CP1610:Introduction to Computer Components

The I/O Subsystem

Parts on the Motherboard

Purpose of the I/O Subsystem:

A computer must have some way of transporting data between the CPU and memory.

It must also have some means of transmitting and receiving data to and from internal and external devices.

Devices which input data to the computer (and send it to either memory, the CPU, or some other component) are called Input devices.

Output devices are devices to which the CPU, memory, or some other component, send data back to the user, or to another remote system (such as another computer, a network, or the Internet.

The purpose of the I/O subsystem (the Input/Output subsystem) of a computer is to:Provide a path through which this data can

be transported;Provide mechanisms by which the flow of

data can be controlled, so that it is used properly, and does not interfere with other data.

This transmission of data is done most effectively by using a bus:A system of traces (or wires) on the

motherboard;Connects to all of the expansion ports (for

expansion cards) and the various other ports that are used to connect and communicate with peripheral devices.

Various Bus Systems

There are various different busses in a computer system, such as:Front Side BusBack Side Bus ISA BusAGP BusPCI BusUSB BusFirewire Bus (IEEE1394)

Front Side Bus

This bus connects the CPU to other devices within the computer system such as the RAM, BIOS, I/O busses, hard disks, etc.

A typical Front Side Bus today is 64 bits wide, and today’s Front Side Busses can achieve data transfer rates of over 8 GB per second..

Back Side Bus

A special, high speed bus that connects the CPU to an L2 or L3 memory cache, it they are present in the system.

ISA Bus

Sometimes referred to now as a legacy bus. ISA stands for Industry Standard Architecture, and it was the industry standard in the early days of personal computers.

It is much slower than newer bus architecture designs, but it is still incorporated into many motherboards in order to integrate older (or legacy) devices.

The ISA bus is an I/O bus that connects the system bus to ISA expansion slots.

Originally the ISA bus was an 8-bit bus, but more recent versions are 16-bits wide with data transmission rates of up to about 8 MHz.

AGP Bus

Accelerated Graphics Port Bus. This is a specialized, high speed I/O bus

used to connect the system bus (and thus the CPU) to graphics output devices (monitors and projectors).

A typical AGP bus can be either 32 bits or 64 bits wide, with data transfer rates on newer versions reaching as much as 2GB per second.

PCI Bus

Peripheral Component Interconnect Bus. An I/O bus used for connecting peripheral devices

(including integrated circuits and expansion cards) to the system bus.

The PCI bus is the most common bus found in computers today, and has replaced the ISA bus and the VESA Local Bus.

A typical PCI bus is either 32 bits or 64 bits wide, and can achieve data transfer rates of up to 133 MB per second.

The PCI bus, as well as the AGP bus, are gradually being replaced by the PCI-Express bus.

USB Bus

Universal Serial Bus. A serial I/O bus designed to allow peripherals

to be connected to the system bus without the need to use expansion cards into the computer’s ISA or PCI busses.

Devices connected via USB are typically PnP (Plug and Play).

USB can achieve data transfer rates ranging from 1.5 MB per second to 480 MB per second.

FireWire Bus

Also known as an IEEE 1394 bus. This is a specialized serial I/O bus used for

high speed data transfers. FireWire is typically used in personal

computers as well as digital audio / video devices.

The latest FireWire technologies can support data transfer rates of up to 3.2 GB per second.

Characteristics of a Parallel Port

In computing, a parallel port (LPT) is an interface from a computer system where data are transferred in or out in parallel, that is, on more than one wire.

A parallel port carries one bit on each wire thus multiplying the transfer rate obtainable over a single cable (contrast serial port).

There are also several extra wires on the port that are used for control and status signals to indicate when data are ready to be sent or received, initiate a reset, indicate an error condition (such as paper out), and so forth.

For the most part, the USB interface has replaced the parallel port - most modern printers are connected through a USB connection, and often don't even have a parallel port connection.

On many modern (2006+) computers, the parallel port is omitted for cost savings, and is considered to be a legacy port.

An LPT port has an 8-bit parallel databus, plus 4 pins for control output (Strobe, Linefeed, Initialize, and Select In), and 5 more for control input (ACK, Busy, Select, Error, and Paper Out).

Characteristics of a Serial Port

In computing, a serial port is an interface on a computer system with which information is transferred in or out one bit at a time (contrast parallel port).

Throughout most of the history of personal computers, this was accomplished using the RS-232 standard over simple cables connecting the computer to a device such as a terminal or modem.

Mice, keyboards, and other devices were also connected this way.

The name "serial" comes from the fact that a serial port "serializes" data. That is, it takes a byte of data and transmits the 8 bits in the byte one at a time.

The advantage is that a serial port needs only one wire to transmit the 8 bits (while a parallel port needs 8).

The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires.

Serial ports lower cable costs and make cables smaller.

Before each byte of data, a serial port sends a start bit, which is a single bit with a value of 0. After each byte of data, it sends a stop bit to signal that the byte is complete. It may also send a parity bit.

Serial ports, also called communication (COM) ports, are bi-directional. Bi-directional communication allows each device to receive data as well as transmit it.

Serial devices use different pins to receive and transmit data -- using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time.

Using different pins allows for full-duplex communication, in which information can travel in both directions at once.

Diagram of the I/O Subsystem