Loss of Serial Communication When Connecting Common Ground

Hi All,

Running it a frustrating problem. When connecting common ground between Arduino and power supply serial communication is lost. TX(4) pin goes to >4V while RX(3) goes to <0.5V. FYI I am using software serial on a Arduino Nano.

The circuit contains the following components.

  1. 14V 30A DC Switching Power Supply (PYRAMID PSV-300)
  2. ECUMASTER EMU (Communication is based on RS232 at 19200. Output levels are <5V)
  3. DC-DC Buck Convertor 12V to 5V. (POLOLU D36V6F5, Also tested with POLOLU D24V22F5)
  4. Arduino Nano
  5. CANBUS Module (Not Shown, Issue is still present with this completely removed)

I have posted some hand drawn schematics to illustrate what wiring worked and what wiring didn’t work.

Troubleshooting that has been completed so far.

  1. If the Arduino Nano is powered via Laptop/Wall wart serial communication works as intended.
  2. The moment I bridge the GND from Arduino to Power Supply serial communication stops working.
  3. Having the Arduino Nano powered via +5V port using a 5V regulator serial communication does not working.
  4. Having the Arduino Nano powered via USB port using a 5V regulator Serial communication does not work.

This circuit is to be installed within a vehicle where the grounds are common hence would like to figure out why serial communication stops working when grounds are common.

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