HP Surestore 125ex Hewlett-Packard SureStore 125ex Optical Jukebox User&#1 - Page 97

Scsi B

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B NEXT PREVIOUS JUMP CONTENTS OPERATING THIS JUKEBOX ON A SCSI BUS 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 7. 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. INDEX 2

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B
O
PERATING
T
HIS
J
UKEBOX
ON
A
SCSI B
US
B
2
NEXT
PREVIOUS
JUMP
CONTENTS
INDEX
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
7.
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.