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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0101:  SCSI
      9  IV  SCSI IDs

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Logical Unit Numbers (LUNs)

Early SCSI devices were limited to 8 unique IDs (0-7). SCSI expanded the capability on a single bus with the use of the Logic Unit Number (expander). LUNs expand the use of a single SCSI ID.

A good example is a tape backup system, with an auto-loader. Lets say the tape drive has the ability to hold 7 tapes in a holder.

By giving the holder a number 1 for Sunday, 2 for Monday, etc., the tape drive would only have a single SCSI ID, yet a way for the auto-loader to know which tape to insert into the tape drive.

Currently, 960 LUN numbers may exist for a single SCSI ID.

[spacer]Multiple SCSI Controllers

Some computer manufactures are not keen on the idea of adding more than 7 devices to a SCSI channel. Or maybe the data flying around on your network is so intensive your SCSI channel would saturate if you added one more device.

Nothing stops you from adding another SCSI controller and starting a new SCSI chain with a whole new set of SCSI Ids for that chain.



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