A port can be defined as a "plug or socket that enables an external device such as a printer to be attached to the adapter card in the computer". All communication between a computer and external devices is the result of properly configured ports. On every computer, a port's connectors are attached to a motherboard. Computers will have both serial ports and parallel ports with each type serving a particular function. Generally, connector shells are nickel alloy plated alloys of steel. The pins/sleeves are gold flashed alloys of steel.
A serial port allows a serial interface hardware device plugged into it communicate with the computer by transmitting one bit of information at a time. Serial interface devices, such as mice, modems, and keyboards, do not require fast data transmission rates. Serial ports often are referred to as communications (COM) ports. It is an external port on the back of the PC that attaches directly to the PCs motherboard. These ports were one of the first ports put on PCs. The serial port connects the PC and peripheral using a nine-wire, or pin, interface, which uses one pin as the ground and the others for sending and receiving data between Data Terminal Equipment (DTE, your computer) and Data Communications Equipment (DCE, peripheral devices such as modems or mice). Older serial ports also used 25-pin connectors, but most new PCs ship with serial ports featuring only nine pins.
The electrical interface used to connect the serial port to the device is the Recommended Standard 232 (RS-232 is a standard interface approved by the Electronic Industries Association (EIA) for connecting serial devices.) or the newer RS-422. <BR>These interfaces are backward compatible, which means you can use an RS-422-capable device with an RS-232 interface. The RS-232 has a few downfalls, including a maximum transmission rate of 20 kilobits per second (Kbps) and the restriction that a device must be located within 50 feet of the system. The RS-422 supports transmissions rates above 20Kbps and longer cables and offers greater immunity to interference that sometimes occurs with an RS-232.
A parallel port lets an external parallel device plugged into it communicate with the computer by transmitting eight bits (one byte) of data simultaneously therefore it is eight times faster than a serial port. Most devices that send or receive large amounts of data, such as printers and scanners, use parallel ports. Parallel ports are often referred to as Line PrinTer (LPT) ports. The parallel port is the largest port on the rear of your PC, comprising 25 lines that include 17 signal lines and eight ground lines. The parallel port, initially designed by Epson Corporation, feature a maximum data transfer rate of 150 kilobits per second (Kbps) in a technology now known as the Centronics standard. With the introduction of two new standards, the Enhanced Parallel Port (EPP) and Extended Capabilities Port (ECP), data transfer rates increased to more than one megabyte (MB) per second. Nearly every PC built in the last five years features an EPP, and the ECP is slowly growing in popularity. The EPP and ECP, also known as IEEE 1284 standard, are compatible with the standard parallel port and support bi-directional communication, unlike the older Centronics technology. Bi-directional signaling means the device can communicate interactively with the PC.<BR><BR> In summary: The standard ports you will find on the back of your PC are the keyboard connector (6-pin round), mouse connector (6 pin round connector), Serial Port 1 connector, Serial Port 2 connector (9 pin connectors, COM1 and COM2), and parallel port connector(LPT1, 25-pin connector.
One of the most common port problems is simply not having enough of them. The USB (Universal Serial Bus) hopes to overcome this problem with the ability to connect up to 127 devices on one port. The USB peripheral bus standard was developed by Compaq, IBM, DEC, Intel, Microsoft, NEC, and Northern Telecom. The advantages of USB in addition to this high number of add-on capability is that the computer does not have to be physically shut down for the new device to be attached and so offers what is now called as plug-n-play capability. USB 1.1 supports a data speed of 12 megabit per second. This speed will accommodate a wide range of devices, including MPEG video devices, data gloves, and digitizers. And now a faster USB port standard, USB 2, is introduced with the capacity of really powering small devices (e.g. webcams, small HDD's).
Courtesy of the French Energy Agency ADEME, Future Electronics project.