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Why two CAN channels are connected to each other?

jani12

Member
Please consider steering electronic control unit (ECU). It has 6-pin CAN connector.
CAN L pin is connected to another CAN L pin in this 6-pin connector.
CAN H pin is connected to another CAN H pin in this 6-pin connector.

What are the reasons for above connections?
 

jani12

Member
That was previously answered, the last time you asked it.
Last time I asked are multiple CAN ports in a ECU connected to same CAN network. I didn't ask if CAN ports in same ECU are connected to each other.

Another possible reason is redundancy
This makes sense to me. This is probably the reason. There are two wires connected to CAN Transceiver CAN L pin and two wires connected to CAN Transceiver CAN H pin. For example, if one of the CAN L wire is cut, CAN communication will be unaffected because you have the other CAN L wire.
The question is how will be system detect that one of the wires is open circuit so low severity diagnostic can be raised. I say low severity because you still have the other CAN L wire.

Also, is there some standard or Functional Safety requirement this wiring configuration or two CAN ports in a Electrical Power Steering (EPS) connected to each other.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Last time I asked are multiple CAN ports in a ECU connected to same CAN network. I didn't ask if CAN ports in same ECU are connected to each other.
???? Same thing? Two ports connected to the same network must also be connected to each other!

If I were designing that type of setup, I'd alternately send on each port and regularly verify the received data from each matched.

No idea on regulations - I work mainly with industrial automation and machine tool control systems etc., it's just a reasonable guess that there would be some form of twin-circuit type failsafe system on controls that could otherwise affect safety in a fault condition.

Same concept as high risk machine "Emergency stop" circuits: Everything has to be double circuit and self-checking, so any single fault does not compromise the safety system and can be detected, preventing a restart until it's fixed.
 

jani12

Member
???? Same thing? Two ports connected to the same network must also be connected to each other!
I agree. I didn't clear state my questions. On March 26, 2022, when I asked this question, I meant each CAN port in the ECU is connected to it's own CAN Transceiver.
In this question, both CAN ports are connected to same CAN Transceiver as follows:

1654364417777.png
 

Diver300

Well-Known Member
Most Helpful Member
I agree. I didn't clear state my questions. On March 26, 2022, when I asked this question, I meant each CAN port in the ECU is connected to it's own CAN Transceiver.
In this question, both CAN ports are connected to same CAN Transceiver as follows:

View attachment 137345
That is done to reduce the need for splices in the wiring loom. Splices in the wiring loom can be very labour intensive to fit. If each wire goes from one module to the next, that eliminates the need for a spice within the wiring loom and leaves only the connector terminals.

Car manufacturers take that to extremes. I recently fixed my daughter's Fiat Cinquecento, where the stress of bending the loom as it passes into the tailgate was too much.

The tailgate contains:-
Rear screen heater
Rear wiper
Tailgate lock release
Switch that detects if the tailgate is locked
Assembly containing rear numberplate light and tailgate release button

So five devices requiring an earth wire.

Each of those had a separate wire routed through the join between the tailgate and the body of the car. I assume that within body of the car they all went to a single ground point.

In spite of the fact that the rear screen heater wire was several times the cross section of the others, all those extra wires were cheaper than assembling a splice, or even earthing the tailgate metal and the devices being earthed to that.
 

Diver300

Well-Known Member
Most Helpful Member
Is splice not needed because adding a second wire to a signal like CAN Low increases the length of the wire?
It's not length, it's the number of connections.

Assuming that there is a wire from A to B that this module has to be added to, the method with a splice would be:-

1. Wire from A to splice
2 Wire from B to splice
3 Wire from splice to module.

If there are two connections on the module, then it will be wired:-

1. Wire from A to module
2 Wire from B to module.

Obviously that would apply to both CAN-H and CAN-L
 

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