Grounding and common-mode in CAN and RS485

Thread Starter

electrosn

Joined Jun 9, 2022
14
Hi,

I am trying to design a system where I have to communicate two nodes, using CAN or RS485, which are powered from isolated supplies. The node grounds cannot be tied together.

1700134898190.png

Supposing I don't use isolated transceivers, is it okay to only transmit using the two differential wires of the RS485/CAN? Since the ground of transceiver #1 is different from the one of transceiver #2, is there any risk of the voltages coming from #1 not being within the common-mode rating of the transceivers of #2, or the other way around? Will EMI be a problem?

I know that using isolated transceivers would solve this problem (since I could then connect the isolated bus ground of both transceivers), but I am trying to have a better understanding of the influence of non-isolated transceivers in isolated systems.
 

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nsaspook

Joined Aug 27, 2009
12,796
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Thread Starter

electrosn

Joined Jun 9, 2022
14
You ask the question knowing the answer.

The common/ground connection is required by the CAN standard with non-isolated transceivers. Will it work in some conditions, maybe. Should you design a out of spec system, no unless you want a crapplication (a nice name for bad engineering), because of the reasons you listed.
https://www.analog.com/en/products/adm3053.html#product-overview
View attachment 307683

https://www.ti.com/lit/an/slla268/slla268.pdf?ts=1700120020064
Yeah, I more or less understand what is the right approach for this, but I am also trying to understand what happens in this situation.

Since the systems are initially isolated but are now connected through the non-isolated 485 bus, what is the common mode voltage seen by the transceivers of one of the systems? Are GND#1 and GND#2 now somewhat shifting in relation to each other?
 

nsaspook

Joined Aug 27, 2009
12,796
Yeah, I more or less understand what is the right approach for this, but I am also trying to understand what happens in this situation.

Since the systems are initially isolated but are now connected through the non-isolated 485 bus, what is the common mode voltage seen by the transceivers of one of the systems? Are GND#1 and GND#2 now somewhat shifting in relation to each other?
It's really impossible to know in a real world environment without a solid connection between GND#1 and GND#2 and could change at any time from working perfectly to smoking transceivers. The TI link explains.
 
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