What is a Serial Communications Interface?

The serial communications interface (SCI) is a means by which computer components can communicate with each other. This can be within a computer itself, such as from an integrated microchip via a circuit board's serial bus or an expansion card or through a cable to an external device such as a keyboard or printer. Serial interfaces are also used for some computer networking technologies.

The details of serial communications interface are discussed in the recommended standard 232 (RS-232), which was originally devised in 1962 by a standards group that eventually became known of as the Electronic Industries Association (EIA). RS-232 describes, at one end of a serial communication, the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE) at the opposite end. The standard further describes voltage levels and other electrical attributes for the signal, pin identifications for the physical interface, circuit functions and more.

From the RS-232 standard, one of the early serial communications interfaces developed is what's known of as a universal asynchronous receiver and transmitter (UART). The first UART provided a means for teletype machines to transfer small, five-bit sequences known as Baudot codes. Later, as digital computer use increased, the American Standard Code for Information Interchange (ASCII) standard described encoding characters in an eight-bit format, which were transmitted serially between computers via integrated circuits and serial interfaces around 1971. Motorola® then coined the serial communications interface phrase for their UART a few years later.

The way a serial communications interface works is by sending groups of data, referred to as words, in either these five- or eight-bit sequences across the wire, or computer bus. The bits are sent one at a time in sequence, with a starting bit that initiates the communication, followed by the data bits and a stop bit that closes the transfer. Depending on the use, a checking bit, called a parity, may also be inserted into the sequence to ensure the data made it through intact. This method of framing the data transfer within a start and stop bit allows for asynchronous communication. The serial interface isn't required to stay in time with a synchronized clock, but is instead allowed to send a frame at any given moment that the recipient device can recognize.

Serial communications interface technology has found use in numerous areas. One very popular method is the universal serial bus (USB) for connecting peripheral devices to a computer. Inside computer enclosures, hard disk drives sometimes use an interface known as serial advanced technology attachment (Serial ATA) for high-speed communication with the computer's processor. Many expansion cards use another type of serial interface called the peripheral component interconnect express (PCI-E). Still, serial communications interfaces remember their roots and are also used in common Ethernet networking environments, as well as high-speed fiber optics.