Re:61 Node CAN Bus

[Funny N.]

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This is a email re-send due to long delay from vector:

Greeting, Ram,
Do you have 60 nodes or just one nodes? I am bit confused here.
Removing all de-coup cap, TVS, and choke for 60 nodes are lots of work. Are you sure there is no short-circuit on all those components you removed? Or some Cap is broken which starts function like a terminal resistor.

Normally a CAN network won't function at all if you didn't put the terminal resistor on the network. However you reported your system works, this is strange. There must be something function as the terminal resistor on your hardware.

 

Funny N.
Au Group Electronics, New Bedford, MA, http://www.AuElectronics.com

[Ram-12]

I just have one Node and CAnalyzer hooked up to that node to monitor the
CAN messages. Initially it was working with all the transient
suppression circuit but as i added a external 120ohm resistor the
differential voltage dropped from 2.5V to 0.5V. Thought there would be
some hardware issue on the suppression circuit (choke + TVS + cap). So
bypassed the circuit and gave an another shot and everything seems to
work fine. I don't have an explanation for this behaviour.

Thanks
Ram

Funny NYPD wrote:

> This is a email re-send due to long delay from vector:
>
> Greeting, Ram,
> Do you have 60 nodes or just one nodes? I am bit confused here.
> Removing all de-coup cap, TVS, and choke for 60 nodes are lots of
> work. Are you sure there is no short-circuit on all those components
> you removed? Or some Cap is broken which starts function like a
> terminal resistor.
>
> Normally a CAN network won't function at all if you didn't put the
> terminal resistor on the network. However you reported your system
> works, this is strange. There must be something function as the
> terminal resistor on your hardware.
>  
> Funny N.
> Au Group Electronics, New Bedford, MA, http://www.AuElectronics.com
>

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[John Dammeyer]

Hi Ram,

My explanation for the behaviour is that there's something wrong with
your suppression circuit regardless of what you think you have schematic
wise.

The bus part of the CAN world is not rocket science.  (Steve C. can
correct me here.  <grin>)

Two nodes on the bus exhibit a very deterministic and predictable static
voltage.  If the CAN_H and CAN_L lines are not at 2.5V relative to
ground with two 120 Ohm resistors from CAN_H to CAN_L in the bus
recessive state then you have a problem.

Disconnect the TTL side of your driver from the CAN processor and force
the RX and TX pins with 4.7K resistors to VCC or GND and you can mimic
the recessive/dominant states.  Forget about distances or the number of
nodes.  If you can't create a dominant state with CAN_H approaching 4.5V
and CAN_L approaching 1.0V then you have an electrical problem on the
bus side.

If indeed removing the suppression circuit makes the voltage correct
then obviously it's a circuit (or component) problem there.

Note I've clearly stated "relative_to_ground".  That implies all your
nodes have a common ground.  If not, common mode voltages can create all
sorts of havoc.  Fix the simple things, the complicated ones often
repair themselves.

Let us know what you find.

John Dammeyer

> I just have one Node and CAnalyzer hooked up to that node to
> monitor the
> CAN messages. Initially it was working with all the transient
> suppression circuit but as i added a external 120ohm resistor the
> differential voltage dropped from 2.5V to 0.5V. Thought there
> would be
> some hardware issue on the suppression circuit (choke + TVS +
> cap). So
> bypassed the circuit and gave an another shot and everything seems to
> work fine. I don't have an explanation for this behaviour.
>
> Thanks
> Ram
>

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Subscribe and unsubscribe at www.vector-informatik.com/canlist/
Report any problems to <canlist-owner@...>