USB was developed as a connection for low bandwidth peripherals such as scanners or keyboards. The transfer speed for each port is a maximum of 12MBits/second. USB is a hot swappable plug and play interface which can daisy-chain up to 127 devices per port. To connect this many devices, a USB port hub can be used.

All devices on the USB chain are powered by the signal cable. If too much power is draw from the line or there are too few free connections for extra devices, a hub can be used. The cable used for USB devices is a 4 connection cable; a single twisted pair for data transfer and 2 wires for power - a ground and a 5 Volt.

Often referred to as LPT ports, parallel ports are able to transmit data one byte at a time, a 8x improvement over serial ports. Like serial ports, parallel ports require an IRQ, and they are also bi-directional. The most common uses for parallel ports are scanners or printers. Computers are able to support up to 3 parallel ports at a time.

There are 3 different versions of parallel ports, Standard Parallel Ports, (SPP); which have a theoretical maximum transfer speed of 300 Kbytes per second, and Enhanced Parallel Ports, which support a theoretical maximum transfer speed of 1.3MBytes/sec. The most advanced version is the Enhanced Capability Port (ECP) which is usually used for advanced printing devices and supports a theoretical 2.7MB/sec.

LPT ports require 4 I/O addresses, each a byte in capacity. Each byte has a specific usage, they are for status, control, data, and reserved. The status byte is used by the device on the LPT port to display its status, like if a printer was out of paper. The control byte is used to control the device, like tell a scanner to scan. The data byte is used to transfer data threw the port. The reserved byte is not used.

Serial ports are used to connect many small peripherals, such as modems or mice, and were once used as a direct connect network. These ports are often referred to as COM ports, and even older computers are able to support up to 4. Serial ports operate by sending data serially, one bit at a time. Because serial ports use an IRQ for all data processing, data is stored in buffers while it is being transferred to minimize the CPU attention. Serial ports are capable of bi-directional transmissions, because there are separate pins for sending and receiving.

Serial ports work by using a UART chip, a Universal Asynchronous Receiver/Transmitter which converts binary signals into signals capable of being transmitted. Serial ports are now being outdated by newer, faster transfer methods. A standard serial port is limited to 115 Kbaud, however some serial card interfaces are able to run at up to 460.8 Kbaud.

Serial ports come in either 25-pin or 9-pin D-shell. The 25 pin connector is the most used, but although they hav 25 pins, the serial port is still only able to use of 9 of them. This is to ensure compatiblity for both the transfer mode and devices, and also allows for the use of a adapter to change between the two versions.

All modems that have a controller have an integrated serial port. This acts just like a regular serial port, mimicking a serial port and external modem. The advantages of this is that just like an external modem, the modem is capable of doing it's own processing and buffering, and maintaining the capability with the external programs. A downside of this is that the modem requires a dedicated IRQ, while some of the newer, non-serial based modems are PCI based, and therefore eligible for IRQ sharing.

The floppy disk interface was designed only for use by a floppy disk drive. There is a floppy disk controller on every motherboard, and each controller is capable of controlling 2 floppy devices, and requires both a DMA and IRQ resource. The floppy cable looks similar to an IDE cable, except that the data wires are switched between the 2 drive connections. A standard floppy cable uses 34 pins. This interface was not designed for speed, but for low level compatibility.

FireWire is a competing standard against USB, which was first developed by Apple Computer, and then finalized by the IEEE1394 Working Group. FireWire is capable of up to 63 hot-swappable devices per port, with a maximum of 16 nodes of up to 4.5 meters of chord for each device. Each port is capable of 400Mbits/second to be distributed to all of the devices. FireWire is also capable of speeds of 100MBits/sec and 200MBits/sec, and is soon going to be expanded to 800Mbits/sec. FireWire is not only capable of low bandwith peripherals, but is also useful for high bandwidth peripherals and devices, such external hard drives.

Like USB, all devices are powered by the data cable. The data cable is a 6 conductor cable. There are 2 twisted pair connections for data travel, one for upstream and one for downstream traffic. There are also 2 connections for power. FireWire is very popular because of its high bandwidth, by it isn't catching on very strong because there is a royalty involved to use it.

SCSI & Fibre Channel | IDE | USB, Parallel, Serial, Floppy, & FireWire