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Input/Output Ports

Whilst detailed knowledge of input/output ports is really the domain of computer technicians, it is important that users have at least a basic understanding of what they are, how they work and what computers systems they use have. This information is for serial and parallel ports and the newer Universal Serial Bus (USB) and IEEE01395 (also known as 'i.Link' or 'FireWire'). SCSI and IDE interfaces are also Input/Output interfaces, but they're important enough to have their own section.

Serial, or communications (COM), ports were originally used for devices that communicated bidirectionally with the computer. Whilst new parallel ports allow bidirectional communication, serial ports are still used for devices such as modems, mice, scanners and so forth. Serial ports are asynchronous, meaning that no clocking signal is set, so data can be sent with any time spacing. 'Serial' means that the data is sent over single wires, one for transmission and one for reception. Serial ports come in 9 and 25 pin standards, with a Universal Asynchronous Receiver/Transmitter (UART) chip responsible for converting parallel data within a PC into serial data and converting serial data when received in parallel data. Some modem manufacturers have recently introduced Enhanced Serial Ports (with the ridiculous acronym, ESP) and Super High Speed Serial Ports (which are called Super ESP).

Parallel ports, in comparison, are usually used for connecting printers to a PC. Originally such ports were one way only (after all, it is rare that your computer needed information from a printer!), but modern parallel ports can send and receive data. The name 'parallel' port comes from the fact that they have eight lines for sending one bit of data (one byte total) simultaneously. Standard bidirectional parallel ports are capable of transmitting and receiving about 150kb/second - many modern machines have an Enhanced Parallel Port (EPP), a standard developed by Intel and others that operates at almost 2mb/second. Microsoft and Hewlett Packard have also developed an alternative - the Enhanced Capabilities Port (ECP) - which is particularly notable for supporting external peripherals such as CD-ROM drives, scanners, high-speed printers and so forth. A common use of bidirectional parallel ports is to transfer data between computers (especially if they both have EPP/ECP), using what a null-modem, or laplink cable. Most contemporary operating systems have easy-to-use software to facilitate this process (e.g., Direct Cable Connection) in Windows95 onwards.

USB and Firewire are two new serial port standards that far outstrip the capabilities of serial and parallel ports and may in time entirely replace them. Whilst parallel may appear quicker, transmitting multiple bits simultaneously, there are limits due to what is called 'skew' and 'jitter'. The former is the tendency for some bits to reach their destination before others. The latter is the tendency for bits to remain within the target voltage longer than necessary. With serial this problem doesn't exist. Contemporary serial technology has only depended on the capacity of the clocking rate to send the bits. Furthermore, at high speeds, parallel signals tend to interfere with each other. This doesn't happen with serial because the data transmitted and data received wires are distinct. Finally, for the reasons just noted, serial is capable of transmission and reception over far greater distances than parallel.

USB is an external peripheral bus standard that eliminates the needs to install cards into the computer and saves system resources such as interrupts (IRQs). USB peripherals can be automatically recognized and configured as they are physically attached, eliminating the need to reboot or run a setup. Up to 127 devices can run simultaneously on a PC through USB, sharing a 1.5 mb/second bandwidth (USB 1.1) and now 60 mb/second (USB 2.0). One must be careful however to ensure that the operating system used supports USB. Windows95 and WindowsNT 4.0, for example, do not support USB, however Windows98, Windows2000 and WindowsXP does. Adapters exist to convert USB to parallel, serial, SCSCI, Ethernet, keyboard/mouse and TV/Video.

Established by the Institute of Electrical and Electronic Engineers (IEEE) in 1995 Firewire, or more officially IEEE-1394, is a high speed (25 mb/sec) specification with future designs to run at up to 125 mb/second. Sixty-three devices can be connected to a Firwire adapter, with future designs of up to 64,000 (yes, you read that right) nodes. Furthermore, it is part of the new SCSI standard with drivers developed by Microsoft for Windows 95/98/2000/XP/NT.