[SI-LIST] Canbus EMC/SI questions?

• From: "Javier del Valle" <javodv@xxxxxxxx>
• To: <si-list@xxxxxxxxxxxxx>
• Date: Mon, 3 Mar 2003 19:15:25 +0100

```Hello to all,

I am a bit confused with the termination of the Canbus. I was wondering if
you experts could help me because I think that the voltage
pattern is a bit similar to LVDS.

A common Canbus voltage pattern is like this:

VOLTAGE
_________ <------------------   3.5Volts
/                   \
---------------------/                      \------------------- CAN_H
---------------------                        -------CAN_L <--  2.5 Volts
\                      /
\__________/<---------------- 1.5 Volts

_____________________________________________  TIME

RECESSIVE          DOMINANT      RECESSIVE

____________             ___________
|   Node 1       |             |   Node 2       |
| __________|             |___________|
|        |                          |         |
|        |                          |         |
|----x---- |-----------------x------|--------|
|               |                                    |            |
Ro           |                                    |           Ro
|               |                                    |            |
|----------x-----------------------x--------|

Ro is 120 Oms. I have also seen instead of using 120 Ohms the circuit below
with R1= 60 Ohms and C1=47nF.

x
|
R1
|
|----C1----GND
|
|
R1
|
x

"ISO 11898 specifies a bus cable including the two signal wires CAN_H and
CAN_L with a nominal characteristic impedance of 120 Ohms.. each bus node
shall be capable of providing a differential output voltage between 1.5 and
3.0 at a resistive load of 60 Ohms".

I understand, correct me if I am wrong, that the differential impedance that
the transceiver see at its output is 60 Ohms  (Because each node sees at its
output the parallel of 120 Ohms and 120 Ohms, then it sees the differential
impedance of 60 Ohms). Then the CANBUS cable should have a "differential"
impedance of 120 Ohms. So I am a bit confused because I think that I
should choose a Canbus cable that provides me with this 120 Ohms
differential impedance. This is different from choosing a cable in which the
CAN_H and CAN_L  lines have a Zo=120 Ohms. In fact, if I want a
differential impedance of 120Ohms then I should choose a cable that has
CAN_HZo= 60Ohms and CAN_LZo = 60 Ohms aprox.

Zdiff=2.Zo(1-k)

Am I wrong or forgetting something?

- Another thing that is confusing me is that the voltage pattern applied to
the differential pair in CANBUS is a mix of an odd and even patter.
(Vref=2.5Volts and X=1Volts)

CAN_H = Vref + X
CAN_L = Vref  - X

So if I use a cable with a differential impedance of 120 Ohms (Zo of each
one near 60Ohms) the odd voltage pattern is mached but what happen with the
even voltage??. Is it better to use a  two pairs cable (one pair for the
CAN_L and CAN_H and the other pair for the GND) and also shielding or
it is the same if I use a single pair for CAN_L and CAN_H and the shielding
of this two pair cable to GND. In my case the cables are used inside a
machine that generate much EMI. Any suggestion or recommendation
for Canbus connection?

The texas canbus transceiver SN65HVD230 for example has a the minimun
differential rise time of 25ns. Then if I use the delay rule of thumb of
5ns/m of cable, and that a line is a transmission line when Tline is > 20% of
rise time. Then when the line is more than one meter I should have in mind
trasmission line concepts. Should I have in mind this kind of things for a
canbus communication at 125Kbps-1Mbps or am I being too pesimistic?.

Sorry for hitting you with so many questions,

Thanks a lot and Best regards,

Javier

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```