SCSI (Small Computer System Interface) has
a long history in the
computing industry.
Since SCSI became an industry standard,
the interface has evolved to keep pace with the demands of the
most sophisticated systems.  The standard recognizes virtually every
peripheral type and takes advantage of newer hardware and more
intelligent controllers.
The data path has been
widened and transfer speeds have been increased to keep pace with
system requirements. Just this year, the STA adopted a new SCSI
technology roadmap that will provide up to 640 Mbytes/second performance,
as well as increased manageability and more features for high-end
desktop connectivity.


 







  
 
     
      


The 7 Generations of SCSI

    		
  

  
 
     Up 
      to 5

      Megabytes

      Per Second

    		
     Up 
      to 10

      Megabytes

      Per Second

    		
     
      
Up 
        tp 20

        Megabytes

        Per Second

      

    		
     Up 
      to 40

      Megabytes

      Per Second

    		
     Up 
      to 80

      Megabytes

      Per Second

    		
     Up 
      to 160

      Megabytes

      Per Second

    		
     Up 
      to 320

      Megabytes

      Per Second

    		
  

  
 
     
     
     
     
     
      

        Ultra2
 
        SCSI

        Fast-40

        SPI-2

        LVD SCSI

    		
    
      Ultra3 or

      Ultra160 SCSI

      Fast-80

      SPI-3

      LVD SCSI		
    
    
      Ultra320 SCSI

      Fast-160

      SPI-4

      LVD SCSI

  

  
 
    SCSI-1

                 		
    SCSI-2

      SE		
    SCSI-3 
      SPI

      Fast & Wide

      SE		
      Fast-20

      Ultra SCSI

      SE		
    
     
     
  

  
 
    SCSI-2

      Fast

      Differential

      HVD SCSI		
    SCSI-3 
      SPI

      Fast & Wide

      Differential

      HVD SCSI		
    Fast-20

      Ultra SCSI

      Differential

      HVD SCSI		
     
     
     
  

  
 
    
    
    
  

  
 
     
      
Narrow 
        (8 bit data) bus only

    		
     
      
Wide 
        (16 bit data)

        and narrow (8 bit data) bus

    		
     
      
Wide 
        (16 bit data) bus only

    		
  

  
 
     
      
The Three electrical levels of SCSI:

SE = Single Ended

HVD SCSI or Differential SCSI = High voltage differential SCSI, based on EIA485

LVD SCSI = Low voltage differential SCSI


Source:  Paul Aloisi, Texas Instruments; March 2000









The storage of preference in Internet servers, Servers, workstations and 
High end PCs.





Ultra320 is only the next step, future steps are planned as the performance of 
the disk drives improve.


SCSI started as a narrow bus (50 pin connector) transferring one byte at a time 
and grew into a wide bus (68 or 80 pin connector) transferring 2 bytes at a 
time in SCSI-3 SPI. LVD SCSI was added in SPI-2 allowing high speed transfers 
with a growth plan. SCSI is basically following the rules of Moore's Law 
doubling performance with each generation. SPI-3 added packetized SCSI that 
reduces the protocol overhead. 


The 80 pin SCA-2 connector integrates power and signal for hot plugging SCSI 
devices into backplanes. They should only be used on backplane systems.


SCSI connects the generations; all 7 generations can run on the same logical 
bus. Expanders Isolate the high speed LVD SCSI bus from the slower single ended 
or HVD bus segment.


Fibre channel and Infinaband make good connection systems for connecting SCSI 
RAID (Redundant array of independent disks) boxes or JBODs (Just a box of 
Disks) to systems. SCSI arrays are used in the SAN. These are not competing 
interfaces, but ways of improving the connections to the SCSI devices and SCSI 
arrays.


Plan your storage future around SCSI and allow it to grow as the technology 
grows. SCSI offers the lowest cost interface for the high performance disk 
drives.




    		
  







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