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A serial port is an interface that allows a PC to transmit or receive data one bit at a time. It is one of the oldest types of interfaces and at one time was commonly used to connect printers and external modems to a PC. Modern serial ports are used in scientific instruments, shop till systems such as cash registers and applications like industrial machinery systems.
Compared to a parallel port, the data transfer rate of a serial port is slower.
Normally, a serial port is a male port, while a parallel port is a female port. System resource configurations are chosen for each port and are identified by COM1, COM2, COM3, COM4, and so forth. Each COM position represents an input/output (I/O) and an interrupt request (IRQ) address. The I/O address transfers and receives data to and from a peripheral device such as a mouse or keyboard.
The serial port standard is RS-232. This standard is used for transmitting serial communication between devices, which are usually called data communications equipment (DCE) and data terminal equipment (DTE). The serial port uses a nine-pin (DE-9) connector or a 25-pin (DB-25) connector. Originally, the standard used 25 pins. Because many of the pins were unused and were quite bulky, the smaller DE-9 connector became popular.
Higher speed communication was needed for serial communication, such as external data storage units. In 1998, the Universal Serial Bus (USB) and FireWire introduced faster interfaces. This new technology could transfer data at high rates on the same bus, called a daisy chain.
Today, the serial port is seldom used but can be found as a communication device for GPS receivers, LED and LCD text displays, bar-code scanners and flat-screen monitors.