CAN Bus Repeater / Opto-isolation question

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.

prerequisite:

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)

Problem:

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.

Jon

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.

Steve

Steven D. Letkeman BSc. President - Zanthic Technologies Inc.

403-526-8318

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