How many CAN buses?

I hadn't quite realized that the CAN bus wiring diagram in the G3X install manual has the GEA 24 attached as a stub.  I'm building an AirCam, so the engines are in the rear behind the wing, while the GEA-24s are in the nose.  That's a length of about 4 meters!  The 912iS harness from Lockwood is designed that way, but now I'm guessing that's asking for CAN bus trouble?

Actually, you are not even close to trouble.  The CAN buss standards say the total buss length, including the sum of the stub lengths, cannot exceed 40 meters.  The G3X install manual is clear about wiring from the GEA-24 to the I/O and FADEC CAN, but it’s easy to miss the Garmin CAN connection. 

I'm looking at Fig 32-2.2 in the G3X install manual.  It shows the CAN bus arranged just as you said in your first reply:  terminated at both HIC A and B, with the GEA 24 off a stub.  It says, though, "CAN node length 0.3 meter maximum".  Still think my 4 meters is ok??

It would be possible, though difficult, to wire the GEA 24 with a short node length to the bus running between HIC A and B, but that would require one 4 m CAN cable running to the nose from  HIC A, and a second from HIC B.  That's what I'll do if necessary.

The node length is the stub length, not the buss length.  As I said before, the stub/node length should be limited to 12” (about .3 meters). I can’t say it would not work at 4 meters, but that would not meet the CAN spec and neither Garmin nor Rotax would not approve it.

I would do just as you said, a separate 4m cable from both HIC-A and HIC-B, then a short node stub where they meet at the GEA-24.  Same for the second engine.  

What you are up against is that a CAN buss must have a 120 ohm terminator (resistor) at each end, and there are only two ends allowed. Think of it as one long cable with terminators on each end, and node stubs in between. The CAN bus does not care where you place the ends, only that the terminators are located there. Node stubs do not get a resistor (terminator), but are limited to .3 meters in length and must lie between the two end terminators. If the stubs are any longer than specified, the buss can see the stub as part of the main buss instead of a node, and you can get more errors. Rotax already has 120 ohm terminators at each end of the buss (HIC-A, and HIC-B), so you are forced to treat the GEA-24 as a node rather than a termination. If the 120 ohm terminator could be removed from the HIC-A or HIC-B termination, then one of them could be treated as a node instead of an end termination.  Then you could install a 120 ohm terminator at the GEA-24 making it the end of the line instead of a node, and you would only need one cable.  However, I don’t see anywhere in the Rotax 912iS installation manual where you can remove the 120 ohm terminator, I suspect they are built into the ECU.  

I would be curious how Lockwood intends this connection to be done.  Is this connection accounted for in the wiring harness?  

Thanks, Jeff.  I'll ask Lockwood what they have in mind for this connection.  They don't supply a harness - just a bunch of unterminated cables, thankfully most with labels.  No circuit diagrams, either!  In any case, there's only a single CAN cable per engine running from the nose back to the engines.