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SCSI. Pronounced “scuzzy”. By Cory Washburn. The SCSI Logo. Small Computer System Interface. What is SCSI?. SCSI is a set of standards for physically connecting and transferring data between computers and peripheral devices.

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Pronounced “scuzzy”

By Cory Washburn

The SCSI Logo

Small Computer System Interface

What is SCSI?

  • SCSI is a set of standards for physically connecting and transferring data between computers and peripheral devices.

  • The SCSI standards define commands, protocols, and electrical and optical interfaces.

  • SCSI is mostly used for hard disks and tape drives, but it can connect a wide range of other devices.

How it Works

  • A computer is full of busses, pathways that take information and power from one place to another. For example, when you plug an MP3 player into your computer, you're probably using a USB port. Your USB port is good at carrying the data and electricity

    required for small electronic

    devices, but that bus isn't big

    enough to support a whole

    computer, a server or lots of

    devices simultaneously. For

    that, you'd need SCSI.

How it Works Continued

  • A SCSI controller, called a host adapter, coordinates between all of the devices on the SCSI bus and the computer. On the controller is the SCSI BIOS, which is a small ROM or Flash memory chip that holds the software needed to control the devices on the bus.

  • Each SCSI device must have a unique ID(UI) in order for it to work right. If the bus can support sixteen devices, their IDs range from zero to 15. While the external devices attach themselves to the controller using a thick round cable, internal devices connect to the SCSI controller using a ribbon cable.

  • Once all the devices are installed on the bus and given a UI, the end of the bus should be closed; else the signal in the bus might reflect back interfering with the communication. A resistor circuit is used to solve this problem so that every device connected using SCSI can be operated.

SCSI Command Protocol

  • Communication takes place between an initiator and a target. The initiator sends a command to the target which then responds. SCSI commands are sent in a Command Descriptor Block (CDB). The CDB consists of a one byte operation code followed by five or more bytes containing command-specific parameters.

  • There are 4 categories of SCSI commands: N (non-data), W (writing data from initiator to target), R (reading data), and B (bi-directional). There are about 60 different SCSI commands in total; some common ones are:

  • Inquiry, Send diagnostic, Start/Stop unit, Read, Write

Why use scsi?

  • Most workstations use the IDE interface. Probably the reason is that IDE drives are cheaper than SCSI type devices. For most workstation usage, the IDE interface is more than adequate. But if high speed disk access is crucial, the choice should be SCSI.

  • Besides the speed issues, another important feature of SCSI interfaces is that backwards compatibility has been incorporated in all new improvements. Different SCSI technologies can be mixed on the same machine.


  • In the 1970's - SASI, the Shugart Associates System Interface was developed as an 8-bit system bus. The SASI interface defined a logical level rather than a device level interface to disk.

  • In 1982 After rejection and improvement in 1978, SASI was again submitted to ANSI which then used it as the foundation for the SCSI standard.

  • By 1983, SCSI had matured to the point that NCR began shipping the first SCSI chipset.

  • In 1986 SCSI-1 was approved by ANSI.

  • In 1994, extensions to SCSI-2 were finalized.

History Continued

  • 2001 SCSI Trade Association started Serial Attached SCSI (SAS), designed to be the evolution of SCSI to satisfy requirements of scalability, performance, reliability and management, while leveraging a common low-cost electrical and physical connection interface from SATA.

  • Internet SCSI (iSCSI) is an official standard ratified in 2003 by the Internet Engineering Task Force that allows the use of the SCSI protocol over TCP/IP networks.

  • 2004 - BiTMICRO Networks announced the availability of the world's first Ultra320 SCSI solid-state flash storage.


  • SingleEndedSCSI is a kind of electrical signal where one wire carries the signal and another wire or shield is connected to electrical ground.

  • High Voltage Differential SCSI (HVD) is a kind of electrical connection using two wires, one of which carries a positive signal and the other a negative signal.This type of SCSI is generally used where long distances are required between end devices. This hardware is very expensive and now almost obsolete.

