Problem of Common Ground wile networking two Motor Control PCBs

[Srikrishna]

Hi!!

My goal is to network two Motor Control PCBs through the CAN protocol.
The problem here is regarding Ground. Is it that all nodes on the CAN bus need to have a common ground?? If is..what is the effect of the common ground on the CAN transceivers present on each board...

 

[ckshivaram]

CAN is a distributed network so there is no possibility of making a common ground.. even the physical layer in can does not have ground....

we cannot run common ground lines if the nodes are kept far from each other... not only in CAN but in any distrbuted network the concept of common groung does not exist.

 

[Srikrishna]

Re: Problem of Common Ground while networking two Motor Control PCBs

Hmm...but assume that each PCB's power supply is different (which is...in my case)..which means that the ground of each board is different....so when we connect them...it is quite obvious that due to ground loop problem...some kind of noise will be generated..which may affect the transmission..isnt it???

 

[ckshivaram]

No... you did not understand my reply.... the basics is in distributed netork protocols like CAN is there is no common ground policy.... the physical layer should define the common ground, which is not defined.. thats the reason you use differential bus in CAN... Please go through CAN SPECs.........

 

[Srikrishna]

I'm using DB9 connector to connect them..and only two pins among the 9 are used for CANH and CANL..so what I get from ur rply is that I need not worry about grounding any of the remaining 7 pins..is it??

 

[ckshivaram]

yes CAN bus does not have a ground line like your UART..............

 

[Srikrishna]

Sir..one final question..as I'm a beginner...so a bit confusing...



here is the image showing our problem.....
there r different power supplies and hence different ground..of MCBs...
ur above post says that there is no common ground policy in CAN..
according to that...this configuration should work for networking without connecting the three parts( shown in the figure)...so..it means..i need to connect only CANH and CANL pins between the parts shown..
is it like this???