Basic SCSI

The word SCSI is an acronym - it stands for Small Computer Systems Interface. In its most basic sense, SCSI is a method of adding parts to your computer.

To add SCSI capabilities to your computer will require a card, cables and the parts you want to add - which might be another CD-Rom drive, more hard drives, a scanner, a back-up tape unit, more floppy drives, or all the above.

Why you might want to add SCSI is a little more complicated. All home computers have limitations built into them:

  • you are restricted to 2 floppy drives for instance;
  • you are restricted to 4 devices on your IDE interface(s) - which could be
    • 3 hard drives and 1 CD-Rom drive;
    • OR 2 hard drives, a CD-Rom drive and a SuperDisk (or Zip) drive;
    • OR a hard drive, a CD-Rom drive, a Zip drive and a tape back-up unit;
    • OR some other combination of any of these items that total to 4 units);
  • you usually have 1 parallel port and that is where your printer is connected (so if you want to add something like a scanner you have to "daisy-chain" - that is, run a cable from your computer to your printer and then a cable from your printer to your scanner).
  • Another limitation is space - your computer case will only accept a certain number of hard drives, floppy drives, etc. - and that is limited by the design of the case.
  • A final limitation is power (from your power supply) - the things you want to add to your computer might be too much of a drain on your computer's power supply and you'd have to replace the power supply with a more powerful one to overcome that limitation.

SCSI was designed to overcome these (and other) limitations. You can add up to 15 devices with each SCSI card (and you can have more than one SCSI card!). The devices you add can be up to 100 feet away from your computer and the external SCSI devices usually have their own power supply so there is no drain on your compter's power supply.

Another limitation that SCSI overcomes is speed inside your computer. Transfer of data between SCSI devices can be very rapid, much quicker than between similar devices on your internal IDE interface, and the SCSI card has its own microprocessor which relieves part of the burden of data transfer from your computer's microprocessor.

The physical parts of SCSI are fairly simple - a card that plugs into one of your expansion slots, cables that connect the card to the SCSI devices you are adding and the SCSI devices themselves.

The cards have progressed as the computers have evolved. The oldest SCSI cards are 8 bit ISA cards, quickly followed by 16 bit ISA cards (still the most common) and now being replaced by the 32 bit/64 bit PCI cards. The cards have SCSI cable connectors on them; almost all have internal connectors, most have external connectors as well, some have internal floppy drive connectors and a few contain 2 (dual channel) internal SCSI connectors. The size and shape of the connectors is determined by the model of SCSI you're using (or are planning to use) and is a little beyond the scope of this article.

(Read the attached article HERE for more information about SCSI).

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