Some Information about the Most Common Computer Ports

When connecting devices to your computer, you should first look at the different types of ports that can be found on it. You may already be familiar with some of the common ones, such as serial, parallel, and USB. This section will discuss IEEE-1394 ports as well, which are often called FireWire.

Serial versus parallel

Two of the most common ports on the back of your computer are the serial ports and the parallel ports. Most computers have one parallel port and two serial ports. They are used to connect different types of devices to your computer.

Parallel ports send data over multiple wires simultaneously, while serial ports only send data over one wire at a time. Since parallel communication allows for multiple streams of data, it provides higher data transfer rates than serial communication. Parallel ports were implemented on the personal computer when it was introduced by IBM in 1981. They used nine wires in cable to connect two devices together. This enabled them, at any given time, to deliver eight bits of data. However, because there was no way to accurately control the flow of the signal down each of the wires in the cable, it was recommended that the length of the cable be less than six feet. This became more of a factor when bi-directional communication was implemented over the parallel port. The standard for bi-directional communication was delivered in the IEEE-1284 specification, which allowed for high-speed communication over the parallel port. This also opened two new specifications for the port: the Enhanced Parallel Port (EPP) and the Extended Capabilities Port (ECP).

The EPP-type parallel port was used primarily for non-printer peripherals, while the ECP-type parallel port was designed to accommodate new high-speed printers and scanners. In order to better handle high-speed data communication, the ECP-type parallel port also implemented the use of a DMA channel. Serial ports, on the other hand, deliver data sequentially down a single wire. Eight bits of digital data are converted into analog data using a system called baud. Baud rate refers to the number of state changes (tones) that are made on the wire in any given second. This is very different from bits per second (bps), which measures the amount of data that is transferred. At one point in time, 300 bps modems communicated at 300 baud, but compression standards adopted by the communications industry allowed more data to be delivered at the same baud rate. Today, 56Kbps (57,344 bps) modems communicate at 9,600 baud.

Depending on the baud rate used to transfer data, the length of the cable can range up to 5,000 feet. For data transfer rates at 9,600 baud, the maximum cabling length is 250 feet. The RS-232C standard, which is used as a basis of serial communication, recommends a maximum cable length of 50 feet.

Universal Serial Bus

Universal Serial bus, or USB, is a new release of the serial standard. The goal of USB was to revolutionize the way serial communication was conducted. In an effort to fulfill this goal, USB uses a new cabling system that allows up to 127 devices to be connected together. It also delivers power to the devices that are connected on this bus. To go along with this new cabling, the specification for USB dictates that all devices should support Plug-and-Play. With these two pieces of the puzzle put together, USB enables you to plug devices in and have them work. Some USB devices that require a large amount of power may use a supplemental power supply, but the USB bus will power most devices.

FireWire (IEEE-1394)

FireWire is an Apple trademark for the IEEE-1394 standard. The 1394 standard implements a version of serial communication across a wiring network that is similar to USB. IEEE-1394 enables the connection of 64 devices on a bus that supports 50 to 400 Mbps. One of the goals of the IEEE-1394 standard was to replace SCSI. On the exam, do not confuse IEEE-1394 (FireWire) with IEEE-1284, which deals with bi-directional communication over the parallel port.

While there have been disk drives implemented through the IEEE-1394 standard, it has seen the most growth in the area of data, voice, and video. While many PCs have been shipping for the last few years with USB ports, IEEE-1394 has not seen the same level of adoption by the PC manufacturing industry. Many device manufacturers are supporting IEEE-1394 in their devices, and interface cards are readily available to be added to your PC.

Keyboard

Since it’s certain you will want to enter data into your computer, the keyboard connector on the back of your computer plays an important role. There are two main types of keyboard connectors: PS2 or mini-DIN 6 and DIN 5. The mini-DIN connector has now been fully adopted by AT PC standard.

Monitor

Since you will also want to see what you are doing, the monitor connector on the back of your PC plays an important role. With the demise of the CGA/EGA standards, and the release of VGA and SVGA, the monitor connector on the back of your computer moved from a DB9 female connector to a HD (high density) DB15 female connector.