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

Operating This Drive 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.

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 adapter,
which links the host computer to the SCSI bus, is also a SCSI device (initiator), and
is usually assigned an ID of 7.
Priority of IDs ascends from lowest to highest but this ascending priority, however,
is in blocks of eight IDs, and the block from 8 to 15 is actually defined to be at a
lower priority than ID 0, the lowest address on the "narrow" portion of the bus. This
is done so that if a narrow device is placed on a wide bus, the wide devices, which
can "see" the lower addresses, will always defer to the lower addresses when they
contend for the bus. Otherwise, a narrow device, which cannot "see" any device at
B-2 Appendix B