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Appendix B B-3

Operating This Jukebox on a SCSI Bus

A Brief Overview of SCSI

Operation on a

SCSI Bus

an ID greater than 7, would always assume it won the contention and would attempt

to talk, perhaps at the same time as a device with an ID above 7 that was contending

for the bus. The following diagram shows the priority scale of IDs when the priority

of the two blocks of eight are reversed.

The diagram below shows the linear addressing of a simple, narrow bus with the

host adapter set at a SCSI ID of 7. The device ID does not determine where the

device is physically placed on the bus.

LUN Addressing

SCSI IDs are the top-level method of addressing devices on the bus. Another

method of addressing is also available; logical unit numbering (LUN). This method

enables a complex device, such as a jukebox, to group its internal SCSI devices to a

single SCSI ID.

The complex device is identified by a SCSI ID, but this complex device assigns

logical unit numbers (LUNs) to each device within it. Up to eight LUNs may be

assigned under a single SCSI ID. The LUNs that can be assigned are from 0 to 7

(LUN 0 is usually retained by the complex device).

8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7

Wide Address Range

Narrow

Addresses

Priority

1 2 ... 86 87 88 89 90 91 92 93 94 95 96 ... 119 120

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HP 220mx Manuale Utente Pagina 91