1-8Adaptec Ultra320 SCSI Cards User’s GuideUnderstanding SCSISCSI (pronounced “scuzzy”) stands for Small Computer SystemsInterface. SCSI is an industry standard computer interface forconnecting SCSI devices (such as a hard disk drive, CD-ROMdrive, or scanner) to a common SCSI bus.A SCSI bus is an electrical pathway that consists of a SCSI adaptercard (such as the Adaptec SCSI Card) installed in a computer andone or more SCSI devices. SCSI cables are used to connect thedevices to the SCSI adapter card. The Adaptec SCSI Card has up totwo independent SCSI buses (channels).For the SCSI bus to function properly, a unique SCSI ID must beassigned to the SCSI card and each SCSI device connected to it, andthe SCSI bus must be properly terminated.SCSI IDsEach device attached to the Adaptec SCSI Card, as well as theAdaptec SCSI Card itself, must be assigned a unique SCSI IDnumber from 0 to 15 for each of the two SCSI buses. A SCSI IDuniquely identifies each SCSI device on the SCSI bus anddetermines priority when two or more devices are trying to use theSCSI bus at the same time.Refer to the device’s documentation to set the SCSI ID. Here aresome general guidelines for SCSI IDs: For internal SCSI devices, the SCSI ID usually is set byconfiguring a jumper on the device. For external SCSI devices, the SCSI ID usually is set with aswitch on the back of the device. SCSI ID numbers do not need to be sequential, as long as theAdaptec SCSI Card and each device has a different number. Forexample, on each of the two SCSI buses you can have aninternal SCSI device with ID 0, and an external SCSI device withID 6. SCSI ID 7 has the highest priority on the SCSI bus. The priorityof the remaining IDs, in descending order, is 6 to 0, then 15 to 8.
