Electronic – Reading UART from device that has floating ground

groundgroundloopsserialuart

I am powering an ESP32 over a less-than-ideal power line, that has 4 Ohms of resistance. In order for 5V to arrive at the device, I apply 7V to the power line, such that when 250mA of current are flowing, 1V is dropped on both the power line and the return path, resulting in 5V difference at the ESP's linear dropout regulator.

I would like to connect my FT232 serial-to-USB converter to the ESP's UART to diagnose the device in this environment, but am afraid I can't because

  • the device's ground is 1V above my laptop's ground
  • the device's HIGH signal would be at 4.3V instead of 3.3V
  • the device's LOW signal might not be low enough to be interpreted by the FT232 (1V)

What options do I have to still be able to read data from this UART port safely?

Best Answer

You could consider:

  • RS-485 drivers in between (full-duplex, or half-duplex if your debug protocols support it)
  • Opto-isolators near the converter
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