How many CAN buses?

CAN 1 from each HIC connector goes to the engine module. However, the GEA-24 only has one input for the Rotax FADEC CAN, whereas the GEA-24B has separate discrete inputs for the HIC-A and HIC-B CAN buss wiring. Both configurations are shown in the G3X installation manual in the wiring examples section.  For the GEA-24 (only one FADEC CAN input) think of CAN buss 1 as a continuous loop between the HIC-A and HIC-B CAN buss terminals. In the middle of that loop, you create a stub that connects to the Engine Interface module. Ideally the stub you create should not exceed 12” (30 CM) in length.  If you are not familiar with creating a CAN buss stub, there are some good videos on YouTube.  Also, you can search “CAN buss architecture” and you will find a lot of information on the internet.  

CAN 2 from each HIC connector goes to a separate SUB-D-DE9 connector, which will be your maintenance ports for connecting the BUDS dongle and software.  Typically each lane has its own SUB-DE9 connector (one originates from each HIC connector).  The connector pin outs for creating two separate maintenance ports are in the 912iS installation manual. See section 77-00-00, “CAN Interfaces” starting on page 15 of that section.  

There is also a separate Garmin CAN that connects specific Garmin Modules and will be needed to connect the GEA-24 to the GDU-460 (or similar).  On the GEA-24 pin out list this is referred to as CAN, and the Rotax CAN input is referred to as FADEC-CAN-2.  

Hi Jeff

Can you comment on "twisted pairs" for readers.  It might help some. I have found it helps. 

Cheers

RW, I’m happy to comment on paired cables. 

There are several types of paired wires:  Unshielded untwisted pair, unshielded twisted pair, shielded untwisted pair, and shielded twisted pair.  There are also different shield types, braided and foil.  Adding twists to a wire pair reduces the effect of electromagnetic radiation from outside sources, and also reduces cross-talk from other wire pairs in the same cable sheath.  The number of twists in the cable also helps determine its data speed because cross-talk is more prevalent at higher transmission frequencies.  

CAN buss cable is a 120 ohm two wire cable with a braided shield and a mild number of twists. Because a CAN buss is a single pair, cross-talk with other pairs is not an issue.  In particular, Garmin approves a cable called Gigaflight CAN24.  When working with this cable to construct the node stubs and terminations, it’s important the shield integrity is maintained and there are no shorts from wire to wire or wire to shield. The shield must be grounded at one end as a minimum, and preferably at both ends and each node between. The only problem with this cable is the wire insulation is a bit thicker, making it difficult to use a pin extraction tool once the termination pin is inserted into the connector body.

Garmin has a great YouTube series on cabling basics.  I recommend watching the entire series (in order) if you are doing your own wiring, but the two below will show you how to construct a CAN node. Watch them in the order as posted below.  

https://www.youtube.com/watch?v=jmVLVhGoGQs

https://www.youtube.com/watch?v=xiwlaF3tdOM&t=15s

Jeff...thanks.  I had it all explained a few years ago to me by Michal Stock, Stock Flight Systems. (I am not so great at electrical, it is my weak area I admit)  We had an issue with the early release of the iS with the single maintenance port type.  Shortly after that Rotax changed and said the single port was not a good idea and wanted to use the dual port.  To read they said a splitter was fine but they did not want to terminate them within the instrument for some reason.  Seems with the BUDS dongle connected via a splitter was not an issue.  

i will check with RS Flight Systems and see if there is any recommendations he might note as he did talk about twisted pairs being used.

Thanks for the links.

Cheers

Just a heads up, a bit off topic however, RS Flight Systems should be releasing their MOSFET regulator very soon.  I am looking forward to that very much.  (lose the diodes and replace with transistors) Should stop most major stator and wire failures that occur in some aircraft types. I have seen the prototype in operation for several  years now and seems to work great. 

The maintenance port architecture is interesting.  Each port alone is not a functioning CAN buss.  There is a 120 ohm terminator on each port (at the ECU) and it’s not until they are joined together at the BUDS adapter that the buss is complete. At that point the 120 ohm terminators on each port are connected together giving the completed buss the required 60 ohm resistance. In other words, the BUDS adapter completes the buss. So it’s really not two maintenance CAN busses, rather they are two ends of the same buss.   At least that’s the way I understand it.  If you measure the resistance between the CAN LO and CAN HI on either port without the BUDS dongle plugged in you will get 120 ohms, which becomes 60 Ohms once connected together.  

I connected to a new RV12 912iS on Sunday that had a single physical maintenance port.  In other words, a single DB9 connector.  In this case six of the nine pins on a single DB9 connector are used, instead of three pins on each of two DB9 connectors. It works the same in that the buss is not completed until the BUDS adapter is pugged in.  The only difference is that you don’t need the “Y” cable. 

I’m looking forward to being an early customer for the MOSFET regulator.  Thanks for the update.