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4. Small Computer System Interface (SCSI) - The SCSI interface is a high speed
parallel interface that transfers eight bits at a time rather than one bit at a
time for the ST506 and ESDI serial interfaces. Thus data transfer rates for
SCSI are measured in mega
bytes
versus mega
bits
and are considerably
faster than those of the serial interfaces. SCSI is also a bus level interface
which makes it very flexible. Since the commands are interpreted by the
device and not the SCSI host bus adapter, new devices (with new
commands) can be implemented and used with standard SCSI adapters. The
device driver then interacts with the device via the new commands. An
example of this would be a CD-ROM device sharing the same adapter as a
hard disk drive. Figure 10 shows a SCSI subsystem with a host bus adapter
attached to an integrated controller and hard disk.
Figure 10. SCSI Disk Interface
The SCSI flexibility and high performance make it very suitable for the server
environment. In fact, SCSI is the most widely used disk subsystem
technology in advanced servers today. All the current IBM PC Servers
except for a few at the low end use this technology. For these reasons, we
will take a closer look at this interface.
1.6.2 SCSI Technology
As previously discussed, SCSI is a bus level interface through which computers
may communicate with a large number of devices of different types connected to
the system unit via a SCSI controller and daisy-chained cable. The attached
devices include such peripherals as fixed disks, CD-ROMs, printers, plotters, and
scanners. The SCSI controller may be in the form of an adapter or integrated on
the planar board.
There are several terms and concepts used in discussing SCSI technology that
require definition.
•
SCSI-I and SCSI-II:
SCSI is a standard defined by the American National Standards Institute
(ANSI). The original SCSI-I standard is defined in ANSI standard X3.131-1986.
Chapter 1. IBM PC Server Technologies 17