KarbosGuide.com. Module 5c1b.

About SCSI - continued

The contents:

  • SCSI is intelligent
  • About the SCSI standards
  • What do you gain with SCSI?
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  • SCSI is intelligent

    SCSI is remarkable in having an intelligent protocol, which assures maximum utilization of the data transfer. The basis of SCSI is a set of commands . Each individual device holds its own controller, which interprets these commands.

    All commands within the SCSI system are handled internally, meaning the CPU does not have to control the process:

    While the read/write head moves across a SCSI disk, the host adapter as well as the CPU can handle other jobs. Therefore SCSI is well suited for multitasking environments.

    About the SCSI standard

    SCSI stands for Small Computer System Interface. It is intended as a universal interface, defined and designed in 1982 by NCR and Shugart Associates. It exists in numerous variations. Here you see some of the more significant editions:

    Standard Year Bus speed Bus width Max. bandwidth
    Standard SCSI 1986 5 MHz
    8 bit 5 MB/sec
    Fast SCSI
    1990 10 MHz
    8 bit 10 MB/sec
    Fast SCSI
    1992 10 MHz
    16 bit 20 MB/sec
    Ultra SCSI 1994 20 MHz
    16 bit 40 MB/sec
    LVD Ultra2 1996 40 MHz 16 bit 80 MB/sec

    Today there are many SCSI standards. Among others, you can come across SCSI-20 and SCSI-40, which refers to the bus speed. The SCSI standard seem to have its own life with plenty of new development.

    LVD Ultra2

    The latest version of SCSI is called LVD ( Low Voltage Differentiale). You also find the term SCSI Ultra 2 - there have always been so many terms... LVD is an improvement to SCSI-3.

    LVD gives twice the bandwidth of the ordinary SCSI-3. Another improvement is the cabling which works up to 12 meters. Traditional SCSI only works within 3 meters. LVD has to compete with FireWire, which also has a powerful bandwidth.

    Adaptec have a SCSI-controller delivering up to 160 MB/sec. This unitcalled Adaptec SCSI Card 3950U2 uses two independent Ultra2 SCSI buses in one card. It connects up to 30 SCSI devices!

    What do you gain with SCSI?

    Expensive but good. SCSI makes the PC a more expensive, but more versatile. The advantages are, that on the same PC you have free access to use many and good units:

  • It is easy to add accessories as DAT streamers, CD-ROM recorders, MO drives, scanners, ZIP- and DVD-drives etc.

  • You can use SCSI hard disks.

  • You can use CDROM drives on SCSI, which may give a better performance.

    The advantages of SCSI hard disks

    SCSI hard disk are generally of higher quality than other disks.Typically, good SCSI disks come with a 5 year warranty. Traditionally they are faster than the EIDE disks. At 10,000 or 14,000 RPM they have shorter seek times. They also have a bigger cache.

    Another advantage is the large number of accessories, which can be attached. If you buy a 18 GB SCSI disk today, you will guaranteed need additional disk storage in a few years. Then you just add disk number two to the SCSI chain, and later number three. The system is more flexible than EIDE, where you can have a maximum of four units incl. CD-ROM.

    The SCSI hard disks can also adjust the sequence in the PC's disk read commands. This allows reading the tracks in an optimal sequence, enabling minimal movements of the read/write head. Quantum calls this technology ORCA ( Optimized Reordering Command Algorithm ). It should improve performance by 20%.

    Finally, the SCSI controller can multitask, so the CPU is not locked up during hard disk operations, which you can experience with IDE.

    SCSI hard disks can achieve substantially larger transfer capacity than the IDE drives, but they have the same bottle necks: the serial handling of bits in the read/write head, where the capacity is highly dependent on the rotation speed.

    SCSI is for servers

    However, today the importance of SCSI is decreasing except for use in dedicated servers. Modern CD-ROM and CD-RW drives work just as good on EIDE as on SCSI. USB has taken over when it comes to controlling units like scanners, cameras and Zip drives.

    Finally, modern EIDE-based harddisks have an extremely high quality compared to the products we had five years ago. Hence, there is no reason to prefer SCSI-based harddisk to the more inexpensive EIDE drives.

    But for servers SCSI still has a market.

    Booting from SCSI disk

    If the hard disk has to be booted, traditionally it has to be assigned ID 0. If the SCSI controller has to control the hard disk, then the PC CMOS setup must be modified, so the (IDE) hard disk is not installed if not both types of hard disks are installed.

    The operating system will find the host adapter after start up and BIOS will be read from the hard disk through the adapter. New BIOSs allow a choice of booting from either IDE or SCSI disk.

    Fast and Ultra Wide

    The newest generation of SCSI hard disks are both fast, ultra and wide. Therefore, the best advice is to buy an adapter like Adaptec 2940UW2, which can handle the newest disks.

    IBM disks

    Allow me to advertise IBM's SCSI disks. They are fantastically good. Unfortunately, not many people know about them. I have had a few of them. They excel in high quality at reasonable prices. The physical construction is very appealing: The electronics are integrated in very few components. Everything exudes quality! And they are very quiet. You simply cannot hear them.

    32 bit problems in Windows 3.11

    Windows 32 Bit Disk Access has given problems with SCSI disks. For a long while, it was impossible to install a 32 bit driver in Windows 3.11 to the SCSI disk. This was solved in 1995 and there have been no problems with Windows and SCSI since then.


    About SCSI: SCSI Pro and DPT offer some information.

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    Learn more

    Read about FireWire in module 5c3

    Read about chip sets on the motherboard in module 2d

    Read Module 4d about super diskette and MO drives.

    Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

    Read module 5b about AGP

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.
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