I read in Kolban's book that TXD(GPIO14) and RXD(GPIO15) Raspberry Pi GPIO pins logic level outputs are TTL compatible meaning highs of +5V. Given that I read everywhere else that all GPIO pins are 3.3V this surprised me a bit.
I did connect my Raspberry Pi Zero over USB2UART connector, first with 3.3V selected on the connector. But with 3.3V the Pi Zero did not boot. Then I turned switch to 5V and Pi Zero booted and I could see the bootup messages and do login in minicom session with 115000 8N1 (right click to see photo in big):
These are the connections:
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USB2UART Pi Zero RXD TXD TXD RXD VC 5V GND GND