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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) ]

Summary of SCSI Protocols and Transfer Modes

For easier comparison, the chart below shows all of the different SCSI transfer modes and feature sets, along with their key characteristics. For your clicking pleasuer, I've also hyperlinked everything that wasn't nailed down ;^)

Transfer Mode

Defining Standard

Bus Width (bits)

Bus Speed (MHz)

Through- put (MB/s)

Special Features

Cabling

Signaling Method

Maximum Devices Per Bus

Maximum Cable Length (m)

"Regular" SCSI (SCSI-1)

SCSI-1

8

5

5

 

50-pin

SE

8

6

HVD

8

25

Wide SCSI

SCSI-2

16

5

10

 

68-pin

SE

16

6

HVD

16

25

Fast SCSI

SCSI-2

8

10

10

 

50-pin

SE

8

3

HVD

8

25

Fast Wide SCSI

SCSI-2

16

10

20

 

68-pin

SE

16

3

HVD

16

25

Ultra SCSI

SCSI-3 / SPI

8

20

20

 

50-pin

SE

8

1.5

4

3

HVD

8

25

Wide Ultra SCSI

SCSI-3 / SPI

16

20

40

 

68-pin

SE

8

1.5

4

3

HVD

16

25

Ultra2 SCSI

SCSI-3 / SPI-2

8

40

40

 

50-pin

LVD

8

12

2

25

HVD

8

25

Wide Ultra2 SCSI

SCSI-3 / SPI-2

16

40

80

 

68-pin

LVD

16

12

2

25

HVD

16

25

Ultra3 SCSI

SCSI-3 / SPI-3

16

40 (DT)

160

At least one of Fast-80, CRC, DV, QAS, Packet

68-pin

LVD

16

12

2

25

Ultra160(/m) SCSI

SCSI-3 / SPI-3

16

40 (DT)

160

Fast-80, CRC, DV

68-pin

LVD

16

12

2

25

Ultra160+ SCSI

SCSI-3 / SPI-3

16

40 (DT)

160

Fast-80, CRC, DV, QAS, Packet

68-pin

LVD

16

12

2

25

Ultra320 SCSI

SCSI-3 / SPI-4

16

80 (DT)

320

Fast-160, ?

68-pin

LVD

16

12

2

25

Some notes on this table:

  • "(DT)" means that transfer mode uses double transition clocking.
  • Throughput numbers are in decimal megabytes per second.
  • To keep the table from getting cluttered, I've only mentioned current cabling, not obsolete cables.
  • The number of devices includes the host adapter.
  • For Ultra and faster speeds, the maximum length of cable for some signaling types depends on the number of devices on the chain; thus, the multiple rows.
  • Ultra320 specifications are not final as of this writing.

Note: The number of devices (8 or 16) includes the host adapter.

Next: SCSI Host Adapters


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