Hewlett-Packard SureStore 125ex Optical Jukebox User's Guide - C1118-90000

OPERATING THIS JUKEBOX ON A SCSI BUS

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the bus, the ID also determines the priority of the device during contention among the devices for use of the bus.

The narrow SCSI bus, with its eight data lines, can communicate with eight devices that have IDs from 0 to 7. The wide

SCSI bus, with its 16 data lines, can communicate with 16 devices that have addresses from 0 to 15. The host bus

adapter, which links the host computer to the SCSI bus, is also a SCSI device (initiator), and is usually assigned an ID of

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.