SCSI 简介

SCSI (pronounced SKUH-zee and sometimes colloquially known as"scuzzy"), the Small Computer System Interface, is a set ofANSI standard electronic interfaces that allowpersonal computers to communicate withperipheral hardwaresuch as disk drives,tape drives, CD-ROM drives, printers, and scanners(扫描仪) faster and more flexiblythan previous interfaces. Developed at Apple Computer and still used in theMacintosh, the present set of SCSIs areparallelinterfaces(并行接口). SCSI ports continue to be built into many personal computers today and are supported by all major operating systems.

In addition to faster data rates, SCSI is more flexible than earlier parallel data transfer interfaces. The latest SCSI standard,Ultra-2 SCSI for a 16-bitbus can transfer data at up to 80 megabytes persecond (MBps).SCSI allows up to 7 or 15 devices(depending on the bus width) to be connected to a single SCSI portin daisy-chain fashion(菊花链方式). This allows one circuit board or card toaccommodate all the peripherals, rather than having a separate card for eachdevice, making it an ideal interface for use with portable and notebookcomputers.A single host adapter(主适配器), in the form of a PC Card, can serve as a SCSI interface for a laptop, freeing up the parallel and serial ports for use with an external modem and printer while allowing other devices to be used in addition.

Although not all devices support all levels of SCSI, theevolving SCSI standards are generallybackwards-compatible（向后兼容）. That is, if you attach an older device to a newer computer with support for a later standard, the older device will work at the older and slower data rate.

The original SCSI, now known as SCSI-1, evolved into SCSI-2, known as"plain SCSI". as it became widely supported.SCSI-3 consists of a set of primary commands and additional specialized command sets to meet the needs of specific device types. The collection of SCSI-3 command sets is used not only for the SCSI-3 parallel interface but for additional parallel and serial protocols, includingFiber Chanel（光纤通道） , Serial Bus Protocol (used with the IEEE 1394Firewire physical protocol), and the Serial Storage Protocol (SSP).

A widely implemented SCSI standard is Ultra-2 (sometimesspelled "Ultra2") which uses a 40MHz clock rate to get maximum data transfer ratesup to 80 MBps. It provides a longer possible cabling distance (up to 12 meters) by using low voltage differential(LVD) signaling(低压差动信号机制). Earlier forms of SCSIs use a single wire that ends in a terminatorwith a ground. Ultra-2 SCSI sends the signal over two wires with the data represented as the difference in voltage between the two wires. This allows support for longer cables. A low voltage differential reduces power requirements and manufacturing costs.

The latest SCSI standard is Ultra-3 (sometimes spelled"Ultra3")which increases the maximum burst rate from 80 Mbps to 160Mbps by being able to operate at the full clock rate rather than the half-clock rate of Ultra-2. The standard is also sometimes referred to as Ultra160/m. Newdisk drives supporting Ultra160/m will offer much faster data transfer rates.Ultra160/m also includescyclical redundancychecking (CRC)for ensuring the integrity of transferred data and domain validation for testing the SCSI network.

Currently existing SCSI standards are summarized in thetable below.

Technology
  Name

Maximum Cable
  Length (meters)

Maximum
  Speed
  (MBps)

Maximum
  Number of
  Devices

SCSI-1

6

5

8

SCSI-2

6

5-10

8 or 16

Fast SCSI-2

3

10-20

8

Wide SCSI-2

3

20

16

Fast Wide SCSI-2

3

20

16

Ultra SCSI-3, 8-bit

1.5

20

8

Ultra SCSI-3, 16-bit

1.5

40

16

Ultra-2 SCSI

12

40

8

Wide Ultra-2 SCSI

12

80

16

Ultra-3 (Ultra160/m) SCSI

12

160

16