IBM 847651U Handbook - Page 145

Installing or removing internal drives of methods to set the SCSI ID, such as jumpers, thumbwheels, or switches. The SCSI device ID is also known as the SCSI address or physical unit number (PUN). The UltraSCSI adapter supports SCSI IDs 0 to 15; ID 7 is reserved for the adapter. The default startup (boot) SCSI ID is 0 for an internal SCSI hard disk drive in Channel A. Use SCSISelect to view the SCSI IDs of SCSI devices in your server. (See "Using the SCSISelect Utility program" on page 62 for more information.) If you install wide (16-bit) SCSI devices, you can set the IDs to any number between 0 and 6, or between 8 and 15. If you install narrow (8-bit) SCSI devices, you can set the IDs to any number between 0 and 6. SCSI logical unit number (LUN) Each physical device has a physical unit number (PUN) and a logical unit number (LUN). If a physical device supports no logical devices, its LUN is 0. Most SCSI devices support only LUN 0. Certain types of physical SCSI devices (bridge controllers) can support more than one logical device. For example, a printer controller might support up to eight printers. The printer controller is a physical device, and each printer is a logical device. If the printer controller is assigned to ID 2, the ID of the first printer appears in the SCSI device configuration as PUN 2, LUN 0. The IDs of the other printers are PUN 2, LUN 1; PUN 2, LUN 2; on up to PUN 2, LUN 7. Termination requirements The SCSI bus must be terminated at both ends only. The UltraSCSI adapter provides termination for one end of the SCSI bus (cable). The SCSI cable comes with a terminator that provides termination for the other end of the SCSI bus. Therefore, any internal SCSI devices that are installed must have their termination set to Disabled. Chapter 4. Installing options 111