SCSI Interface
The Small Computer Systems Interface (SCSI),pronounced “skuzzy” (rhymes with
fuzzy), is not an interface standard in the way that the IDE/ATA (Integrated Drive Elec-
tronics/AT Attachment) architecture is. SCSI is made u pof a collection of interface stan-
dards covering a range of peripheral devices, including hard disks, tape drives, optical
drives, CD-ROMs, and disk arrays. The SCSI bus is capable of connecting many devices,
both internal and external, to a single SCSI controller and share a common SCSI bus
interface.
Like IDE/ATA devices, SCSI controllers are built into the devices. As SCSI devices
are added to the SCSI bus, each device is assigned a unique device number to differenti-
ate it from the other devices. The SCSI controller communicates with the devices on the
bus, by sending a message encoded with the unit’s device number, which is also included
in any replay sent by the device. A SCSI bus must be terminated to prevent unclaimed or
misdirected messages from bouncing back onto the bus.External SCSI Connectors
There are several different SCSI standards available, each with its own protocols and con-
nectors. Table 19-6 lists the various SCSI standards that are in use and the external con-
nector used by each to connect an external SCSI device to the bus. Figures 19-18 and 19-19
illustrate the connectors referenced in the table.
Some early SCSI standards used 25-pin Centronics and DB-25 connectors. The SCA
(Single Connection Attachment), which is a high-density connector that also includes the
power connection as well, is now being used for some higher-end SCSI systems.(^504) PC Hardware: A Beginner’s Guide
SCSI Standard External Connector Internal Connector
SCSI – 1 50-pin Centronics 50-pin IDC
SCSI – 2 50-pin high density 50-pin IDC
Ultra SCSI 50-pin high density 50-pin IDC
Fast SCSI 50-pin high density 50-pin IDC
Wide SCSI 68-pin high density 68-pin high density
Fast Wide SCSI 68-pin high density 68-pin high density
Ultra SCSI - 3 68-pin high density 68-pin high density
Ultra2 SCSI - 3 68-pin very high density 68-pin high density
Table 19-6. SCSI Standards