Paspberry Pi GPIO Pinout

It seems that the serial port from the Pi 3B & 3B+ is no longer a hardware UART. The UART is a software emulation, which causes problems which you don't have with a hardware UART.

26 pins are only on the first two pi's; rev.1 and rev.2.
40 pins are on Pi model B+ and up. Including Pi Zero.

Click to enlarge

Pi Zero W Additional Connections pin-out

You may notice two sets of thruhole pads labeled TV and Run.

The Run pins

Connected to the Broadcom reset pin and will reset the Pi. This is a dirty shutdown though and can give you a corrupt SD card!
Only use this if your Pi is locked down. Btw, the Run pins are on other Pi models as well.

The TV pins

Allow you to connect a RCA jack to the board for composite video output.
The square hole (left) is signal. The round hole (right) is ground.

Pi 3B+ PoE header

The 4-pin header is below the mounting hole on the right.