SCSI ID’s are used to uniquely identify each device on the SCSI bus. A SCSI bus may have as many ID’s as the number of bits in the width of the bus. An 8-bit (or Narrow) SCSI bus has 8 possible ID’s ranging in value from 0 to 7; while a 16-bit (or WIDE) SCSI bus has 16 possible ID’s ranging in value from 0 to 15. Devices may be assigned any ID in the range but controllers should be limited to ID’s 0 through 7 to insure proper operation with any 8-bit (or Narrow) devices that might be installed. The default ID assigned to the host adapter and boot device varies with adapter implementation (see your controller documentation for more information).

ID values not only identify the device or controller but also determine the ‘priority’ that a device or controller receives on the bus. ID’s 0-7 are the highest priority with 7 being the highest;
followed by 8-15 with 15 being the highest. SCSI ID priority is as follows:

HIGH <-----------  PRIORITY       --------------->LOW
 7  6  5  4  3  2  1  0  15  14  13  12  11  10  9  8

Important: SCSI ID’s define priority, not physical location on the cable.

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