Garmin CAN Bus - single point of failure?

[jdeas]

Originally Posted by KatieB

Kindof off-topic, but since we're talking CAN Bus... What exactly happens if a node is too long?

I have not worked on the Garmin implementation but in general, the speed of the CAN bus determines the maximum cable length. This is nessisary for CAN to resolve communication conflicts if more than one device tries to use the bus at the same time (auto negotiate without any loss of data).

If the wire is too long, bus conflict errors can result. Garmin most likely has some secondary error correction on top of this.

[KatieB]

Originally Posted by jdeas

I have not worked on the Garmin implementation but in general, the speed of the CAN bus determines the maximum cable length. This is nessisary for CAN to resolve communication conflicts if more than one device tries to use the bus at the same time (auto negotiate without any loss of data).

If the wire is too long, bus conflict errors can result. Garmin most likely has some secondary error correction on top of this.

If this conflict scenario happens, can it show up as an intermittent network error, or would the system just not work at all?

[N96TJ]

Originally Posted by KatieB

Kindof off-topic, but since we're talking CAN Bus... What exactly happens if a node is too long?

This is a simple question with a can of worms answer.

Longer nodes degrade the signal quality and if it degrades enough there will be transmission errors. These errors will not sneak through unnoticed, the CAN bus will detect them and tell the sending device to try again. As long as there are not too many errors you will never know there was a problem.

When you start getting too many errors then good data is not getting through, you just have a lot of failed retries. The CAN bus never gives up, it keeps trying but eventually the Garmin software says "enough, this isn't working" and gives you the "red X".

So how long is too long? The early G3X manuals specified the maximum node length at 1 meter and say "shorter is better". The latest versions say the maximum is 1 foot. I will bet there are many 1 meter nodes out there working just fine, and a few that didn't.

It is tricky because the signal integrity is effected by the entire network, not just one node. If all your nodes are short except for one you might be fine. If they are all 1 meter long then maybe not. I made all mine about 6 inches.

[KatieB]

Tom, thanks, that helps me imagine what's going on.

I work with the G3X Touch system every day at my day job. We haven't had any known problems with the CAN bus so far after 6 completed airplanes, but I know there is still a lot to learn. So, I've signed up for the G3X Touch installation class and looking forward to it very much!

[g3xpert]

Hello all,

The title of this thread is a bit off the mark... although CAN is the primary interface to connect devices in the G3X system, of course we provide the most critical components in a G3X system (namely the ADAHRS, and optionally the engine interface) with redundant mechanisms for communicating with the pilot's display screen.

The CAN network interface has been around for more than 30 years, and has been used in millions of automotive and industrial applications. If you drove a car today, you probably used a CAN bus. In a proper installation, which is easily achievable by following a set of simple principles, CAN is highly reliable and fault-tolerant.

In the latest revision of the G3X installation manual (Rev Z), the section on CAN network installation and troubleshooting (section 2.3.1.3) has been rewritten to be more user-friendly, including plenty of examples. We encourage anyone with questions about the G3X CAN bus to check it out.

- Matt

[dcollins]

In my day job as an embedded programmer I've worked with CAN bus for almost 20 years. It's exceedingly robust, and you can violate lots of rules and it still works (I've seen nasty unplanned "star" networks in piles of wires on the floor - also at a NMEA2000 "plug fest" where a dozen companies plugged into the same bus at the same time, physically strung out in a hotel ball room, I've seen huge numbers of devices communicating simultaneously). Although obviously doing it by the book is best.
At a hardware level, CAN devices track message failures and the hardware will stop attempting to transmit after a certain number of failures. Sometimes network protocols will tell you to restart the device (like ISOBUS, the CAN network for farm tractors). But that has to be a deliberate software effort. I've seen CAN networks function with missing terminators, or terminators in the middle of the network, or extra terminators. If you wire it as Garmin tells you (ISO 11898 standards), it's nearly impossible to kill it, outside of major wiring failures.

[SMO]

Originally Posted by dcollins

...... it's nearly impossible to kill it, outside of major wiring failures.

As long as you define getting one strand of the wire shield contacting the bus wire as a "major wiring failure". Since fixing that issue it has been rock solid.

[lr172]

Originally Posted by SMO

As long as you define getting one strand of the wire shield contacting the bus wire as a "major wiring failure". Since fixing that issue it has been rock solid.

I believe that just about anyone with electrical knowledge, and even most without, would consider introducing an un-specified ground into a circuit as a MAJOR wiring failure. One strand or 100 strands, a ground is a ground.

Larry

[paulmiller]

It is also called a "short circuit". Fortunately, this was not on a power cable. Building a CANBUS harness and checking the hi and lo and shield connections from node to node is quite simple. In fact, checking all your wiring for continuity and shorts is pretty simple. A short circuit is the exact opposite of the goal to isolate and insulate individual wired paths so I'd say it was more of an error by the builder and not a failure where something broke.

Yes, I've had CANBUS problems when I first started. I stripped the signal wires using 24 slot and I had 22 wire and I also had an old batch of solder sleeves that did not flow properly. it did not pass the first power-up.

CANBUS is great on the G3X when built as directed.

[bnt83]

Can you wire a CANBUS through disconnects like at a wing root or is everyone running one-piece wires from each equipment's connector?