Multiple isolated devices and GND - Page 1
Forum section: electronics / projects

author: jgauthier
author_rank: Newbie
timestamp: August 10, 2022, 02:00:41 pm
content: 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.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 11, 2022, 01:12:11 pm
content: B and C will be possibly connected so you don't have to make one more connection between A and B. The less ground connections - the better.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 11, 2022, 02:25:50 pm
content: Untrusted in what ? network, operating system ?you can power them thru isolated power supplies instead of isolating connections if they became at same ground levels when connected / communicating ... you have fully isolated dc-dc converters ??Normally the ground is the common point in many things in communication, but not necessarily connected to the psu linesare they automotive or industrial ??define device a will power b thru gpio ?? is it an raspberry ?i think you do know you can have usb / rs232 / ttl or cmos isolators thru opto couplers or specialized isolator ic's

author: Terry Bites
author_rank: Super Contributor
timestamp: August 11, 2022, 06:44:22 pm
content: If you power something with iso power then you have to communcate with it over isolated lines. If not, the power isolation serves no purpose.If you dont trust it then really isolate it.Digital isolation is simple- optos are slow but things like iCouplers are fast and simple to use. You'll find made for purpose bidirectional versions for coms.https://www.analog.com/en/parametricsearch/11037#/https://www.ti.com/isolation/digital-isolators/products.htmlYou want to add protection on lines within the iso side circuit. ie signal to iso gnd and iso power.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 12, 2022, 05:52:10 am
content: Quote from: Terry Bites on August 11, 2022, 06:44:22 pmThis.A lot of even senior engineers don't get this, so I'm going to say it again.If you cross a "galvanically isolated" barrier withabsolutely anythingother than a galvanically isolated signal then you haveno isolationat all. None. Nothing. What you have might or might not be useful, but it ain't isolation anymore. That's why best practice for isolated barriers on PCBs is to pull back solder mask so you can see clear through the PCB and verify immediately with your own eyes that only isolators ever make the crossing.

author: jgauthier
author_rank: Newbie
timestamp: August 12, 2022, 12:57:00 pm
content: Coromonadlix> Untrusted device. As in: It's not mine. It's someone else's. Untrusted is just a term - I mean I trust them. But it's not a known device, not my design, and not provided by me. So I call it "untrusted".Since A and C are powered through the same PSU, and B is not - if I use GND for comms between B and C it's then connected to A.Which I felt like was dangerous waters.I have like 12V power source. A is powered through a 3.3V regulator or DC/DC converter.Device C is a 9V system.When I say A will power C I mean A will trigger a GPIO (not a pi) that will flip a relay, that will power device C from the 12V source.I have recently read about some possible isolation ICs and I am digging into those.Terry Bites>Thank you. That was my gut feeling. I will look at the poroducts you mentioned.exmadscientist> Thank you! This exactly why I asked the question. I was putting this down and when I saw the GNDs my brain went "Hm, nope. This is going to bind A and B and I do not want that"