Setting Priorities

The UltraSCSI specification lets you attach up to 16 devices to a single cable, though currently you can put only eight 16-bit-wide UltraSCSI drives on a bus. You must also set each device with a unique SCSI ID (0-15). Because they share a single data channel, however, only one device can take charge of the bus at a time. When two SCSI devices vie for control of the bus, their SCSI IDs determine who wins according to a priority scheme.

The highest-priority ID on the SCSI bus is 7. You normally assign this ID to the host adapter. Next in priority, from highest to lowest, are IDs 6-0 and then 15-8. (For 32-bit SCSI devices, though none are yet available, IDs continue to decrease in priority from 23-16 a nd then 31-24.)

The justification for this odd sequence is backward-compatibility between older narrow (8-bit) and newer wide (16-bit) SCSI devices. SCSI devices signal on the appropriately numbered data line to assert their ID, which is why an 8-bit bus supports only eight devices. Because IDs greater than 7 are invisible to narrow-SCSI devices, all devices with IDs greater than 7 must yield to ensure that a conflict doesn't occur on the bus.

Although it provides backward-compatibility, this priority scheme can have a negative impact on overall system performance. Narrow--and therefore slower--devices, because of their lower ID numbers, will automatically preempt use of the bus by faster wide devices with addresses greater than 7.

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SCSI ID Priority

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SCSI ID priority from top to bottom.