PC Hardware

Logical Unit Numbers (LUN)

It is possible to have a single SCSI ID support more than one device. Logical unit numbers (LUNs) can be used to provide a unique identifier for up to seven subunits per ID number. These are used primarily in hard disk drive arrays to create one large logical drive out of several smaller physical drives. LUNs require highly specialized software and are most often found in network servers running NetWare, Windows NT, or UNIX.


Whenever you send a signal through a wire, some of that signal will reflect back up the wire, creating an echo. To terminate a device simply means to put a terminating resistor on the ends of the wire. The purpose of this terminator is to prevent the occurrence of this echo. Two kinds of termination are used in SCSI technology: active and passive. Most older (and all SCSI-1) devices use passive termination. Proper termination of a SCSI device requires special consideration. Older hardware can be damaged by improper termination but, more often, lack of proper termination will result in a boot failure or the failure of the system to recognize a device that has been connected to the SCSI chain.

On most devices within a computer, the appropriate termination is built into that device. On other devices, including SCSI chains and some network cables, termination must be set during installation. The only absolute termination rule is that both ends of the chain must be terminated and that devices that are not on either end must not be terminated. Most SCSI devices come equipped with some form of termination. For most internal products, jumpers can be set to enable termination and connectors can be attached to cables that lead to one of the two SCSI connectors on an external device. Internal Ultra-SCSI 80 and Ultra-SCSI 160 drives do not have termination options on the actual devices. Their termination is handled by a termination block on the end of the cable.

Most new SCSI host adapters are equipped with autotermination circuitry, which polls the chain and sets the proper termination at their ends (or middles). On older cards, you might have to set jumpers. Check the manual for any SCSI device you are installing for instructions on how to set termination and ID before powering it up.

Lesson Summary

The following points summarize the main elements of this lesson:

  • SCSI was introduced in 1979 as a system-independent means of mass storage.

  • A SCSI chain is a series of devices that work through a host adapter.

  • SCSI chains can have up to 8 devices, including the host adapter (or 16, depending on the configuration) connected together.

  • SCSI chains must be terminated on both ends.

  • SCSI is used with many different types of peripherals, including printers, scanners, hard disk drives, and tape units.

  • Bus mastering is a method used by SCSI to transfer data independently of the CPU.

  • RAID uses several SCSI hard disk drives to provide improved performance and fault tolerance for data storage.