There are quite a few standards to choose from when considering a serial communications protocol and in many MCUs, multiple standards are supported on the same IC. RS232 or UART is one of the most popular standards and is the one that will be covered in this course. The Serial Peripheral Interface (SPI) is a synchronous serial interface that uses four (or three) wires to communicate between what is considered a master device and a slave device. The data lines are for the data input, data output and the clock. The fourth wire is for the chip select such that multiple slave devices can be on the bus. The SMBus is a two wire bus that consists of one wire for data input and output and then the clock. Multiple devices can be added on the bus as the protocol provides an addressing scheme for data transfers. Serial ATA is a serial form of the parallel bus used in the PC for things like hard disk drive connections.