Karbosguide.com - Module 5c1a.

About SCSI


The contents:

  • The host adapter
  • 7 units in a chain On the following page:

  • SCSI is intelligent
  • About the SCSI standards
  • What do you gain with SCSI?
  • Next module: USB
  • IEEE1394 FireWire
  • Device Bay
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  • Introduction

    SCSI (Small Computer System Interface) is high end technology. It is a technology which provide means for data exchange among hardware devices such as drives, tape streamers and scanners. SCSI is especially used in high end PCs such as network servers or just powerful workstations.

    SCSI might be compared to the EIDE interface, which also uses a host adapter controlling drives. However SCSI has two major advantages over EIDE:

  • A SCSI host controls 7 or 15 devices (using only one IRQ).

  • The SCSI system holds its own computer power, thus freeing the CPU from workload.

    If you are critical about your PC power, the SCSI would be worth considering.


    The host adapter

    A SCSI system is built around a central, intelligent controller called the host adapter . A host adapter can control several SCSI units:

  • Many units on the same host adapter.

  • Many types of drives : Hard disks, CD and DVD drives, Zip drives, MO drives, etc.

  • Tape streamers (DAT and others).

  • Scanners.

    The host adapter has its own BIOS separate from the PC's. When you boot the PC, you will see the adapter communicating with connected SCSI devices.

    The adapter is rather expensive. Currently, the best for ordinary use is called Adaptec 2940 U2W (priced at around $200). It is PCI based, so you could use it in your next PC too. I have had good experiences with ASUS motherboards in versions with onboard SCSI controller. That is the most easy solution - to have the controller onboard.


    7 units in a chain

    The regular SCSI 2 system can handle 8 devices including the adapter itself. SCSI Wide handles 15 devices. Each device has to be assigned a unique number going from ID 0 to ID 7. The SCSI devices can be internal (installed inside the PC cabinet) or external . The host adapter is a device itself. Typically, the host adapter will occupy ID 7.

    Here is an illustration of a SCSI string with host adapter (ID 7) and five units (ID numbers 0, 1, 2, 4, and 5):

    Here you see the BIOS report of a found SCSI chain:

    Terminators in both ends

    The last unit in both ends of the SCSI chain must be terminated. This means that there must be resistors (jumpers or switches) attached to two of the units.

    If you only use two devices, you do not have to worry about it. The host adapter is one end of the chain and the other device is the other end. With three or more units you have to take care of termination:


  • Next page
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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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