A UART is one of the most common serial communication subsystems. To transmit data with a UART, one or more bytes are written to an internal shift register by a CPU from where they are then serially sent out; a single output is sent out at a previously defined clock rate. Another UART (programmed to run at the same clock rate as the first), with its single input connected to the output of the first, detects the start of this bit stream, and it sequentially reads the bits into its own internal shift register. Once all the bits are sent, the receiving UART signals its CPU to let it know there's data ready, and the CPU then reads it out of the receive register.

A UARTs transmit output is referred to as TX, and its received input as RX. They often have some additional signals for more advanced handshaking, called flow control, which help ensure synchronization. We'll only cover the RX and TX signals here, as we won't be using the flow control signals in any of the demos in this book. Synchronization...