There is only one 1-channel UART port on the original Raspberry Pi. Once it is used for terminal debugging, no other serial device is allowed. With this shield you have additional 2-channel UART, plus 8 programmable GPIOs. - Compatible with Raspberry Pi Zero/Zero W/Zero WH/2B/3B/3B/3B+. - onBoard SC16IS752 - up to 16 of these modules are stackable - integrated multi-LEDs for indication of the UART operating status - reserved I2C control pins, allows working with other control boards specifications - Operating voltage: 3.3 V - Expansion chip: SC16IS752 - Control interface: I2C - Mounting hole size: 3.0 mm - Dimensions: 65 x 30 mm Scope of delivery 1x Serial expansion HAT 1x RPi Screw Package Documentation and downloads can be found here: https://www.waveshare.com/wiki/SerialExpansionHAT