G3X and the CAN

Very excited to get my new G3X Touch installed and running.

Found I was getting intermittent Network Errors on several of the LRU's. Clearly I had a CAN bus problem.

My setup was a string starting at the GSU25, GDU460 etc. etc. then finally a GSA28. Following the detailed guidance in the install manual, I terminated the GSU25 with the Plugin terminator, and the GSA28 with the strap from pins 3 and 4. All the nodes were constructed in the connectors and were no longer than about 2". Should be all good, no such luck.

After much investigation it would appear that one of the terminators must be performed by a GDU. The interconnect diagram does show the GDU terminated but my assumption that this is just the way the diagram was drawn.

After removing the terminator off the GSU25 and terminating the GDU460 leaving a node of about 6", all seems ok, no more network errors.

I wasn't able to find anywhere in the install document indicating a requirement that the GDU must be a terminator.
Am I missing something in the document?

Newbie here too.

We just got ours turned on a few days ago. I don't know all I should, but we did not used the GDU at the end. We elected to used the Auto-pilot servos as the two ends of the buss as they are good for that with no need to jumper anything other than for roll, pitch or yaw. I would send an E-mail to the G3X guys. They are nice to work with and care about your problem.
Yours as always. R.E.A. III #80888

The CAN bus does not need to be terminated at a GDU, sounds like you have some other issue going on. Here are some CAN bus checks you can look at, complimnents of the G3X team:

Here are our general CAN bus troubleshooting tips.

1. Make sure that the CAN bus is daisy-chained from CAN device to CAN device around the system and that CAN devices are not connected via a single point like a star or routed through a hub device. This can cause unwanted signal reflections and “orphan” some devices on the bus and prevent their communication.
2. Make sure the CAN bus is terminated only at 2 places and only at the extreme ends of the CAN bus.
3. With power removed, remove the connector and meter across the pins associated with CAN Hi and CAN Lo for any device on the CAN bus for which the bus is not terminated and verify it reads 60 ohms. This will verify that the CAN backbone is properly terminated at each end (two 120 ohm terminators in parallel).
4. Verify that CAN Hi and CAN Lo signals are not crossed on any connector and that neither signal is shorted to ground which can happen when shorted to the CAN shield braid.
5. Power up only PFD1 and each of the single CAN devices at once and verify that communication is strong (0% CAN error rate on config page with device highlighted). Sometimes a device will communicate with PFD1 when it is the only powered device on the CAN bus when some of the above issues are present.
6. Verify that a fully powered and operating system has a steady state 0% CAN error rate for each device as shown on the config page.

At SteinAir, we've been using the autopilot servos as the terminating ends of the CAN backbone for a while now and it's been fine. This pretty much eliminates "out and back" backbone runs from devices mounted away from the main panel. When the G3X was new, 1 meter was the maximum node length but Garmin advised us later on that was causing problems. Now we run the backbone to each device and just drop a 2-3" node and move on. We occasionally have CAN problems on first power-up but it is almost always a mis-wire or a short to the shield (which is why we test). I had a bad GSU-25 AHRS (verified by substitution) but that is the only time a unit has caused us a CAN problem.

I assume "terminating the CAN bus at the autopilot servos" is when the roll servo is in the wing.

When the roll servo is under the pilot seat it would be logical to have it attach to the CAN backbone as the feed passes thru the fuselage on its way to terminate at the pitch servo.

The only G3X CAN Bus device that a termination cannot be made at is the GI-260. I've attached some slides from the Aircraft Electronics Association G3X Touch Installation class that may help with some can questions.

Slide 7 shows how we typically do a CAN bus node. This keeps the node length down to a few inches. (Note: we normally leave them a little bit longer, they were cut short for purposes of the picture)
Slide 10 shows how we typically terminate a CAN bus at a display.
Slide 21 shows the CAN bus routing in one of our company airplanes.

CAN Terminator plugs can be used at the GEA24, GAD29 and GSU25.
A wired termination can be made at GDU37X, 4XX, and GSU73 units by jumpering the CAN Lo to a termination pin
A wired termination can be made at the GSA28, GTR20/200 by jumpering two pins.

Hope this helps.

When I fired up my G3X system last month for the first time, I noticed a problem with the autopilot pitch servo not updating its firmware. I placed a call to the G3X boys at Garmin, and worked through the problem via telephone. As it turns out, I had inadvertently reversed the Canbus wiring on the leg between the roll and pitch servos (the roll and pitch servos are the last two devices in the chain in my setup, with the Canbus terminations at the GSU on one end, and the pitch servo on the other.) During the troubleshooting process, we also discovered that the bus resistance was 60 ohms, when it should have been 120 ohms. Although this did not effect the operation of the bus once the reversed wiring was corrected, it was not a normal condition. When we removed the terminator from the GSU, the bus resistance read the normal 120 ohms - which should not happen. Then it clicked: I had been missing one terminator when I made the GSU connections, so I had assumed that I had misplaced it somewhere in my multiple electronic parts boxes. However, I'd forgotten that I had preinstalled a terminator on the GSU prior to bolting it onto the longeron, and in its installed position, you cannot see the connectors on the GSU. Yup, I had installed two terminators on the GSU. Yea, I felt dumb.....

Reviving an old thread here. I saw a couple of people above mention terminating the CAN bus on each end at the AP servos. Where are you mounting the GMU 11 in these installations? I've seen people putting it in a wingtip or in the tail, and in either of those locations it seems to be a more logical CAN termination point than the AP servo?

David, yes, that is what I did in my last installation, the GMU 11 was in the right wing tip and the roll servo in the left wing so I used the GSA 28 roll servo and the GMU 11 as the terminating LRUs.

The GMU11 has only been availalbe about a year, before that the GMU22 did not connect to the CAN bus so the AP servos were the logical termination points.

Walt said:

The GMU11 has only been availalbe about a year, before that the GMU22 did not connect to the CAN bus so the AP servos were the logical termination points.

Click to expand...

Walt, thanks for that context. It explains a lot!

Slides

avionicsr said:

The only G3X CAN Bus device that a termination cannot be made at is the GI-260. I've attached some slides from the Aircraft Electronics Association G3X Touch Installation class that may help with some can questions.

Slide 7 shows how we typically do a CAN bus node. This keeps the node length down to a few inches. (Note: we normally leave them a little bit longer, they were cut short for purposes of the picture)
Slide 10 shows how we typically terminate a CAN bus at a display.
Slide 21 shows the CAN bus routing in one of our company airplanes.

CAN Terminator plugs can be used at the GEA24, GAD29 and GSU25.
A wired termination can be made at GDU37X, 4XX, and GSU73 units by jumpering the CAN Lo to a termination pin
A wired termination can be made at the GSA28, GTR20/200 by jumpering two pins.

Hope this helps.

Click to expand...

Pahan,

The slides are no longer available. Any chance you could fix the links?

Thanks,
Michael-