Serial Port Definition

A serial port is an interface on a computer system with which information is transferred in or out one bit at a time.

An interface is a connection between two subsystems. In this case, it is an electrical connector built into a computer into which another another, external connector can be easily inserted (and removed) by a user to allow data to be transferred to or from peripheral devices (e.g., display units, keyboards, mice, printers and scanners).

A bit is the most basic unit of information in computing and communications. Each bit has a value of either zero or one. This value is stored as an electrical charge in a single capacitor within a memory device (i.e., a RAM) or a CPU (central processing unit) or as the magnetization of a tiny area of magnetic material on a hard disk drive (HDD) or floppy disk.

In contrast to serial ports, a parallel port consists of multiple wires over which bits are transferred in parallel, i.e., simultaneously. There are usually several extra wires on the port that are used for control signals to indicate when data is ready to be sent or received.

Throughout most of the history of personal computers, serial connections have been made using the RS-232 standard. This standard was originally specified as a 25-pin D-type (because its shape resembled the upper case letter D) connector. However, these connectors were large and awkward. Also, most of the pins remained unused (and unnecessary) because data is sent one bit at a time, and thus only one wire is needed for data in each direction plus a few control signals. Consequently, it has become common to use other connectors for these ports, particularly the much smaller, nine-pin DE-9 (also called DB-9 or D-sub 9) connectors, which also conform to the RS-232 standard.

Recent advances in electronics have made possible more reliable and higher-speed serial communications, and thus RS-232 is being supplanted by newer standards such as Universal Serial Bus (USB) and Firewire. These allow the efficient connection of devices that would not have been practical over slower serial connections, such as external data storage units (e.g., external hard disk drives and USB key drives) and video devices.

In Linux, serial port devices are usually designated /dev/ttyS*, where the asterisk represents an integer beginning with zero. In the older Microsoft Windows operating systems and MS-DOS, serial ports are referred to as COM1, COM2, etc.

The trend for external busses is to employ a serial architecture. A bus is a subsystem that transfers data and/or power between computer components inside a computer or between computers.

Parallel busses, which are used by SCSI (small computer system interface) and ATA (advanced technology attachment) ports, have 16 or more parallel wires that are used to send bits simultaneously. They are much faster than serial busses for identical clock speeds. However, it is considerably easier to increase the clock speed for a serial connection than for a parallel connection, in large part because parallel signals tend to interfere with each other at high clock speeds.

The clock speed or clock rate is the fundamental rate in cycles per second, measured in Hertz, at which a computer performs its most basic operations, such as adding two numbers or transferring a value from one CPU (central processing unit) register to another. A register is a very small amount of very fast memory that is built into the CPU in order to speed up its operations by providing quick access to commonly used values.

Created August 19, 2005.
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