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Introduction to SCSI Concepts


Brief Introduction to SCSI Concepts

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									Introduction to SCSI Concepts

       The Small Computer System Interface (SCSI) is a computer industry standard for connecting
        computers to peripheral devices such as hard disk drives, CD-ROM drives, printers….
       The SCSI standard specifies the hardware and software interface at a level that minimizes
        dependencies on any specific hardware implementation
       A SCSI device refers to any unit connected to the SCSI bus, either a peripheral device or a
       Each SCSI device on the bus is assigned a SCSI ID, which is an integer value from 0 to 7 that
        uniquely identifies the device during SCSI transactions.
       When two SCSI devices communicate, one device acts as the initiator and the other as the

SCSI Bus Signals
/BSY   Busy             Indicates that the bus is in use.
/SEL   Select           The initiator uses this signal to select a target.
/C/D   Control/Data     The target uses this signal to indicate whether the information being transferred
               is control / Data
/I/O   Input/Output The target uses this signal to specify the direction of the data movement with
               respect to the initiator.
/MSG Message            This signal is used by the target during the message phase.
/REQ Request            The target uses this signal to start a request/acknowledge handshake.
/ACK Acknowledge This signal is used by the initiator to end a request/acknowledge handshake.
/ATN Attention          The initiator uses this signal to inform the target that the initiator has a message
/RST Reset              This signal is used to clear all devices and operations from the bus, and force the
                        bus into the bus free phase.

SCSI Bus Phases

Bus free.
This phase means that no SCSI devices are using the bus, and that the bus is available for another SCSI

       This phase is preceded by the bus free phase and permits a SCSI device to gain control of the
        SCSI bus
       During this phase, all devices wishing to use the bus assert the /BSY signal and put their SCSI ID
        onto the bus (using the data signals).
       The device with highest SCSI ID wins the arbitration.

Selection. This phase follows the arbitration phase. The device that won arbitration uses this phase to
select another device to communicate with
Reselection. This optional phase is used by systems that allow peripheral devices to disconnect and
reconnect from the bus during lengthy operations.

Command. During this phase, the target requests a command from the initiator.

Data. The data phase occurs when the target requests a transfer of data to or from the initiator.

Status. This phase occurs when the target requests that status information be sent to the initiator.

Message. The message phase occurs when the target requests the transfer of a message. Messages
are small blocks of data that carry information or requests between the initiator and a target. Multiple
messages can be sent during this phase.

Together, the last four phases (command, data, status, and message) are known as the information
transfer phases

        SCSI commands are contained in a data structure called a command descriptor block (CDB),
         which can be 6, 10, or 12 bytes in size. The first byte specifies the operation requested, and the
         remaining bytes are parameters used by that operation.
        SCSI messages are small blocks of data, often just one byte in size, that indicate the successful
         completion of an operation (the command complete message), or a variety of other events,
         requests, and status information. All messages are sent during the message phase.

SCSI Types

SCSI has three basic specifications:
        SCSI-1: SCSI-1 is now obsolete. It featured a bus width of 8 bits and clock speed of 5 MHz.
        SCSI-2: This specification included the Common Command Set (CCS) -- 18 commands
         considered an absolute necessity for support of any SCSI device. It also had the option to double
         the clock speed to 10 MHz (Fast), double the bus width from to 16 bits and increase the number
         of devices to 15 (Wide), or do both (Fast/Wide). SCSI-2 also added command queuing,
         allowing devices to store and prioritize commands from the host computer.
        SCSI-3: Includes a series of smaller standards within its overall scope. A set of standards
         involving the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate
         with each other, has continued to evolve within SCSI-3. Most SCSI-3 specifications begin with the
         term Ultra, such as Ultra for SPI variations, Ultra2 for SPI-2 variations and Ultra3 for SPI-3
         variations. The Fast and Wide designations work just like their SCSI-2 counterparts. SCSI-3 is the
         standard currently in use.

                            Bus           Max. Bus Lengths, Meters (1)   Max.
                                    Bus                                 Device
        STA Terms                  Width,
                                    bits                               Support(7
                            Max.          Single-
                                                    LVD       HVD          )

SCSI - 1 (2)                   5         8         6         (3)         25            8
                                10          8         3         (3)       25           8
Fast SCSI (2)

Fast Wide SCSI                  20         16         3         (3)       25           16

                                20          8        1.5        (3)       25           8
Ultra SCSI (2)

                                20          8         3          -           -         4
Ultra SCSI (2)

                                40         16         -         (3)       25           16
Wide Ultra SCSI

                                40         16        1.5         -           -         8
Wide Ultra SCSI

                                40         16         3          -           -         4
Wide Ultra SCSI

                                40          8        (4)        12        25           8
Ultra2 SCSI (2,4)

                                80         16        (4)        12        25           16
Wide Ultra2 SCSI (4)

                               160       16(6)       (4)        12        (5)          16
Ultra 160 SCSI (6)

                               320       16(6)       (4)        12        (5)          16

Controllers, Devices and Cables

        A SCSI controller coordinates between all of the other devices on the SCSI bus and the
         computer. Also called a host adapter
        The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that
         contains the software needed to access and control the devices on the bus.
        Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example,
         if the bus can support sixteen devices, their IDs, specified through a hardware or software setting,
         range from zero to 15


        Internal devices connect to a SCSI controller with a ribbon cable
        External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial
         Attached SCSI devices use SATA cables.)
        The cable itself typically consists of three layers:
         Inner layer: The most protected layer, this contains the actual data being sent.
         Media layer: Contains the wires that send control commands to the device.
         Outer layer: Includes wires that carry parity information, which ensures that the data is
       Different SCSI variations use different connectors, which are often incompatible with one another.
        These connectors usually use 50, 68 or 80 pins.
       Once all of the devices on the bus are installed and have their own IDs, each end of the bus must
        be closed. We'll look at how to do this next.


       If the SCSI bus were left open, electrical signals sent down the bus could reflect back and
        interfere with communication between devices and the SCSI controller.
       The solution is to terminate the bus, closing each end with a resistor circuit. If the bus supports
        both internal and external devices, then the last device on each series must be terminated.
Types of SCSI termination can be grouped into two main categories
       Passive termination is typically used for SCSI systems that run at the standard clock speed and
        have a distance of less than 3 feet (1 m) from the devices to the controller.
       Active termination is used for Fast SCSI systems or systems with devices that are more than 3
        feet (1 m) from the SCSI controller

SCSI also employs three distinct types of bus signaling, which also affect termination. Signaling is the
way that the electrical impulses are sent across the wires.
       Single-ended (SE): The controller generates the signal and pushes it out to all devices on the
        bus over a single data line. Each device acts as a ground. Consequently, the signal quickly
        begins to degrade, which limits SE SCSI to a maximum of about 10 ft (3 m). SE signaling is
        common in PCs.
       High-voltage differential (HVD): Often used for servers, HVD uses a tandem approach to
        signaling, with a data high line and a data low line. Each device on the SCSI bus has a signal
        transceiver. When the controller communicates with the device, devices along the bus receive the
        signal and retransmit it until it reaches the target device. This allows for much greater distances
        between the controller and the device, up to 80 ft (25 m).
       Low-voltage differential (LVD): LVD is a variation on HVD and works in much the same way.
        The big difference is that the transceivers are smaller and built into the SCSI adapter of each
        device. This makes LVD SCSI devices more affordable and allows LVD to use less electricity to
        communicate. The downside is that the maximum distance is half of HVD -- 40 ft (12 m).

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