A Brief Overview Of Scsi; General; The Scsi Bus; Initiators And Targets - HP SureStore 220mx User Manual

Operating This Jukebox on a SCSI Bus

A Brief Overview of SCSI

A Brief Overview of SCSI

General

The Small Computer System Interface (SCSI), is a contention-based bus
that accommodates different speed devices without impacting the
devices with faster transfer speeds. This specification was defined by the
American National Standards Institute (ANSI) in 1986. The specification
defines both the physical medium and the command set used to transfer
information. Later developments are known under the newer
specifications, SCSI-2 and SCSI-3.

The SCSI Bus

The common SCSI buses are 50-pin and 68-pin. The 50-pin bus, which
uses eight of its lines for data transmission, is called a "narrow" bus. The
narrow bus can support eight devices. The 68-pin bus, which uses 16
lines for data transmission is called a "wide" bus. The wide bus can
support 16 devices.
Some devices may have 68-pin connectors, but operate with a "narrow"
bus. (See the next section in this Appendix, "The SCSI Bus and This
Jukebox".

Initiators and Targets

SCSI devices on the bus are either "initiators" or "targets." An initiator
(usually the host computer) originates a transaction and the target
(usually a peripheral device) fulfills the request.
Initiators and targets identify themselves on the bus by a SCSI ID. The
ID is designated by the user and is set electronically or manually
depending on the device. In addition to identifying a device on the bus,
the ID also determines the priority of the device during contention
among the devices for use of the bus.
The narrow SCSI bus, with its eight data lines, can communicate with
eight devices that have IDs from 0 to 7. The wide SCSI bus, with its 16
data lines, can communicate with 16 devices that have addresses from 0
to 15. The host bus adapter, which links the host computer to the SCSI
bus, is also a SCSI device (initiator), and is usually assigned an ID of 7.
B-2 Appendix B