HP 220mx HP SureStore 220mx Optical Jukebox User's Guide - Page 83

Appendix B, A Brief Overview of SCSI

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Operating This Jukebox on a SCSI Bus A Brief Overview of SCSI Priority of IDs ascends from lowest to highest but this ascending priority, however, is in blocks of eight IDs, and the block from 8 to 15 is actually defined to be at a lower priority than ID 0, the lowest address on the "narrow" portion of the bus. This is done so that if a narrow device is placed on a wide bus, the wide devices, which can "see" the lower addresses, will always defer to the lower addresses when they contend for the bus. Otherwise, a narrow device, which cannot "see" any device at 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. SCSI and This Jukebox [1] Priority - the order at which contentions on the bus are resolved [2] Narrow Addresses - IDs from 0 to 7 available on a narrow bus [3] Wide Address Range - IDs from 0 to 15 available on a wide bus The following diagram below shows the linear addressing of a simple, narrow bus with the host bus adapter set at a SCSI ID of 7. Seven other target devices are set to the remaining IDs. Note that the ID of the device does not determine where the device is physically placed on the bus. [1] Host Bus Adapter - the connecting point for all SCSI devices on the bus Appendix B B- 3

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Appendix B
B-3
Operating This Jukebox on a SCSI Bus
A Brief Overview of SCSI
SCSI and This Jukebox
Priority of IDs ascends from lowest to highest but this ascending priority,
however, is in blocks of eight IDs, and the block from 8 to 15 is actually
defined to be at a lower priority than ID 0, the lowest address on the
“
narrow
”
portion of the bus. This is done so that if a narrow device is
placed on a wide bus, the wide devices, which can
“
see
”
the lower
addresses, will always defer to the lower addresses when they contend
for the bus. Otherwise, a narrow device, which cannot
“
see
”
any device at
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.
[1] Priority - the order at which contentions on the bus are resolved
[2] Narrow Addresses - IDs from 0 to 7 available on a narrow bus
[3] Wide Address Range - IDs from 0 to 15 available on a wide bus
The following diagram below shows the linear addressing of a simple,
narrow bus with the host bus adapter set at a SCSI ID of 7. Seven other
target devices are set to the remaining IDs. Note that the ID of the device
does not determine where the device is physically placed on the bus.
[1] Host Bus Adapter - the connecting point for all SCSI devices on the
bus