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External connectors
I/O ports
To accommodate different types of input and output hardware, a computer has several places where the input/output, as well as storage-hardware is connected to the processing unit. These places, connected to the motherboard, are called I/O (Input/Output) ports.
Three types of I/O ports are available: serial, parallel and USB. Serial ports have been around for decades. Parallel ports are a more recent invention and are much faster than serial ports. USB ports are only a few years old, and will likely replace both serial and parallel ports completely over the next several years.
Serial and parallel ports
Considered to be one of the most basic external connections to a computer, the serial port has been an integral part of most computers for more than 20 years. Although many of the newer systems have done away with the serial port completely in favour of USB connections, most modems still use the serial port, as do some printers, PDAs and digital cameras. Few computers have more than two serial ports.
Data is sent between the computer and the attached hardware one bit at a time with a serial port. When a parallel port is used, the entire bit pattern for a single character is sent at the same time. The advantage to using a parallel port is that it is faster than a serial port, as it sends several bits simultaneously. However, the I/O and storage hardware must be physically close to the computer. While serial ports do not provide as high a transmission speed, the peripherals can be further away. Serial ports are used to help send data over telephone and other communication lines.
While USB is becoming increasingly popular, the parallel port is still in use to connect a range of peripherals.
When a PC sends data to a printer or other device using a parallel port, it sends eight bits of data (one byte) at a time. These eight bits are transmitted parallel to (beside) each other, as opposed to the same eight bits being transmitted serially (all in a single row) through a serial port.
USB ports
All computers today come with one or more Universal Serial Bus connectors. These USB connectors let you attach everything from mice to printers to your computer quickly and easily. Connecting a USB device to a computer is simple; find the USB port on your machine and plug the USB connector into it.
The Universal Serial Bus allows you to connect up to 127 devices to a computer. Each device can consume up to a maximum of 6 megabits per second of bandwidth.
Just about every peripheral made now comes in a USB version. These include printers, scanners, mice, joysticks, flight yokes, digital cameras, webcams, scientific data acquisition devices, modems, speakers, telephones, video phones, storage devices and network connections.
If it is a new device, the operating system auto-detects it and asks for the driver disk. If the device has already been installed, the computer activates it and starts talking to it. USB devices can be connected and disconnected at any time.
Many USB devices come with their own built-in cable, and the cable has an "A" connection on it. If not, then the device has a socket on it that accepts a USB "B" connector.
The USB standard uses ‘A’ and ‘B’ connectors to avoid confusion. ‘A’ connectors head ‘upstream’ toward the computer, while ‘B’ connectors head ‘downstream’ and connect to individual devices.
Most computers that you buy today come with one or two USB sockets. With so many USB devices on the market today, you easily run out of sockets very quickly. The easy solution to the problem is to buy an inexpensive USB hub which can support up to 127 devices.
A hub typically has 4 new ports, but may have many more. You plug the hub into your computer, and then plug your devices (or other hubs) into the hub. By chaining hubs together, you can build up the number of available USB ports on a single computer.
The ‘Spectronics’ website has an excellent table with images and explanations of the external connectors for devices. Go to:
http://www.spectronicsinoz.com/ 
>>Library>>type in ‘connectors’.
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