It is a port on a computer that allows external devices to be connected simply and easily. It is a protocol for transferring data to and from digital devices. It is known as a plug and play interface between a computer and a low-speed peripheral, such as a mouse, keyboard, digital camera, printer or scanner. Unlike devices connected via SCSI ports, USB devices can be added to and removed from the computer without having to reboot the computer
USB v3.0 has 4.8 gbps
PCI (Peripheral Component Interconnect)
Developed by Intel(1993)
Used for interfacing processor with its chipset
Very successful, widely used
Much faster than ISA(Industry Standard
FEATURES OF PCI BUS
Synchronous Bus Architecture.
64 Bit Addressing.
Linear Burst Mode Data Transfer.
PCI operates at 66MHz and at 33MHz.
The very well known connection port available on your PC is the USB
USB is an acronym for Universal Serial Bus
The change that has been caused by this standard and its wide
application is definitely mentionable.
USB was created by a core group of companies that consisted of
Compaq, Digital, IBM, Intel, Northern Telecom and Microsoft.
FEATURES OF USB
A maximum of 127 peripherals can be connected to a single USB
USB device has a maximum speed up to 480 Mbps (for USB 2.0).
Length of individual USB cable can reach up to 5 meters without a
hub and 40 meters with hub.
USB acts as "plug and play" device.
USB can draw power by its own supply or from a computer. USB
devices use power up to 5 voltages and deliver up to up to 500 mA.
If a computer turns into power-saving mode, some USB devices will
automatically convert themselves into "sleep" mode.
APPLICATIONS OF USB
USB connections are used to connect computer peripherals such
as mouse, keyboards, digital cameras, printers, flash drives,
joysticks, scanners, modems, storage devices, etc.
It has become common on other devices such as phones, PDAs,
Small Computer Systems Interface
Developed by Shugart Associates (1981)
Originally called Shugart Associates Systems Interface
Set of standards for physically connecting and
transferring data between computers and peripheral
SCSI ports allows data to be transmitted in a “daisy
Up to 8 or 16 devices can be attached on a single SCSI
SCSI uses handshaking signal between devices
Bus Free Phase
• In a busy system, the SCSI bus may be free for as little as 1.2 ps while there is no
device requesting the bus . if the bus is free it could remain free indefinitely.
• A device can arbitrate and be granted the bus in 3.6 ps or less. Devices that lose
arbitration can try again when the bus is in free state.
• The selection phase can select the target in 580 ns. If the target does not respond the
bus waits for 250 ms and then enters the free phase.
• The message phase is the first information transfer phase in the connection, which allows
the initiator to send an Identification message to the target. In SCSI messages are always
• The target responds to the Inquiry data, during the command phase
Data In Phase
• The data transfer can be done synchronously if both the initiator and the target have
previously established a synchronous data transfer agreement.
• The target finally sends a single status byte asynchronously during status
Message In Phase
• The last information that is transferred in the established connection is typically the
command complete message during the message in phase.
Bus Free Phase
• The target device releases the BSY signal and returns to the Bus Free phase. The
bus is now available.