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I'm in a bit of a quandary here but it must be a common one. I have two boards powered from the same battery through a "Y" cable; both boards have voltage regulators on-board. These boards are connected by UART rx and tx. My initial impulse was to add a ground connection for the UART, indeed both boards have a ground pin next to their rx, tx pins. But now I wonder: would this be the wrong thing to do? None of these connections is longer than 20cm/8in.

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  • \$\begingroup\$ the question is what these boards do. Generally, you probably already have a ground connection through your "Y" cable, which connects the grounds of the on-board voltage regulators. \$\endgroup\$ – Marcus Müller Dec 17 '18 at 17:33
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    \$\begingroup\$ Depends. If the circuits always share the same ground, not adding another one will eliminate the ground loop which might be an issue. \$\endgroup\$ – Eugene Sh. Dec 17 '18 at 17:34
  • \$\begingroup\$ on the other hand, if a ground loop becomes an issue, it means you have a (AC or DC) potential difference between the two boards – which might or might not interfere with the UART operation. So, really depends. \$\endgroup\$ – Marcus Müller Dec 17 '18 at 18:25
  • \$\begingroup\$ To sum up, you should not definitely not want to make a ground connection along with TX and RX. You might want to, you might want not to, but you haven't provided enough information. \$\endgroup\$ – Elliot Alderson Dec 17 '18 at 18:40
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There's no definite answer for this, specially without knowing what it is.

If you don't add the GND connection:

  • Return current of UART signals will go through the "Y" cable. Huge loop, possibility to radiate or suffer from EMI.
  • A high current or high frequency in a branch of the "Y" cable could make the grounds of the boards to be on a different voltage, possible errors in transmission

If you add it:

  • Possibility for a ground loop, which could cause EMI problems
  • You might make a stronger ground than the "Y" and some high current or high frequency could flow there distorting the signal.

Without knowing what it is, I recommend adding the ground wire to the UART interface and also adding a strong ground connection between the boards. The strong connection will carry all the high currents and high frequencies. The "UART GND" will only carry the return currents of the UART signals as it is supposed to.

Be warned, my guess on what your setup is could be wrong and so could be my answer. If you give more details on what the boards are, you can get a more detailed answer.

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  • \$\begingroup\$ I actually have three instances of this issue. I have two of blue pill processor board to dual motor controller, and one of blue pill processor board to stepper controller. The motor controllers can go to about 5 A stall; the stepper controller draws less than 0.5 A. (Blue pill is a cheap as dirt, common as dirt STM32F103 board). \$\endgroup\$ – Colin C Jan 1 at 0:41

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