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External Buses in Hardware

August 6, 2009
It is used to connect to external peripherals, such as printers and input devices. These ports may also be based upon expansion cards, attached to the internal buses.
A parallel port is a type of interface found on computers for connecting various peripherals.
It is also known as a printer port or Centronics port . The IEEE 1284 standard defines the bi-directional version of the port.

The serial port is an Asynchronous port which transmits one bit of data at a time (contrast parallel port).
It is usually referred to as a DB9 or DB25 connection, both of which adhere to the RS-232c interface standard defined in ISO 2110 and ISO 4902.
D represents the shape of the connector if placed vertically as shown in the below diagram.
The number 9 / 25 indicating the number of pins found on the connector.
DB9 Serial connections are now commonly found on modern PCs where DB25 is commonly found on older computers.
USB interface has replaced the serial port as of 2007, most modern computers are connected to devices through a USB connection.
Serial ports are still used in network equipment (such as routers and switches) for configuration. In these areas they are simple, cheap and allow interoperability between devices.
Devices that can be connected through a serial port are listed below:
Mouse, Modem, Network, Printer..etc

DB-9 Pin out

DB-25 Pin out
Universal Serial Bus (USB) is a serial bus standard to interface devices.
USB allows peripherals to be connected using a single standardized interface socket and to improve plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping).
Features of USB
It provides power to low-consumption devices without the need for an external power supply.
It allows many devices to be used without requiring any individual device drivers to be installed.
USB replaced all legacy varieties of serial and parallel ports.
Devices that can be connected through a serial port are listed below:
Mouse, Keyboards, PDAs, Game pads, Joysticks, Scanners, Digital cameras, Printers, Personal media players, and flash drives, connecting several printers to one computer
USB lock software can lock out memory devices and still allow other USB peripherals to function.
The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. Notable members have included Agere, Apple Inc., Hewlett-Packard, Intel, NEC, and Microsoft.
o Firewire
FireWire is Apple Inc.‘s brand name for the IEEE 1394 interface. It is also known as i.LINK (Sony’s name) and DV (Panasonic’s name).
It is a serial bus interface standard, for high-speed communications and isochronous real-time data transfer, frequently used in a personal computer (and digital audio/digital video).
FireWire has replaced Parallel SCSI in many applications, due to lower implementation costs and a simplified, more adaptable cabling system.
FireWire is also available in wireless, fiber optic, and coaxial versions using the isochronous protocols.
Year Created
Created by
1 bit
No of devices
400/800/3200 Mbit/s
SCSI (Small Computer System Interface) is a set of standards for physically connecting and transferring data between computers and peripheral devices.
The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives.
SCSI is an intelligent interface: It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner
SCSI is a peripheral interface: 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host.
SCSI is a buffered interface: It uses hand shake signals between devices, it can transfer only real data and does no error checking and correction. All those are done in a buffer.
SCSI is a peer to peer interface: Communication can be from host to host, host to a peripheral device, peripheral device to a peripheral device
SCSI is most commonly pronounced as “scuzzy”.
    • PS/2 (being phased out)
The PS/2 connector introduced for mice and keyboards
The PS/2 connector is used for connecting a keyboard and a mouse to a PC compatible computer system.
Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987.
The PS/2 mouse connector generally replaced the older DE-9 RS-232 “serial mouse” connector, while the keyboard connector replaced the larger 5-pin DIN used in the IBM PC.
If a PS/2 mouse is connected to a PS/2 keyboard port (or if a PS/2 keyboard is connected to a PS/2 mouse port), the mouse (or keyboard) will not be recognized by the computer. Wrong connection into the incorrect PS/2 port can damage sensitive parts of the computer’s motherboard.
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