CAN Bus Repeater / Opto-isolation question

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Maybe someone can shine a light on a basic CAN bus problem. I think i'm
trying to do something that isn't working with a CAN bus , and that i'm
missing something basic about CAN.


I'm having ground loop problems with my CAN bus (probably due to
different earth potentials) So i thought of isolating all nodes on my
bus (I intend to use the Analog Devices iCoupler)

Such a device is working with normal CMOS signal levels, so normally
you would place it between the CAN-controller and the CAN-tranceiver.

As i already have some (commericial) nodes that have CAN-tranceivers
integrated, i thougth of converting it back to logic levels with
another CAN-Tranceiver. (before isolating and going back to CAN)


I made a small test, with two tranceivers, TX and RX cross-wired.
Placing both tranceivers in line with my bus. CAN coming in on one,
shifting to 5V, getting CAN out on the other (as you would to with for
example two RS485 tranceiver)

And as i was supposing, this isn't working... I think i realy skipped
something on the basics of a CAN bus.

Can anyone explain this ? I maybe thought, as the Tranceivers are just
level shifting, that this could work...

If this isn't working. Would there be another solution in isolation a
CAN device (that already has a tranceiver build in) from the bus ?

Thanks to all.


Re: CAN Bus Repeater / Opto-isolation question
Hi Jon,  You are correct in thinking that the isolation happens at the
logic side, normally a couple of opto couplers are placed between
the CAN logic levels and the CAN tranceiver.  The tranceiver
then requires an isolated power supply, either supplied through
a small DC/DC on each board or by running a common power
supply through the cable that all the boards share.  This works
because all the transceivers have to share a common ground
anyways (seperate, of course, from the board ground)
  I believe that the reason your simple converter isn't working is
that if the one side goes dominant it is past through to the other
side, which in turn passes it back again, keeping both buses
stuck in the dominant state.  There is a CiA document on CAN
physical layers that shows a block diagram of a repeater with
delays built in to keep this from happening.  If you can't find it
on their site I can send it to you directly.  There isn't much information
but maybe enough to get you going in the right direction.
  The other option is to use a small processor with two CAN
ports and transfer the data back and forth through the micro.


Steven D. Letkeman BSc.
President - Zanthic Technologies Inc.
403-526-8318 Embedded micro-controllers and CAN interfaces Automated lighting systems

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