Assignment_v S.hymavathi M.tech(c.s.e) R.no:09641d5804 Scsi Bus :

ASSIGNMENT_V S.Hymavathi M.TECH(C.S.E) R.No:09641D5804 SCSI BUS : 1) The acronym SCSI stands for Small Computer System Interface. It refers to a standard bus defined by the American Natinal standards institute(ANSI) Under the designation X3.131[2]. 2) In the original specifications of the standard, devices such as disks are connected to a computer via a 50- wire cable,which can be up to 25 meters in length and can transfer data rates up to 5 megabytes. 3) A SCSI bus may have 8 datalines,in which case it is called a narrow bus transfers data one byte at a time. The SCSI connectormay have 50,68,or 80 pins ,because SCSI has various options. 4) Transfer Rate Of SCSI Bus : The maximum transfer rate in commercial devices that are currently available varies from 5 megabytes to 160 megabytes.The most recently version of the standard is intended to support transfer rates up to 320 mega ytes , and 640 mega bytes. The maximum transfer rate on a given bus is often a function of the cable and the number of devices connected ,with higher rates for a shorter cable and fewer devices. 5) To achieve the high data transfer rate ,the bus length is typically limited to 1.6 m for SE signaling and 12 m for LVd signaling. The maximum capacity of the SCSI Bus is 8 devices for a narrow bus, and 16 devices for a wide bus.

6)

The SCSI bus is connected to the processor bus through SCSI controller , as shown in the below figure.

Main Memory

Processor

Bridge

Additional memory

SCSI controller

Ethernet interface

Usb controller

ISA interface

IDE Disk Video Disk controller

Disk1

cd-Rom controller

Disk2

CD-Rom

Key board

Game

An Example of a computer System using different interface standards

7) This controller uses DMA to transfer data packets from the main memory to the device ,or vice versa. A packet may contain a block of data, commands from the processor to the device , or status information about the device. 8)

Operation Of the SCSI Bus :

To know the operation of the SCSI Bus , let us consider how it may be used with a disk drive.The communication with adisk drive differs substantially from the communication with the main memory . Data are stored on a disk in blocks.called sectors .where each sectors may contain several undred bytes.These data may not be necessarily be stored in contigious sectors. Some secors may be defective and must be skipped. 9) In disk drive a read or write request may result in accessing several constraints of the mechanical motion of the disk, there is a long delay on the order of the everal milliseconds, before reaching the first sector to or from which data are to be transffered. Then a burst of data are transffere at high speed .Another delay may ensue ,follwed by aburst data.A single read/write request may involve several such bursts .Hence the SCSI protocol is designed to facilitate this mode of operation.

10) A controller connected to a SCSI bus is one of two types an initiator or a target .An initator has the ability to select a particular targer and send commands specifying the operations to be performed. The disk controllers operates as a target . It carries out the commands it receive from the initator. The intiator establishes a logical connection has been established it can be suspended and restored as needed to transfer commands and bursts of data while particular connection is suspended,other devices can be use the bus to transfer information . 11) This ability to overlap data transfer request is one of the key features of the SCSI bus that leads to it’s high performance. 12) Data transfers on the SCSI bus are always controlled by the target controller. To send a command to a target, an initiator request controller of the bus and after winning arbitration , selects the controller.It wants to communicate with and hands control of the bus over to it.Then the controller starts a data transfer pperation to receive a command from the initiator.

13)

THE OPERATION OF SCSI BUS ACCORDING TO H/W:

Bus Signal : There are different signals shown in below table Category

Name

Data -DB(0) to –DB(7) byte of Information during the

Function Datalines:carry I

Information transfer And identifies device during Arbitration selection and Reseletion phases. -DB(p) Phase is Not free

-BSY

Information type -C/D Asserted during Transfer of

Parity bit for the databus Busy: asserted when the bus

control/data: control information.

-MSG

message:idicates

that the information Being transferred is a message Handshake to Request

-REQ

request: asserted by atarget a data transfer

cycle. -ACk

asserted by the

initiator when it has completed a operation Direction of transfer assrted to

–I/O

input/output: Indiacate an i/p

operation.

14) Here _ sign indicates that the signals are active ,or that dataline is equal to 1,when they are in the low voltage state. The bus has no addresslines,Instead ,the datalines

are used to identify the bus controllers involved during the selection or reselection and arbitration.

15)

Phases Of SCSI Bus:

The main phases involved in the operation of the SCSI bus are arbitration , selection,information transfer,and reselection. Arbitration: The bus is free when the _BSY signal is in the inactive state. Any controller can request the use of the bus while it is in this state.When 2 or more cotrollers may generate such a request at same time ,an arbitration scheme must be implemented. A controller requests the bus by asserting the _BSY signal. And by assrting its associated dataline to identify itself .The SCSI bus uses a simple distributed arbitration scheme. It is shown in below figure.

____ DB2 ____ DB5 ____ DB6

____ BSY

____ SEL free

Aribitration

Selection

Phases involved in the operations ofthr SCSI bus 16) Assume that controller 6 is an initiator that establishes a connection Controller 5 . After winning arbitration controller 6 proceeds to the selection phase, in which identifies the target.

17)

Selection:

Having won arbitration ,controller 6 contiues to assert _BSY and _DB6 .Any other controller that may have been involved in the arbitration phase ,such as controller 2 in the figure must stop driving the data lines.

Once the _SEL line becomes active, if it has not already done so.After placing the address of the target controller on the bus ,initiator releasest he _BSY signal. 18) The selected target controller responds by asserting –BSY this informs the initiator that the connection it is requesting has beenestablished,so that it may remove the address information from the datalines .The selection process is now complete and the target controller is asserting _BSY signal.

19)

Information Trarnsfer:

The information transfer between two controllersmay consist of commands from the target to the initiator to the target states , responses from the target to the initiator or data being transferred to or from the i/o device . Handshake signaling is used to control Information transfers at the end of the transfer , the target controller releases –BSY signal ,thus freeing the bus for use by other devices. 20)

Reselection:

When a logical connection suspended and the target is ready to be restore it,the target must first gain control of the bus .It’s starts an arbitration cycle ,and after winnig arbitration ,it selects the initiator controller.The initiator now asserting –BSY .Before data transfer begins,the initiator must hand controller over to the target . This is achieved by having target controller assert –BSY after selecting the inititor the target respose with states information and data transfer operations .The latter are controlled by the target , because it is a target that knows when data is available ,when to suspend and reestablish connections

Circuit for SCSI Bus ` D

sel

Q

A DFF > T R Qc Clk1 C

G

D

Q DFF

Clk2

RST

F > T R

Qc