SCSI Interface Small Computer Standard Interface
SCSI Connectors
There are a number of connectors, including high-density, low density, Mac, PowerBook,
Wide, and SCSI-3
Typically either low density (centronics) or high density (SCSI-2) connectors on most
devices.
Mac and PowerBook are Apple specific, and older host adapters have the low density,
newer adapters have the high density or WIDE and UltraWide connectors.SCSI Zip drives
use a 25 pin DB-25 Macintosh cable.
Power book has a proprietary connector
The Adaptec 2940UW controller has 1 -68pin internal and 1-50pin internal, 1-68pin UW
external
Differential SCSI allows the chain to extend beyond 6m.
SCSI chain
The SCSI devices are daisy-chained with up to 7 devices such as hard disks, removable
disks, CD-ROM, CD-R, tape drives, jukeboxes, autoloaders and scanners.
SCSI ID
Each device is assigned its own unique SCSI ID, set by jumpers, toggles or switch on the
SCSI device.The device with SCSI ID 7 (usually the host adapter) has the highest priority
on the bus. The priority of the other IDs, in descending order, is 6, 5, 4, 3, 2, 1, 0.
Wide SCSI bus has 16 available SCSI IDs, the priority of the IDs (highest to lowest) is 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.
There are physical limitations on the overall length of SCSI chains of
6 meters for SCSI
3 meters for FastSCSI
Differential SCSI allows the chain to extend beyond 6 meters
SCSI Termination
Both ends of the SCSI chain require termination. If the chain consists exclusively of external devices the chain is terminated at one end by the
host adapter ( usually self detecting/terminating, but older host adapters may require jumper settings)
Many SCSI devices have selectable termination on them, set by jumpers, toggles or switch
If the last device at the end of the chain does not provide termination, a terminator is plugged into the unused SCSI connector.
If the chain is long or has a high number of devices, an active terminator vs. passive terminator is advised.
Internal drives are typically terminated by setting jumpers
The device with SCSI ID 7 (usually the host adapter) has the highest priority on the bus. The priority of the other IDs, in descending order, is
6, 5, 4, 3, 2, 1, 0.
On a Wide SCSI bus, which has 16 available SCSI IDs, the priority of the IDs (highest to lowest) is 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.
SCSI Performance
The increase in drive performance and demand has led to performance increases to
SCSI-1 Original SCSI commands set
SCSI-2 32-bit data transfer
SCSI-3 support up to 16 or 32 devices
FastSCSI 10MB/s
FastWIDE SCSI 20MB/s
UltraSCSI 20MB/s
UltraWide 40MB/s
Fiber channel
for information regarding connectors try
DataSure's Host adapters Page
Buslogic
DPT
ATTO Technologies
CD-Recordable Page
Software
Most of the host adapters have there own software, or drivers for there target platform
Software solutions are available to automate storage and retrieval and HSM
Drive manufactuers provide drivers if needed for target platforms, and third party solutions are availble.
Backup software usually lists supported devices, so check for compliance and compatability with OS, Version#, host adapter, devices, etc.
Thanks for visiting our Guide to Mass Storage pages, and our SCSI connection page. This page was last updated
This page, and all contents, are Copyright © 1994 -1998, all rights reserved by DataSure Services , Victoria, Canada. 
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