Hardware Reference
In-Depth Information
have been standardized under the names Fast SCSI (10 MHz), Ultra SCSI (20
MHz), Ultra2 SCSI (40 MHz), Ultra3 SCSI (80 MHz), Ultra4 SCSI (160 MHz),
and Ultra5 SCSI (320 MHz). Each of these has a wide (16-bit) version as well. In
fact, the recent ones have only a wide version. The main combinations are shown
in Fig. 2-22.
Name
Data bits Bus MHz MB/sec
SCSI-1
8
5
5
Fast SCSI
8
10
10
Wide Fast SCSI
16
10
20
Ultra SCSI
8
20
20
Wide Ultra SCSI
16
20
40
Ultra2 SCSI
8
40
40
Wide Ultra2 SCSI
16
40
80
Wide Ultra3 SCSI
16
80
160
Wide Ultra4 SCSI
16
160
320
Wide Ultra5 SCSI
16
320
640
Figure 2-22.
Some of the possible SCSI parameters.
Because SCSI disks have high transfer rates, they are the standard disk in
many high-end workstations and servers, especially those that run RAID configu-
rations (see below).
SCSI is more than just a hard-disk interface. It is a bus to which a SCSI con-
troller and up to seven devices can be attached. These can include one or more
SCSI hard disks, CD-ROMs, CD recorders, scanners, tape units, and other SCSI
peripherals. Each SCSI device has a unique ID, from 0 to 7 (15 for wide SCSI).
Each device has two connectors: one for input and one for output. Cables connect
the output of one device to the input of the next one, in series, like a string of cheap
Christmas tree lamps. The last device in the string must be terminated to prevent
reflections from the ends of the SCSI bus from interfering with other data on the
bus. Typically, the controller is on a plug-in card and the start of the cable chain,
although this configuration is not strictly required by the standard.
The most common cable for 8-bit SCSI has 50 wires, 25 of which are grounds
paired one-to-one with the other 25 wires to provide the excellent noise immunity
needed for high-speed operation. Of the 25 wires, 8 are for data, 1 is for parity, 9
are for control, and the remainder are for power or are reserved for future use. The
16-bit (and 32-bit) devices need a second cable for the additional signals. The
cables may be several meters long, allowing for external drives, scanners, etc.
SCSI controllers and peripherals can operate either as initiators or as targets.
Usually, the controller, acting as initiator, issues commands to disks and other
peripherals acting as targets. These commands are blocks of up to 16 bytes telling
the target what to do. Commands and responses occur in phases, using various
