IS8030 SCSI Small Computer System Interface

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IS8030SCSI(Small Computer System Interface)

• Michelle L. Allen
• Lectra Lawhorne

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OUTLINE

• Introduction
• Evolution
• Unique Features
• Specifications
• Pros of SCSI
• Cons of SCSI
• Conclusion

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INTRODUCTION

• Developed to meet needs for a flexible, faster
  interface
• Provides the medium between PCs and peripheral
  devices
• Unlike other interfaces (IDE for example)
• it consists of a separate
• bus provided through the
• host adapter thus
• eliminating bottlenecks
• on system resources

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EVOLUTION

• 1979 Shugart started developing a new universal
  interface to use logical addressing instead of
  head/cylinder/sector addressing
• 1981 Presented to ANSI and the project was
  named SCSI
• 1984/1985 Designers presented the draft
  proposal for approval
• 1986 SCSI-1 became official
• Apple Computer was one of the first to use SCSI
  as a key component of Mac computers
• Adoption of SCSI slow to start due to
• legacy systems and resistance to change
• Manufacturers were hesitant to adopt due to cost
  and effort to create
• Manufacturers feared the technology might become
  obsolete
•  

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EVOLUTION (Cont.)

• SCSI is ever-evolving
• Family of different standards that build on each
  other(SCSI-1, SCSI-2, SCSI3, Fast SCSI,
  Fast/Wide SCSI, Ultra SCSI, Wide Ultra SCSI,
  Ultra2 SCSI, Wide Ultra2 SCSI, and more)
• SCSI-1 addressed only the physical and electrical
  specifications - need for a common set of
  commands so the different manufacturers hardware
  could work collectively
• CCS (Common Command Set) developed
• Manufacturers claimed devices SCSI-2 compliant
  but were not proved that SCSI-1 works with
  SCSI-2

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UNIQUE FEATURES

• Separate bus that is attached to system bus
  through an adapter
• Built in controller interprets demands
• CPU and system bus demands for devices are done
  through interaction with the host adapter
• Adapter handled the commands leaving and does not
  utilize the CPU
• Only the adapter takes up a system IRQ instead of
  each device (better scalability)
• Each device can support 8 devices thus
  improving scalability (though devices are not
  taking advantage of this yet)
• Command queuing Can send 256 commands on bus to
  SCSI device
• The device can process commands in any order it
  desires
• Frees bus for other devices and commands

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SPECIFICATIONS

• Initiators are devices that request I/O processes
  and targets are those devices that perform the
  actions requested by the initiator.
• Each device attached to the SCSI bus may be an
  initiator and the bus is only used for
  transmitting data and commands
• The transfer of data and commands occurs in eight
  phases
• Bus-Free Checks if bus is free to use
• Arbitration Bus placed in busy state higher ID
  wins in contention
• Selection Target states it is active too,
  remembers initiator for broken connections
• Command 4 -7 share the data commands
• Data also termed Data In about connection and
  status
• Message also termed Data Out information for
  each device
• Status
• At completion the initiator will reset the bus
  control signal to a ready state
• During selection an intentional or error break
  can occur an intentional would be for memory
  access which is time consuming so it frees the
  bus for utilization

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PROS of SCSI

• Flexible device attachment (7 or 15 devices per
  SCSI bus)
• Longer cable lengths allowed (up to 12 meters)
• Support for almost any peripheral device (disks,
  tape, CDROM, optical, scanner, etc.)
• Interface and protocol is carefully specified by
  ANSI, and maintains high performance standards
• Most adapters can do scatter/gather DMA which is
  a necessity in virtual memory systems (like Unix,
  NT, 2000)
• Development cycles for SCSI devices is very
  short, thus the latest generation of SCSI
  peripherals keeps pace with the latest generation
  of computer systems
• The intelligent interface places commands onto
  the SCSI device, thus freeing the CPU for other
  tasks. (IDE takes up to 95 of the CPU during
  data transfers, while SCSI only uses about 5 of
  the CPU for buffer management.)
• Extends concept of multitasking to peripheral
  devices

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CONS of SCSI

• Generally more expensive, due to more complex
  firmware, extra testing is required, and
  increased performance typically costs more in the
  computer market.
• Slightly more complicated to install, due to
  termination requirements and if one uses multiple
  buses.
• Terminology and connection protocol options are
  overwhelming.
• Some people point to the need to set IDs in SCSI
  as making it more complicated, but it's really no
  more complicated than choosing master/slave
  jumpers in IDE.

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CONCLUSION

• The SCSI interface is complex yet very versatile
• Its increased scalability and increased speed
  will allow for computers to attach more devices
• With its separation from the main system's bus
  and CPU it provides a unique and excellent
  solution to limited system resources
• Even though other interfaces are still used,
  SCSI's benefits will increase its popularity
• Its constant evolution and development is a
  testament to its effectiveness.