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Look at the ground wires connected to the POSITIVE terminal output.I have a need to read the signals being sent between two boards in a lighting fixture.

In preparing to do this I have read much about the chips, RS485, and analysers/decoders that are out there. I am getting close to purchasing a USB to RS485 adapter and software to use it with - but have been unable to find critical information specific to my situation:

My desktop computer is negative grounded and the device is positive grounded.

I don't have a laptop that can be used for this purpose (which I have read could be used if it wasn't plugged into mains when connected to the fixture.)

Stuck with just the desktop to use, it seems to mean I'll just short it all out if I connect the grounds - possibly damaging the fixture, my desktop, the adapter, or any combo of these.

How do you properly set up for reading the signals with a -GND desktop and the +GND device?

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  • \$\begingroup\$ There are isolated RS-485 to USB adapters. Out of curiosity on my end, is there a datasheet or manual for the positive ground device? I've never seen one before. \$\endgroup\$ – DKNguyen Feb 20 at 21:09
  • \$\begingroup\$ Your question is unclear and a schematic would help. Are you saying that these two devices are powered from the same power supply but that the device has its positive supply connect grounded? This seems most unlikely for anything more modern than an early transistor radio. You can add a schematic using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. \$\endgroup\$ – Transistor Feb 20 at 21:13
  • \$\begingroup\$ @Transistor Um, sorry but how? The desktop is grounded internally like pretty much any desktop is - negative grounded, and the fixture is grounded to the positive DC output AND actual earth, positive grounded. They are two distinct items, and I'm going to connect them. What can I add to make it more clear? \$\endgroup\$ – 111936 Feb 20 at 21:27
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    \$\begingroup\$ Those datasheets in your link...those are chips on the PCB? You listed two RS485 transceivers. One isolated and one not. If your board is positive grounded it would make sense that the designer built in the ability to use RS-485 without needing an isolated RS-485 transceiver since they would run into the same issue you are running into. They didn't use an expensive isolated RS-485 converter with built-in isolated power supply for no reason. \$\endgroup\$ – DKNguyen Feb 20 at 22:20
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    \$\begingroup\$ Furthermore, the fact they have a second non-isolated one elsewhere suggests it's safe to connect or isolated some other way. Your drawings indicate there are isolators right beside the unisolated RS-485 transceiver so it's possible they are isolating that transceiver as well, but only you can tell for sure via testing electrical connections. BTW RS-485 does not always need GND connected though it's good practice if your transceivers are not isolated. But in your case one definitely is and the second looks like it may be so you wouldn't connect grounds when using the RS485 if they are. \$\endgroup\$ – DKNguyen Feb 20 at 22:21
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There are isolated USB-RS-485 converters you can buy. They allow data flow while keeping the grounds (indeed, all signals) on both sides galvanically isolatd from each other. They have their limits to how much isolation they can provide between both ends though (usually limited to ONLY a few hundred or thousands of volts).

If the adapter requires a power input on both sides, ensure that the respective power supplies used are also isolated (like a wall-wart with a transformer inside and the like), especially on the end your your device is on or else you'll run into the same issue as before.

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  • \$\begingroup\$ As written, it reads like a comment. Please rephrase and add more info to clarify that this fully addresses the issue? Future readers would appreciate reading WHY it is that simple to address the problem of differing grounds, I'd think. We noobs appreciate it whenever answerers get verbose! \$\endgroup\$ – 111936 Feb 20 at 21:35
  • \$\begingroup\$ As requested... \$\endgroup\$ – DKNguyen Feb 20 at 21:37

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