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.
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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.
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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.
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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.