Linux+: Hardware Part 16 – Small Computer Systems Interface (SCSI)

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  1. Jarret W. Buse

    Jarret W. Buse Well-Known Member Staff Writer

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    Linux+: Hardware Part 16 – Small Computer Systems Interface (SCSI)

    The Small Computer System Interface (SCSI) is a high-end and more advanced bus system then the Integrated Drive Electronics (IDE). Unlike IDE, which allows for two devices on a cable, SCSI allows for multiple devices per cable. With SCSI, the controller is placed on the SCSI Controller card which can be built-in to the motherboard or added as an expansion card.

    NOTE: For SCSI Basics, see the article Small Computer System Interface (SCSI): Basic Primer.

    There are nine SCSI Transfer Modes which are supported by a SCSI Specification:
    1. Original SCSI (SCSI-1) – SCSI-1/SCSI-2 spec
    2. Wide SCSI – SCSI-2 spec
    3. Fast SCSI – SCSI-2 spec
    4. Fast Wide SCSI – SCSI-2 spec
    5. Ultra SCSI – SCSI-3 spec or SP1
    6. Wide Ultra SCSI – SCSI-3 spec or SP1
    7. Ultra2 SCSI – SCSI-3 spec or SPI-2
    8. Wide Ultra2 SCSI – SCSI-3 spec or SPI-2
    9. Ultra3 SCSI – SCSI-3 spec or SPI-3
    Before we look into these types let’s look at signaling methods since each Transfer Mode can use different signaling methods. There are three signaling methods:
    1. Single-ended (SE)
    2. High Voltage Differential (HVD)
    3. Low Voltage Differential (LVD)

    In Single-ended signaling method a varying voltage is sent over a wire to transmit data. A positive voltage represents a one, or an on-bit, and zero voltage represents a zero, or an off-bit. Another wire is to ground and is used to determine if the varying value is positive or zero.

    With High Voltage Differential and Low Voltage Differential signaling, voltage is sent through two wires. The bits are determined by the difference of voltage on the two wires. If zero volts are sent on the two wires then it represents an off-bit. If one wire is a positive voltage and the second wire is of equal but negative volts, the bit is a one or an on-bit. HVD uses voltages over 5 volts while LVD uses voltages under 5 volts. HVD and LVD allow for better and faster signaling to contribute to longer cable lengths.

    The Original SCSI controller had a bus width of 8 bits and could transfer data at 5 MB/s. With SE there could 8 devices per bus with a maximum cable length of 6 meters. HVD can also achieve 8 devices but with a maximum cable length of 25 meters.

    NOTE: The maximum device count includes the controller card. Cable length is the length of all the cables from one terminated device on one end to the other terminated device on the other end.

    Wide SCSI allows for a bus width of sixteen bits (2 bytes) with a throughput of 10 MB/s. If it uses SE signaling there can be 16 devices and a maximum cable length of six meters. HVD also has a limit of 16 devices but a 25 meter cable length.

    Fast SCSI has an eight bit bus width and a throughput of 10 MB/s. SE signaling allows 8 devices and only a cable length of 3 meters. HVD also supports 8 devices but a cable length of 25 meters.

    Fast Wide SCSI has a 16 bit bus width and can support data speeds of 20 MB/s. The SE and HVD signaling can support up to 16 devices, but SE cables can only be a maximum of 3 meters in length. The HVD cables can be 25 meters in length.

    Ultra SCSI has an 8 bit bus width with a data throughput of 20 MB/s. SE signaling has two possible configurations: 8 devices with a cable length of 1.5 meters, or 4 devices with a cable length of 3 meters. The HVD signaling method can support 8 devices with a cable length of 25 meters.

    Wide Ultra SCSI has a bus width of 16 bits and a throughput of 40 MB/s. SE signaling has two possible configurations: 8 devices with a cable length of 1.5 meters, or 4 devices with a cable length of 3 meters. The HVD signaling method can support 16 devices with a cable length of 25 meters.

    Ultra2 SCSI has a bus width of 8 bits and a throughput of 40 MB/s. The LVD signaling method supports either 8 devices and a cable length of 12 meters, or 2 devices and a cable length of 25 meters. HVD can have 8 devices on a cable of a maximum length of 25 meters.

    Wide Ultra2 SCSI has a bus width of 16 bits and a throughput of 80 MB/s. The LVD signaling method supports either 16 devices and a cable length of 12 meters, or 2 devices and 25 meters. HVD can have 16 devices on a cable of a maximum length of 25 meters.

    Ultra3 SCSI has a bus width of 16 bits and a data throughput of 160 MB/s. The LVD signaling method (there is only the LVD signaling method for Ultra3) supports either 16 devices and a cable length of 12 meters, or 2 devices at a maximum cable length of 25 meters.


    NOTE: When the ‘Wide’ term is used it means the data bus has a 16 bit width and also doubles the throughput of the non-Wide SCSI type which is 8 bits. For example, Fast SCSI has a throughput of 10 MB/s, but Fast Wide SCSI is doubled to 20 MB/s. The ‘Ultra‘ term also doubles the throughput of the non-ultra and non-wide version. For example, Ultra SCSI has throughput of 20 MB/s while the Fast SCSI is only 10 MB/s.

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