The second-most popular hard disk interface used in PCs today is the Small Computer
Systems Interface, abbreviated SCSI and pronounced "skuzzy". SCSI is
a much more advanced interface than its chief competitor, IDE/ATA,
and has several advantages over IDE that make it preferable for many situations, usually
in higher-end machines. It is far less commonly used than IDE/ATA due to its higher cost
and the fact that its advantages are not useful for the typical home or business desktop
user.
In terms of standards, SCSI suffers from the same problem that IDE/ATA does:
there are too many different ones and it can be hard to understand what is what.
Fortunately, this situation is coming under control now. Also, SCSI standards aren't as
much of a problem as they are for IDE/ATA, because in the SCSI world, each SCSI protocol
has a name that indicates rather clearly what its capabilities are, and there is much less
reliance on using the name of the standard to infer transfer rates and other
characteristics. Unfortunately, there is still a lot of confusion if you try to figure out
the standards themselves and what each one means. And there are still manufacturers
playing fast and loose with how they label their drives.
SCSI is a much higher-level protocol than IDE is. In fact, while IDE is an interface,
SCSI is really a system-level bus, with intelligent controllers on each SCSI device
working together to manage the flow of information on the channel. SCSI supports many
different types of devices, and is not at all tied to hard disks the way IDE/ATA is--ATAPI supports
non-hard-disk IDE devices but it is really a kludge of sorts. Since it has been designed
from the ground up as almost an additional bus for peripherals, SCSI offers performance,
expandability and compatibility unmatched by any other current PC interface.
In this section of the site I describe the SCSI interface in detail. I begin with an
overview of the interface and a brief history. I then discuss SCSI standards, focusing on
the ones of most relevance to today's PC SCSI user. I then examine the various protocols
and features of the interface, followed by the popular transfer modes and feature sets
used. I then discuss practical implementation matters, including SCSI host adapters, cable
and connector types, and configuration issues.
Note: Despite the fact
that IDE stands for "integrated drive electronics", and that IDE and SCSI are
"competing" interfaces, SCSI devices all have integrated drive controllers. IDE is really a misnomer for the IDE/ATA interface.
Note: Most of the
attention in this section is oriented towards "regular" parallel SCSI, as
commonly used on PCs. The SCSI-3 standard also defines several
other "cousins" of parallel SCSI; these are mentioned briefly, but parallel SCSI
is the focus, since it continues to be the most widely implemented.
Next: Overview and History of the SCSI Interface
