SCSI Information
From Documentation
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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
| Pins | Bus Speed MHz | Bits | Throughput MB/sec | Max. Cable Length | ||
|---|---|---|---|---|---|---|
| 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 |
Definitions
Single Ended (SE)
- Positive data bits are referenced to ground
- Maximum cable length is 6 meters
- Externally terminate bus with an active terminator.
Differential (Diff)
- Positive data bits are referenced to negative data bits
- Maximum cable length is 25 meters
- Must use a differential terminator.
Narrow devices
- 3 address bits for target id's
- 4 2 1
- Supports target id's 0-6 and the controller is target id7.
Wide devices
- 4 address bits for target id's
- 8 4 2 1
- Supports target id's 0-6, 8-15 and the controller is target id7.
SCSI Rules
- Never mix Single-ended and differential devices on a SCSI bus.
- 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.
- Convention is to externally terminate the SCSI bus, NOT internally on the devices.
- 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.