  • The original standard (SCSI) has been rewritten twice to give us what is now called SCSI-1, SCSI-2, and SCSI-3.

  • Fast SCSI – Doubles clock speed of bus

  • Wide SCSI – Doubles amount of devices that can be connected, by increasing the bus width

  • Ultra SCSI – Doubles clock speed of bus

SCSI 1 and 2

  • SCSI-2 improved upon SCSI-1, but also tried to standardize the command set. It added Fast SCSI, Wide SCSI, active termination, differential signaling (HVD), command queuing, and more. Command queuing was introduced to allow multiple command requests between devices on the bus. Also, more devices were supported on the bus to keep up with the current technology, such as Zip drives. Fast SCSI doubles the bus speed. Wide SCSI doubles the width of the bus. If you combine Fast and Wide SCSI, you're looking at 20mb/s transfer speeds.

SCSI-1 is what started it all, and is now obsolete. SCSI-1 involved the specs for the basic 8-bit SCSI bus, with about 5 MBs transfer rate.


  • SCSI-3 differs from SCSI-2 in that it contains many more new features, technologies, and command sets. This interface has evolved into SPI-2, SPI-3, and SPI-4.

  • The original SPI worked with parallel SCSI as well as Fast SCSI. Transfer rate was increased to 40mb/s.

  • As well as doubling the transfer rate, SPI-2 added even more changes, which included Low Voltage Differential Signaling (LVD), multimode devices, and new connectors. LVD has pretty much replaced HVD in modern SCSI systems, and it's even required for the fastest SCSI buses. Ultra2 SCSI and Wide Ultra2 SCSI are the informal marketing terms for devices that use the SPI-2 standard.

  • The SPI-3 standards again doubled the transfer rate to 160 MB/s. Lots of performance enhancements were added as well as removing some obsolete standards.

Ultra SCSI


  • The newest type of SCSI, called SAS, uses SCSI commands but transmits data serially. SAS uses a point-to-point serial connection to move data at 3.0 gigabits per second, and each SAS port can support up to 128 devices or expanders.

  • The first level of Ultra is just called Ultra and has a data transfer rate of 20 MB/s w/o a wide bus

  • The second is called Ultra2 with 40 MB/s. Ultra2 is also the last type of SCSI to have a narrow bus width of 8 bits

  • The third is called Ultra3 and has a rate of 160 MB/s

  • The fourth is Ultra320 and this one doubled the last with a 360 MB/s transfer rate


  • Short for Internet

    SCSI, it’s an IP-based

    standard for linking

    data storage devices

    over a network and

    transferring data by

    carrying SCSI

    commands over IP

    networks. iSCSI

    supports a Gigabit

    Ethernet interface.



ACARD Technology CorporationAdaptec, Inc.Amphenol CorporationCompaq Computer CorporationDallas SemiconductorIBM Corp.LSI Logic CorporationLinfinity Microelectronics Inc.Maxtor Mylex CorporationQLogicQuantum Corp.Seagate TechnologyTexas InstrumentsWieson Electronic Co., Ltd.

  • For SPI adapters, the prices range from $4 to over $100

  • Termination Interfaces are usually only a few dollars

  • Hard drives and other devices using SCSI have varied prices

  • SCSI host adapter prices can range from $20 to 1000’s of dollars




# of Devices

Bus Width

Bus Speed





8 bits

5 MHz

4 MBps




8 bits

5 MHz

5 MBps




16 bits

5 MHz

10 MBps




8 bits

10 MHz

10 MBps




16 bits

10 MHz

20 MBps




8 bits

20 MHz

20 MBps




16 bits

20 MHz

40 MBps




8 bits

40 MHz

40 MBps




16 bits

40 MHz

80 MBps




16 bits

40 MHz

160 MBps




16 bits

80 MHz

320 MBps


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