A serial communications interface (SCI) is a device that enables the serial (one bit at a time) exchange of data between a microprocessor and peripherals such as printers, external drives, scanners, or mice. In this respect, it is similar to a serial peripheral interface ( SPI ). But in addition, the SCI enables serial communications with another microprocessor or with an external network. The term SCI was coined by Motorola in the 1970s. In some applications it is known as a universal asynchronous receiver/transmitter ( UART ).
The SCI contains a parallel-to-serial converter that serves as a data transmitter, and a serial-to-parallel converter that serves as a data receiver. The two devices are clocked separately, and use independent enable and interrupt signals. The SCI operates in a nonreturn-to-zero ( NRZ ) format, and can function in half-duplex mode (using only the receiver or only the transmitter) or in full duplex (using the receiver and the transmitter simultaneously). The data speed is programmable.
Serial interfaces have certain advantages over parallel interfaces. The most significant advantage is simpler wiring. In addition, serial interface cables can be longer than parallel interface cables, because there is much less interaction (crosstalk) among the conductors in the cable.
The term SCI is sometimes used in reference to a serial port. This is a connector found on most personal computers, and is intended for use with serial peripheral devices.