Seagate ST39251LC Instrukcja Użytkownika Pobierz pdf (Strona 6)

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SCSI-3

Before Adaptec and later SCSITA codified the terminology, the first parallel SCSI devices that exceeded the SCSI-

2 capabilities were simply designated SCSI-3. These devices, also known as Ultra SCSI and fast-20 SCSI, were

introduced in 1996. The bus speed doubles to 20 MB/s for narrow (8 bit) systems and 40 MB/s for wide (16-bit).

The maximum cable length stayed at 3 meters but single-ended Ultra SCSI developed an undeserved reputation

for extreme sensitivity to cable length and condition (faulty cables, connectors or terminators were often to

blame for instability problems). SCSI-3 has many advances over SCSI-2 such as Serial SCSI. This feature will allow

data transfer up to 100MB/s through a six-conductor coaxial cable. SCSI-3 solves many of the termination and

delay problems of older SCSI versions. SCSI-3 eases SCSI installation woes by being more plug-and-play in nature,

such as automatic SCSI ID assigning and termination. SCSI-3 also supports 32 devices while SCSI-2 supports only

SCSI-3 changed the document structure, SCSI-3 is not one document with all the different layers and electrical

interfaces, but a collection of documents that cover the physical layer, the basic protocol specific to that

electrical interface, the primary command set layer (SPC) and the specific protocol layer. The specific protocol

layer contains the Hard Disk interface Commands in the Block Commands (SBC), Steam Commands for tape

drives (SSC), Controller Commands for RAID arrays (SCC), Multimedia Commands (MMC), Media Changer

Commands (MCC) and enclosure services commands (SES) for example. There is an overall architectural model

(SAM). Elements of SCSI-3 are in use today in the forms of Ultra-Wide and Ultra SCSI drives. Ultra SCSI delivers

20MB/s over the 8-bit bus. Ultra-Wide SCSI incorporates the 16-bit bus, and the speed reaches to 40MB/s.

Different SCSI specifications:

Name

Specification

# of Devices

Bus Width

Bus Speed

MBps

Asynchronous SCSI

SCSI-1

8 bits

5 MHz

4 MBps

Synchronous SCSI

SCSI-1

8 bits

5 MHz

5 MBps

Wide

SCSI-2

16 bits

5 MHz

10 MBps

Fast

SCSI-2

8 bits

10 MHz

10 MBps

Fast/Wide

SCSI-2

16 bits

10 MHz

20 MBps

Ultra

SCSI-3

SPI

8 bits

20 MHz

20 MBps

Ultra/Wide

SCSI-3

16 bits

20 MHz

40 MBps

1 2 3 4 5 6 7 8 9 10 11 ... 52 53

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