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Definition: serial port

A socket that connects to a serial interface (one bit following another over one line). Serial ports are widely used by sensors for data acquisition, and they were standard on early computers for connecting a modem, mouse and other peripherals. Old Macs used the serial port for printers. Also known as an RS-232 port, serial ports provide very slow speeds and have been superseded by USB on desktop computers. USB-to-serial adapters are available for old peripherals.

DB (D-Sub) Connectors and COM Ports
A serial port on a PC is a male 9-pin connector (DE-9 D-sub). Early PCs had two 9-pin connectors or one 9-pin and one 25-pin (DB-25). On a PC, serial ports are called "COM ports," identified as COM1, COM2, etc. See COM1 and D-sub connectors.

Serial, Parallel and Game
Early PCs were fitted with one or two serial ports, one parallel port and one game port on the motherboard. On the first PCs, the ports were contained on a stand-alone expansion card plugged into the ISA bus. Contrast with parallel port. See serial interface and RS-232.

Faster, But Still Serial
The USB and FireWire (IEEE 1394) interfaces were added to PCs in 1998, offering a quantum jump in transfer rate, plus the ability to daisy chain many peripherals on the same bus. USB and FireWire are also serial interfaces. See USB and FireWire.

In Transition
As USB ports (left) began to proliferate, the serial and parallel ports (right) were included for a while, but eventually gave way entirely to USB.