AmÃ©rica latina

SCSI , Small System computer Interfaces English , is a standard defining a data-processing Bus making it possible to connect a computer to peripheral S or even another computer.

The standard describes the mechanical, electric specifications and functional calculuses of the bus.

History

Bus SCSI was initially created by the Shugart company, of the name of its founder Al Shugart, a pioneer of data processing, former engineer at IBM (specialist in the hard drives), under the name of SASI (Shugart Associates Systems Interface) in 1979.

Shugart binds with NCR Corporation in 1981 in the hope to make a standard ANSI of it. A technical committee is created, it will improve bus SASI appreciably, re-elects it SCSI and the X3.131 standard describing it will be born in 1986.

The first standard defined too many optional parameters for the standard (in particular in the command set, but also in the material possibilities as for the bits of parity etc), and the incompatibilities between manufacturing are numerous.

Improvements were made in 1994 with standard SCSI-2 and recently with standard SCSI-3, reducing these problems of interworking.

Specificity

What differentiates this bus of the others is that it off-sets the intelligence towards the peripheral itself. Of this fact the orders sent to the peripheral can be complex, it is under the control of the peripheral that they (possibly) will be broken up into simpler sub-tasks, which is extremely advantageous if one works with Operating systems multi-task.

This interface is thus faster, more universal and more complex than the interface E-IDE whose principal disadvantage is to monopolize a considerable percentage of the Processeur, which constitutes a handicap when many data flows are simultaneously open.

More “intelligent” and less dependant with respect to the central processing unit, interface SCSI can manage internal and external peripherals very varied, such as hard drives, scanner S, engravers, streamers, etc

Peripherals concerned

Standard SCSI-2 specifies that the bus can connect between them:

of the computers

with peripherals such as:

hard drives
readers of Diskette S
readers of magnetic bands
Printing S
readers of optical disk (WORM)
readers of optical disk (CD-ROMs)
scanner S
Changer of media
peripheral of communication

It is noted that the standard does not restrict the use of the bus to the interconnection of a computer with peripherals, but that it can be used between computers, or to divide peripherals between computers.

Standard SCSI-3 is more general practitioner. One will refer in the page of the technical committee to have the detail of it.

Modes of transfer

Improvements of the SCSI-3 compared to the SCSI-2

The SCSI-3 presents like enormous change the appearance of a drunk series, in a technology until now parallel exclusively . It results in also some innovations and improvements in the parallel interface.

Parallel interfaces

Ultra 3

The frequency of the drunk is doubled. One thus passes from 40Mhz to 80 MHz. Peripherals SE are not any more compatible with these frequencies with causes of the phenomena of Réverbération

Ultra 160

The interface Ultra 160 uses the LVD, it is not absolutely compatible any more with SCSI-1 and 2. Ultra the 160 makes heavy modifications in the management of the transfer of information.

Implemented of CRC. The bus being in LVD, there is larger speed and thus of much greater risks of errors in the transmissions. This is why one added the generation of a CRC in order to be able to check the integrity of the data.

Validation Field. The chart host negotiates with the peripheral best speed to be used (a little with the image what the Modem S do), which prevents that one communicates more quickly with the peripheral only it of it is not able (what before made it inaccessible)

Double Clocking Transition. One transfers the data on two sides (amount and descendant), which makes it possible to double the flows. This is why one speaks about ultra160 (80Mhz * 2)

Ultra 320

Interface 320 makes additional modifications compared to the ultra160, which makes it possible to further increase the work frequency to 160Mhz and to increase the productive flows.

Packetized SCSI. The ultra320 introduces a protocol of management of packages of data. What results in to reduce the number of orders to be transferred at the same time as the data. One can also transmit several orders in only one package.

QAS (Quick Arbitration and Selection). New method of management of the bus which reduces the number of orders necessary and thus the traffic " inutile"

Read and Write Dated Streaming. One can send several packages of sharp and await the ordering of confirmation of reception only at the time of the reception of all the packages, which reduces the traffic " once again; inutile"

Flow Control. Preprocessing of the data to be transferred in a pile FIFO, which makes it possible to gather data and to carry out a transfer in gust when a package is ready.

