Does a UART connection require shared ground?

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I’m a novice with electronics but not Linux. This situation has had me scratching my head for two nights.

I connected a Pi Pico to a Raspberry Pi 4 via UART, and managed to receive messages sent from the Pico on the Pi successfully. The Pico was connected to my PC via USB; the Pi was connected to a power supply. Then I busied myself with other parts of the project. When I came back, and for many hours, I simply could not get the serial connection to work. Either I received serial ‘noise’ in picocom, or nothing at all.

Finally, I supplied juice to the Pico directly through the Pi via the Pi’s GPIO, and disconnected the Pico from my PC. Lo and behold, the UART worked faultlessly!

Does a UART connection need shared ground? It worked before, but perhaps that was a fluke? If I hadn’t seen it work earlier, I wouldn’t have spent hours trying to replicate the exact conditions I’d had before to get it working again!

I admit I don’t really understand the principle of shared ground in the first place.serialuartpi-picoShareEditFollowCloseFlagasked 9 hours agolowercasename17155 bronze badges

• 1Try your original configuration with a ground connection between Pi and Pico – because yes, you should have a common ground – Bravo 8 hours ago
• Ah, (1) I think if you optically isolate the Tx/Rx signals of each side (eg. Rpi and Pico), then you don’t need any common ground at all. (2) Of course Tx/Rx each side has a local common ground, but the two local grounds need not be common, ie, shorted or connected together. (3) Actually for industrial/noisy and/or long wire/distance communication, it is a good idea to totally optically isolate all signals. (4) Eg if Rpi and Pico are powered by two separate 12V/24V stepped down to 5V) Lipo power banks, then Rpi and Pcio local ground need not be directly or indirectly connected. – tlfong01 11 mins ago   
• (5) You might like to look at the schematic diagram of the short answer part of my TLDR answer to the following EESE Q&A: JD-Vcc Relay Problem: electronics.stackexchange.com/questions/505318/…. (6) EE pros always try to use optical isolation to avoid common ground. The reason is that mains or other sources of noise/voltage spikes (eg back EMF) always travels along the common ground wiring. – tlfong01 1 min ago   Edit   

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You need a Gnd (or similar) connection in ANY CIRCUIT. That is why they are called circuits; there MUST be a complete path for current to flow.

You say your circuit works; often there are indirect paths e.g through Ethernet or USB cables etc. It is a poor practice to rely on accidental indirect paths.ShareEditFollowFlaganswered 7 hours agoMilliways51.2k2424 gold badges8484 silver badges168168 bronze badgesAdd a comment