SCSI Information

This page was last modified 00:23, 19 February 2008.

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Technical Note

Please note that this article recreates Technical Note 212

In the event you find that your backups are not operating as expected, for example, lower than expected throughput or SCSI errors in your System Messages file, refer to the tables below to confirm your SCSI configuration.

SCSI Chart

Bus Speed
Max. Cable
SCSI-1 50 5 8 5 SE - 6m
HVD - 25m
SCSI-2 Fast 50 10 8 10 SE - 3m
HVD - 25m
Wide 68 5 16 10 SE - 6m
HVD - 25m
Fast Wide 68 10 16 20 SE - 6m
HVD - 25m
SCSI-3 Wide Ultra 68 20 16 40 LVD - 12m
HVD - 25m
Wide Ultra2 68 40 16 80 LVD - 12m
HVD - 25m
Ultra3 68 40 16 160 LVD - 12m
Ultra320 68 80 16 320 LVD - 12m


Single Ended (SE)

Differential (Diff)

Narrow devices

Wide devices

SCSI Rules

  1. Never mix Single-ended and differential devices on a SCSI bus.
  2. If you mix narrow and wide devices on a SCSI bus...
    • Only use one 50-68 pin SCSI adapter cable
    • If the controller is wide then the narrow device should go at the end of the SCSI chain
    • If the controller is narrow then the wide device should go at the end of the SCSI chain.
  3. Convention is to externally terminate the SCSI bus, NOT internally on the devices.
  4. You can mix fast and slow SCSI devices on a fast SCSI bus, but you will get SCSI errors when it changes the transfer tate of the SCSI bus back and forth between 5MB/sec and 10MB/sec. The fix...
    • Put fast SCSI devices on a fast controller and slow devices on a slow controller
    • Modify the kernel to force the SCSI bus to only run at 5MB/sec rate.