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My question is related to the common ground in a UART communication. So far, what I understand how communication in general works is: Distinguishing the voltage between two cables (one is a GND and one is the data wire).

But if they have two different grounds and two different supply voltage like for example one device has 5V and the other device has 12V.

How can they create a common ground, for a stable data transmission??

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  • \$\begingroup\$ Your question is unclear. Why should there be two different GNDs even for two different supply voltages? GND is the common potential of the two transmitters, so this is ONE wire. \$\endgroup\$
    – Stefan Wyss
    Commented Jun 22, 2018 at 13:00
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    \$\begingroup\$ "How can they create a common ground?" - with a wire ... \$\endgroup\$
    – brhans
    Commented Jun 22, 2018 at 13:30
  • \$\begingroup\$ If one is at 12V and the other at 5V, then it seems one is "true" RS232 at +/-12V and the other is unbuffered at 0-5V. Don't connect them together. If the 5V one is +/-5V, it may be buffered. \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Jun 22, 2018 at 13:43
  • \$\begingroup\$ @CristobolPolychronopolis That's a reaching assumption. First RS232, then +/-5V (what exactly is that?). I'd rather assume OP is not mistaken and they both are indeed UARTs, despite rather naive question. \$\endgroup\$
    – Maple
    Commented Jun 22, 2018 at 20:32

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Grounds should be established by connecting them together. If they cannot be connected together, galvanic isolation can be employed. The signal levels over cables are typically defined by a standard such as RS-232C (1969).

Discrete circuits or chips such as the antediluvian MC1488/MC1489 and more modern parts such as MAX3232 can be used to convert between TTL/CMOS levels (as would come out of an MCU directly) and RS-232 levels (they also invert), as you would send over a cable into the outside world.

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  • \$\begingroup\$ Thank you very much. It helped me to understand better how these things work:) \$\endgroup\$
    – LeSchwambo
    Commented Jun 23, 2018 at 6:58

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