firewall, the Canbus ground/shield is a closed loop across the length of the Canbus and all LRU's connected?? ie the ground/shield for the Canbus is not connected to the main aircraft ground block on the firewall correct??
 
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jelliott82 said:
Thanks all, really appreciate your help ... I only need to go back and sort Backshell grounds for the elevator and aileron servos so thats not so bad. Pleased I asked!

And a final ground related question ... Unlike all other grounds/shields in the aircraft that are coming back to a ground block on the firewall, the Canbus ground/shield is a closed loop across the length of the Canbus and all LRU's connected?? ie the ground/shield for the Canbus is not connected to the main aircraft ground block on the firewall correct??
Click to expand...
Hello Jonathan,

All of the metal D-Sub connectors on the G3X Touch CAN bus devices are internally connected to the separate power ground leads run to each device, so in effect, everything is connected to the common aircraft ground block including the CAN bus shield.

Lots of good grounding present to keep everything quiet and at the same ground potential.

Garmin has shielded labs where we place entire wired systems and study EMI/RFI and the indirect effects of lightning (IEL). Even though grounding the CAN bus shield at each device on the CAN bus is not as common in the industry as just grounding the shield at each end, our testing found that the shielding works a little better when also grounded at each CAN device.

On one of the other threads someone suggested that one of the other systems on the market was better because it used almost no shielded wire and was likely easier and less expensive to install. Aircraft environments are not kind to electronics, and you want to do everything possible to prevent the aircraft wiring from becoming a path to negatively affect the avionics.

Our success in moving G5 and G3X Touch systems into certified aircraft and not having limitations on doing things like flying coupled ILS/LPV approaches down to minimums is due in part to the attention given to using sound practices and shielded wiring throughout the system.

Thanks,
Steve
 
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Hi Steve

Ok that is great, Thank you. Appreciate your help.

Regards,
Jonathan

g3xpert said:
Hello Jonathan,

All of the metal D-Sub connectors on the G3X Touch CAN bus devices are internally connected to the separate power ground leads run to each device, so in effect, everything is connected to the common aircraft ground block including the CAN bus shield.

Lots of good grounding present to keep everything quiet and at the same ground potential.

Garmin has shielded labs where we place entire wired systems and study EMI/RFI and the indirect effects of lightning (IEL). Even though grounding the CAN bus shield at each device on the CAN bus is not as common in the industry as just grounding the shield at each end, our testing found that the shielding works a little better when also grounded at each CAN device.

On one of the other threads someone suggested that one of the other systems on the market was better because it used almost no shielded wire and was likely easier and less expensive to install. Aircraft environments are not kind to electronics, and you want to do everything possible to prevent the aircraft wiring from becoming a path to negatively affect the avionics.

Our success in moving G5 and G3X Touch systems into certified aircraft and not having limitations on doing things like flying coupled ILS/LPV approaches down to minimums is due in part to the attention given to using sound practices and shielded wiring throughout the system.

Thanks,
Steve
Click to expand...
 
I have a somewhat related can bus question. When using the mil spec wire instead of the garmin wire 15 ohm resistors are required. My question is are they required at each LRU, or just the two terminating ends?

Jeff
 
Wichawk said:
I have a somewhat related can bus question. When using the mil spec wire instead of the garmin wire 15 ohm resistors are required. My question is are they required at each LRU, or just the two terminating ends?

Jeff
Click to expand...
Hello Jeff,

These inline resistors are only required for G5 units installed in certified aircraft.

There is no such requirement for G5/G3X installations in non-certified aircraft.

Thanks,
Steve
 
A question for G3xpert Steve...

Two years ago I installed two G5's and a GMU 11 in my certified Cessna 150. Then last year I installed a GMA 350, GTN 650, GNC 255, GTX 345, and GAD 29. I do not recall the requirement for inline resistors in the CANBUS lines. Is this a new requirement going forward or is this something that needs to be retrofitted? And in what installation document would I find this information? The only information I recall is the placement of two termination resistors at the ends of the CANBUS.
 
noelf said:
A question for G3xpert Steve...

Two years ago I installed two G5's and a GMU 11 in my certified Cessna 150. Then last year I installed a GMA 350, GTN 650, GNC 255, GTX 345, and GAD 29. I do not recall the requirement for inline resistors in the CANBUS lines. Is this a new requirement going forward or is this something that needs to be retrofitted? And in what installation document would I find this information? The only information I recall is the placement of two termination resistors at the ends of the CANBUS.
Click to expand...
Hello Noel,

That is one well equipped C150!

The current Rev. 21 G5 STC Installation Manual can be downloaded from the manuals section of the G5 certified web page.

STC Service Bulletin 1910 was issued in April 2019 regarding these additional installation instructions and contained the following compliance information. It doesn't seem that your C150 would require these changes.

COMPLIANCE
Mandatory for G5 Electronic Flight Instruments installed in single engine Class 1 aircraft certified with FIKI or planning to install a G3X system and maintain the G5 as a standby instrument. Optional for all other installations.
​

Please contact us directly if you have further questions.

Thanks,
Steve
 
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