Pre-compensation. The increase in the frequency means the fall of the amplitude of the signal. A shift which would not have had any front consequence becomes awkward. With the initialization of the bus, one thus tests each wire and one creates a precompensation so that at the time of the sending of data on the bus, all the data arrive at the same time at destination (even if some wire are faster than others)

Interfaces series

Improvements compared to the parallel interface

With the explosion of the frequencies, the problems of shift between the signals and their sensitivity to the noise and the stray capacities become problematic and cause restriction in the face of the cables. One thus passes on drunk series (only one wire) what avoids the problems of induced currents.

The reflection on interfaces series was started before the development Ultra 160 and 320. But the flows offered are such as these technologies remain completely viable. The goal of the interface series is to pack up the orders and data SCSI and to transfer them via only one wire, while preserving compatibility SCSI (in order to keep advantage in particular possibility of storage of the orders, very useful in multitask) One finds many advantages in these technologies, one can quote in particular:

of architectures point-to-point
double access: one can reach a point in two different ways, which introduces a better resistance to the breakdowns on the bus
larger Possibilités on the levels of the systems RAID, one can go up to more than 100 hard drives thanks to SA or Fiber Chanel against 5 with a traditional interface SCSI

…

Technologies used

One can quote like technologies series using orders SCSI:

Improvements of the SCSI-2 compared to the SCSI-1

Standard SCSI-2 of 1994 is an improvement of the SCSI-1. Certain points were improved or made compulsory. Theoretically SCSI-1 and SCSI-2 have a downward compatibility. One can note that the use of the bits of parity was made compulsory with the SCSI-2

Fast SCSI

One uses synchronous transfers of high banc, which allows rate of transfer of 10Mo/s on a cablâge 8bits, and of 20 or 40 Mo if one is on 16 or 32bits (with a frequency of 10Mhz)

Wide SCSI

One with the possibility of working with broader buses of 16 or 32 bits, which allows more important flows. One used a cable has (50 pins) for the transmissions on 8bits, the SCSI-2 had envisaged a cable B (68 pins) for the broader buses. But it did not meet of real success, one prefers the cable P to him (68 pins also) defined in the SCSI-3. For the 32bits, the cable P must be used with a cable Q (68 pins him also)

Queue of the orders

The SCSI-1 made it possible to send the orders only one to one. The SCSI-2 makes it possible to send up to 256 orders a peripheral. They will be stored and treated in the optimal order by the peripheral. This makes it possible to increase the performances of work of the peripheral and takes an interest extremely when one works with operating systems multi-task (Linux, Ms-Windows NT etc) which can have to make several requests simultaneously with a peripheral (several accesses in reading on a Hard drive for example)

Types of wiring

Data transmission

Singled Ended (SE) : One works as an asymmetrical mode. For each signal to be sent by the bus corresponds a single wire.
High Differential Voltage (HVD): One works as a differential mode. For each signal, one uses two wire: the first carries the signal, the second carries his opposite. The receiver treats the difference between the two
Low Voltage Differential (LVD): Idem HVD, but with smaller tensions, allowing cables longer (better tolerance with the stray capacities and the Noise)

Resistances of termination

Passive resistance: At the end of the cable, one establishes two resistances of 220 and 330Ω, is 132Ω. The 220 is connected to the mass, the 330 with the food which is provided by one of the components of the bus (which should thus never be extinct if it is wanted that the bus functions). But at too high frequencies, this type of resistance of termination causes reverberations of the signal. The passive termination is limited to small lengths of cables, and insufficient for Fast and Wide SCSI-2. It thus should be avoided, which is not difficult because there is not practically more manufacturer who uses it.
active Resistance (or Alternative2): Resistance of 110 Ω obtained by voltage regulators. It avoids the phenomena of reverberation. The active termination is strongly advised by standard SCSI-2. It offers regulators of voltage so that signal SCSI always finishes on a level of correct voltage.
forced perfect Resistance: Alternative of Alternative2 which allows longer cables. The termination Forced Perfect Termination is a variation improved of the active termination. There are several versions: FPT-3, 18,27…

See too

taken SCSI with 68 or 80 pins

External bonds

In English:

Al Shugart
SCSI Standards Structures
Norme SCSI-2
Excellent article of popularization on bus SCSI

In French:

SCSI my love

Random links: