4.2SCSI Bus
4.2.1System configuration
(1)System configuration
The SCSI bus connects up to eight SCSI devices, each linked with a daisy chain. The both ends of the daisy chain require a terminator.
Each SCSI device operates as an initiator or a target, so that a series of operations are performed between a pair of initiator and target pair.
The system may be configured with any combination of initiators and targets as long as the number of the initiators and targets combined does not exceed eight.
(2)Addresses of SCSI devices
Every SCSI device on the bus is assigned a unique address (SCSI ID) that corresponds to the data bus bit number. ID#7 through ID#0 correspond to DB7 through DB0. The SCSI ID provides identification for specifying particular SCSI device when an initiator selects a target or when a target reconnects an initiator.
SCSI ID also represents the priority for using the bus in the arbitration phase. (A description regarding the bus phase is given later.) Priorities are given in the descending order of data bus bit numbers (DBn), with the highest priority placed on ID#7 (DB7) and the lowest priority on ID#0 (DB0).
(3)Peripheral equipment
With the basic specification, an initiator can designate up to eight peripheral devices (logical units) belonging to a single target, where the peripheral devices are used as the I/O units of the initiator. Logical units are identified and selected by specifying their LUNs (logical unit numbers) in the IDENTIFY message or command (CDB: command descriptor block).
This scanner is equipped with a target and a logical unit, and its LUN is 000.
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