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Multiple isolated devices and GND

jgauthier

Mar 22, 2013
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Struggled with the subject. I'll try to explain.

I have three devices. I will refer to them as A,B, and C.

Device A will power device C. This will happen through a GPIO level trigger, through some relays etc.
Device B needs to communicate to/from Device C via TTL level UART comms. Minimally, that is 3 lines. TX,RX, GND.

However, device C GND will come through the power switching circuit through Device A.

Should I ultimately connect A and B GND together? B is not my device, and "untrusted".

Thanks for the input.
 

crutschow

May 7, 2021
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If the modules sent signals to each other and/or share a power supply, then they all need to share a common ground.
 

jgauthier

Mar 22, 2013
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Good point of clarity. Devices A, and C, will definitely share a power supply.
Device B does not share a power supply with Device A nor C, but needs to communicate via UART to C.

This then create a common ground between A and B, which have no reason to be connected. I just wanted to make sure that was going to be alright.
 

jgauthier

Mar 22, 2013
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Thanks for your feedback. That was my initial plan, but I wanted to get a second opinion.
 

Harald Kapp

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Nov 17, 2011
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I see no problem with connecting all the common grounds together.
When device B is otherwise totally isolated from A and C, there may (or may not) be issues with connected grounds. It depends on the specific configuration, the devices, clock frequencies involved, length of interconnect etc.
Should you encounter issues with a common grond for A, B and C, you can always use an isolated UART connection using e.g. optocouplers or magnetically coupled interface ICs. Lots of circuits available on the net.
 
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