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CAN Bus-Identification

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Sathiesh Kumar

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Hi,

I have a doubt whether CAN Bus is a normal wire which can be used with CAN Protocol or whether its a specially designed bus...

Can any one help me...

Sathiesh Kumar.V
 

Papabravo

Well-Known Member
CAN can use any kind of wire you like. It is only if you want to push the limits of the technology that special purpose cabling is used. In the special purpose network cables used for CAN, the cables are characterized and the values are consistent from one spool to the next.
 
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Oznog

Active Member
CANbus is by definition a differential pair, essentially two wires plus ground. They generally need to be a twisted pair.

Assuming this is PIC, the PIC's CANbus implementation requires a simple CAN transceiver chip. You might think you can take the CAN Tx out of one PIC and feed it into the CAN Rx of another chip and vice versa, but no, that's impossible. Unlike the UART, the CAN protocol will not support this type of connection.
 
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Papabravo

Well-Known Member
CANbus is by definition a differential pair, essentially two wires plus ground. They generally need to be a twisted pair.

Assuming this is PIC, the PIC's CANbus implementation requires a simple CAN transceiver chip. You might think you can take the CAN Tx out of one PIC and feed it into the CAN Rx of another chip and vice versa, but no, that's impossible. Unlike the UART, the CAN protocol will not support this type of connection.
Some CAN controllers may be connected together on a single board, because they have differential inputs and outputs. Examples are the 82C200 and the SJA1000. The OP's question was about the wire. I have used untwisted hookup wire and speaker cable to make "short" networks. For installations we used specialized cables with foam dielectric, individually foil shielded, with a drain wire, and an outer braid jacket.
 

Oznog

Active Member
What are you trying to connect? A PIC with a CANbus port, or CAN controllers, or what?
 

NHN

New Member
I've used some basic .35-.5mm electrical wiring for canbus in my car mate, cat5 stranded network wiring will do I should think no?
 

Papabravo

Well-Known Member
I've used some basic .35-.5mm electrical wiring for canbus in my car mate, cat5 stranded network wiring will do I should think no?
Almost anything will work for a short network. On a long network there are at least two things to watch out for.
  1. Transmit levels for a dominat bit vs. Receiver thresholds for a dominant bit
  2. Reflections at high baudrates affecting the voltage level at the sample point.
 
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Oznog

Active Member
CANbus should be wired in a line, with no "star" branch connections. And the ends must be terminated with a specified resistance to avoid causing reflections back down the bus.
 

Papabravo

Well-Known Member
CANbus should be wired in a line, with no "star" branch connections. And the ends must be terminated with a specified resistance to avoid causing reflections back down the bus.
Mostly nonsense. Trunkline/dropline is a perfectly reasonable toplogy for a "long" network of say 500 meters @ 125KBaud with drops up to 3 meters. On a very short(<1Meter) network you want only a single terminator equal to the parallel combination of the terminators you would use on the long network, since reflections are no problem at all.
 
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Oznog

Active Member
I'm just going by what is stated in the CANbus specification. I do distinctly recall that branches were a no-no (but that's not saying it'll always be a problem), and that the end of the string needed termination.
 

atferrari

Well-Known Member
In real circuits I hade the chance to see, short stubs are common. I personally understand that "adding" nodes would be impractical if the "line" should be extended for every newcomer to the already working circuit.

I admit my lack of practical experience other than having worked out a running CAN network with three PICs on my bench.
 
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