This is the Dell Logo This graphic produces space between objects Buy Online or Call 1-800-WWW-DELL 
Dell Storage Technology Foundations Online Training Course
This graphic produces a rounded corner This button takes the user back to the previous pagePrevious   NextThis button takes the user to the next page.
This is a placeholder for Dell's Customer Training logo
IndexThis is an arrow graphic for navigation
Course OverviewThis is an arrow graphic for navigation
Storage MediaThis is an arrow graphic for navigation
Storage InterfacesThis image is a down arrow for navigation
This image creates a rounded corner This graphic creates extra space This image creates a rounded corner
This graphic creates extra space
IDE/ATA
IDE/ATA Evolution
IDE Configuration
IDE Cabling

SCSI
SCSI Operation
SCSI Signaling
Termination & Width
SCSI IDs

Fibre Channel
FC Operation
FC Topologies
FC Components

Review
Section Review
 This graphic creates extra space
This image creates a rounded corner This graphic creates extra space This image creates a rounded corner
Data AvailabilityThis is an arrow graphic for navigation
Data RecoveryThis is an arrow graphic for navigation
Storage ArchitectureThis is an arrow graphic for navigation
Storage ManagementThis is an arrow graphic for navigation
Dell Storage Product LineThis is an arrow graphic for navigation
Course ReviewThis is an arrow graphic for navigation
Course Feedback
Contact Dell

Storage Interfaces

SCSI IDs

Each device on a SCSI channel must have a unique SCSI ID. The SCSI ID does not have to be sequential, meaning it does not have to match the device's physical location on the SCSI chain. SCSI IDs start at 0. The SCSI controller counts as a device and as such, is assigned a SCSI ID. Including the SCSI controller ID, wide SCSI will have 15 IDs available, while narrow SCSI will have seven.

SCSI ID Priority
The SCSI ID assigned to a device determines its priority during device arbitration. If two devices are competing for control of the SCSI bus, the device with the highest SCSI ID will "win". The lower ID device will have to wait. You can set a device to have a higher priority by assigning it a higher SCSI ID; the higher the SCSI ID, the higher the device priority. Additionally, devices are addressed by their SCSI ID. Setting devices to the same ID will result in "confusion" on the SCSI bus which is why each device must have a unique ID.

Most SCSI controllers default to the highest SCSI ID. The SCSI ID with the highest priority is 7. Here's why:

    Narrow SCSI has the IDs of 0-7 available. The highest ID is 7 making it the highest priority. Wide SCSI has the IDs of 0-15 available. ID 7 remains the highest available ID, because the wide controller's IDs are separated into two sequences, 0-7 and 8-15. The first sequence of numbers, 0-7, has a higher priority than the second sequence of numbers, 8-15.

    So, for a wide controller, the device IDs in order of priority are: 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8. For a narrow controller, the device IDs in order of priority are: 7, 6, 5, 4, 3, 2, 1, 0.

Priority setting can be critical in data intensive environments. Slower devices such as tape drives and CD-ROM drives might need a higher priority if mixed with faster devices such as hard drives. Otherwise they could be "crowded" off the SCSI bus by the faster devices.

This graphic, located at the bottom of the page, includes navigation buttons allowing you to return to the Previous page, or continue to the Next page. The Previous button is on the left, and the Next button is on the right. Text links are available at the top right of the page.

Copyright 1999-2002 Dell Computer Corporation. All rights reserved.