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CAN BUS

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user9253

Joined: 28 Mar 2008
Posts: 1935
Location: Riley TWP Michigan

Posted: Wed Sep 04, 2019 6:26 am Post subject: CAN BUS

The Rotax 912iS engine outputs engine parameter data via CAN Bus A and 

via CAN Bus B.  The SkyView System Installation Guide - Revision AE 

https://www.dynonavionics.com/includes/guides/SkyView_System_Installation_Guide-Rev_AE_v15_4.pdf

on page 7-37 has a wiring diagram showing SV-EMS-

221 pins 36 and 37 connected to both CAN Bus A and CAN Bus B wired in 

parallel.  I know nothing about a CAN Bus.  But it seems to me that if Rotax 

had intended for both CAN Buses to be connected in parallel, that they would 

have provided only one CAN Bus instead of two.  Can someone explain why it 

is OK to connect the two Rotax CAN Buses in parallel?  Thanks

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_________________
Joe Gores

gilles(at)elixir-aircraft
Guest

Posted: Wed Sep 04, 2019 7:10 am Post subject: CAN BUS

Le 04/09/2019 à 16:26, user9253 a écrit :

 	  Quote: 	
		   

   and 37 connected to both CAN Bus A and CAN Bus B wired in

 parallel.  I know nothing about a CAN Bus.  But it seems to me that if Rotax

 had intended for both CAN Buses to be connected in parallel, that they would

 have provided only one CAN Bus instead of two.  Can someone explain why it

 is OK to connect the two Rotax CAN Buses in parallel?  Thanks



Joe and all,
	




Two years ago I had the opportunity to design the wiring for a 912iS 

equipped aircraft.

Here is what I was told by a person having participated in the design of 

the Rotax Can Bus system :



- Rotax did not design their CAN Bus systems, but outsourced the design.



- At the beginning Rotax advised it was OK to connect both CAN Buses, so 

Garmin and other avionics manufacturers chose this path.



- Somewhere along the line Rotax was instructed about the disadvantages 

of connecting both buses, and so changed its installation instructions.



- Garmin and other manufacturers didn't change the desing of their 

products, keeping the both-connected approach.



FWIW,

-- 

Best regards,

Gilles

http://contrails.free.fr

http://lapierre.skunkworks.free.fr

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Sep 04, 2019 7:43 am Post subject: CAN BUS

 	  Quote: 	
		  Can someone explain why it 

 is OK to connect the two Rotax CAN Buses in parallel?  Thanks



 --------

 Joe Gores

 	


   The CAN bus has been around for a very

   long time. See:



  https://tinyurl.com/lc2mygs



   Originally crafted for the automotive

   world, it has proven to offer great

   utility just about everywhere lots

   of electro-whizzies need to talk to

   each other on a 'party line' bus.



   Not all CAN busses are the same.

   Each whizzy needs to be 'educated' as

   to the protocol for transmitting and

   receiving messages/data while behaving

   in a neighborly fashion with respect

   to 'collisions' on the bus.



   So the idea that two otherwise independent

   CAN systems can be tied together is

   not far fetched as long as every

   whizzy on BOTH busses share the same

   messaging protocols.



   For example, you might purchase an

   oil pressure instrument originally

   intended for service in a over-the-road

   truck with an expectation that it

   would accurately display oil pressure

   on your engine. 



   Yeah . . . maybe . . . assuming the bus

   messaging protocols are the same. Then

   there are risks that more than one 'talker'

   with the same message i.d. would

   promulgate confusion amongst 'listeners'

   having an interest in those messages.



   Proceed with caution.



 

   Bob . . .

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glastar(at)gmx.net
Guest

Posted: Wed Sep 04, 2019 7:53 am Post subject: CAN BUS

Hi Joe,



the 912iS has two ECU's each of em having its own CAN  connection. Now

as the Dynon EMS 221 has only one CAN Bus interface you need to hook up

both in parallel, the CAN Bus "packet ID" will tell which ECU it's

coming from as it is a "broadcast" protocol all packets should arrive at

the 221, if you want to have more redundancy you might be able to hook

up two EMS 221 in order to have both ECU's on different engine boxes

(check with Dynon).



As it is a bus it will work with one EMS only.



Hope it helps a bit to understand.



Cheers Werner



On 04.09.2019 16:26, user9253 wrote:

 	  Quote: 	
		   



 The Rotax 912iS engine outputs engine parameter data via CAN Bus A and

 via CAN Bus B.  The SkyView System Installation Guide - Revision AE

 https://www.dynonavionics.com/includes/guides/SkyView_HDX_Pilots_User_Guide-Rev_D_v15_4.pdf

 on page 7-37 has a wiring diagram showing SV-EMS-

 221 pins 36 and 37 connected to both CAN Bus A and CAN Bus B wired in

 parallel.  I know nothing about a CAN Bus.  But it seems to me that if Rotax

 had intended for both CAN Buses to be connected in parallel, that they would

 have provided only one CAN Bus instead of two.  Can someone explain why it

 is OK to connect the two Rotax CAN Buses in parallel?  Thanks



 --------

 Joe Gores





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