DTR, and GND wires. This cable connects the Arduino to the ESP8266 WiFi module. The TX and RX lines are then tapped into. Each wire is routed to the RX pin of two different serial to USB adapters.

Wyatt]’s CPUVolt library provides a fast way to measure voltage using no external components, and no I/O pin. It only applies to certain microcontrollers, but he provides example Arduino code ...