Serial Ports
A serial port can be used to connect devices that use a serial interface such as a modem, scanner, mouse, etc. Generally, a PC can identify up to four serial ports, but the typical computer contains only two, referred to as COM1 and COM2. Serial ports are sometimes called the RS-232 ports because they use the RS-232C standard as defined by the Electronics Industry Association (EIA). Unlike the parallel port, a serial port transmits data bits one after the other (serially) over a single line. USB 2.0, the most recent version, will run at a speed of 450 Mbps.

A serial port uses a DB-9 (9-pin) connector on new computers. Older computers used a larger, 25-pin connector for the serial port interface. The mouse is sometimes used in serial port 1, called COM1, which is a 9-pin male connector. The modem is typically used in serial port 2, called COM2, which is also a 9-pin male connector. Both serial ports are located in the back of the computer system.

Parallel Ports
A parallel port is a socket on the computer that is used to connect a printer or other peripheral device such as a portable hard disk, tape backup, scanner, or a CD-ROM. The parallel port contains eight lines for transmitting an entire byte (8 bits) across the eight data lines simultaneously. The parallel port interface offers 8-bit parallel data words and nine I/O control lines at a DB-25 socket (25-pin female connector). Figures and show female and male DB-25 parallel ports. The DB-25 socket can be found on the back of the computer unit. Parallel ports can be configured as LPT1, LPT2, or LPT3.

Parallel ports were originally used for printers, but today they can be used for both data input and output devices. This type of parallel port is called bi-directional and is often used for the rapid transmission of data over short distances. The newer, enhanced parallel ports can be converted from uni-directional to bi-directional through the CMOS setup screen. In older PCs, the parallel printer interface was located on the back of the video adapter card, on a multiple I/O card, or on a dedicated parallel printer card. Today, on Pentium system boards, the parallel port is located directly on the back plate of an I/O card, or is connected through a ribbon cable to the 25-pin connector on the back of the unit. When connecting an external device such as a printer to the computer parallel port, avoid using a parallel cable longer than 15 feet. A shorter cable will reduce the chance for errors and will ensure data integrity.

PS/2 Ports/6-pin Mini Din, 5-Pin DIN
PS/2 keyboard or PS/2 mouse ports are used to connect the PC to its keyboard and mouse. Though both ports look identical, the mouse (green) and keyboard (purple) ports are not interchangeable. Usually both ports are color coded or labeled to avoid any confusion. The cable that connects the PS/2 keyboard or mouse uses a PS/2 type connector, also known as a 6-pin mini DIN connector The PS/2 (or 6-pin mini-DIN) has become very popular since its introduction by IBM in 1987 with the IBM PS/2. However, the 5-pin DIN XT/AT connector type ports and serial mice are still quite common. The 5-pin DIN AT connectors are typically used to connect the AT keyboard directly to the motherboard. One advantage of the PS/2 port is that a mouse may be connected to the computer without using a serial port.

Universal Serial Bus (USB)
This external port allows user to connect up to 127 external PC peripherals, including USB keyboards, mice, printers, modems, scanners, and external disk drives. USB is an emerging technology. It offers a data transfer rate of up to 12 Megabits per second. The latest version, USB 2.0 is much faster and transfers data at a rate of 450 Mbps. Seven USB devices can typically be connected directly to the computer using the standard USB four-pin connector. By using external hubs, a networking device that is discussed in a later chapter, each of the seven devices can be connected to the others, creating a daisy chain of 127 devices. USB devices can be hot-plugged (hot-swapped). This means that they can be attached while the computer is already powered up and running. USB devices are also Plug-n-Play.

USB devices are classified as Full-speed or Low-speed devices based on their communication capabilities. A cable serving a Full-speed device has a length limit of 5 meters (16 feet 5 inches). Conversely, the length limit for cables used between Low-speed USB devices is 3 meters (9 feet 10 inches). USB was introduced in the late 1990s and was not supported by MS Windows 95 and NT 4.0. Support for USB is one of the reasons Windows 98 or 2000 is preferred. This port may eventually replace everything except the VGA port. Most PC USB peripherals can also be used on a Macintosh, although some lack the necessary drivers.

FireWire
FireWire, also known as i.LINK or IEEE 1394 is a high-speed, platform-independent communication bus that interconnects digital devices such as digital video cameras, printers, scanners, digital cameras, and hard drives. Developed by Apple, FireWire was designed to allow peripherals to seamlessly plug into a computer. The benefits of FireWire include compatible, smaller connectors, hot-plug connection, shared memory, a single connection, backward compatibility, and speed. FireWire can support up to 63 devices using cable lengths up to 4.5 meters or 14 feet (the length limit for cabling). Like USB, these devices can be "Hot-swapped" or "hot plugged".

FireWire is based on a shared memory model that allows devices to directly access locations in memory instead of having to wait for information to flow in a stream. FireWire is much faster than the original version of USB, transferring data rates up to 400 Mbps.