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When powering the Betz board from its USB plug with JP4 connected and the UART powered (Arduino disconnected), I get working clean serial communications in Putty from that same USB plug. In this state, voltage on the 3v3 pin reads about 2.5 volts, and voltage on the 5v pin reads about 4.8 volts.

When connected to the Arduino, with JP4 disconnected and the UART unpowered, powering the Betz board and the Arduino both from my Pololu power supply which supplies 4.95-5.05 volts to the 5v pin, I get either nothing on the Arduino serial port from the Betz board, or expected text mixed with garbage characters on the serial port. Voltage on the 5v pin reads 4.95-5.05 volts, and voltage on the 3v3 pin reads 1.37 volts.

My TX pin from the Arduino to the Betz board is going through a 10k+10k 50% voltage divider. The RX pin coming from the Betz board to the Arduino is not.