Specifications

Page 2 DocSTAR Level 2 Service Training Workbook
Section 1: SCSI Configuration Overview
? This section will cover the basic theory of SCSI configuration and it’s application in
DocSTAR host systems.
The Small Computer System Interface (SCSI) has been around for over 25 years. It was
developed to provide the capability of attaching many devices and communicating at high
speed. For the most part, SCSI devices communicate much faster than their IDE/EIDE
(internal) or parallel (external) counterparts. However, recent advancements in IDE/EIDE
technology, particularly Ultra DMA 33 & 66, are challenging that statement.
SCSI technology still provides an advantage in the fact that the SCSI host adapter off-
loads a lot of the workload from the main system processor and is not as vulnerable to
interruptions in the transfer of data. It also has the upper hand in the number of devices
that can be connected (8 w/standard SCSI or 16 w/wide SCSI) and the fact that it allows
for internal and external connections.
For these reasons, SCSI technology remains the interface of choice for high-performance
considerations. The gain in performance comes at a higher price and greater degree of
complexity in configuring the SCSI Bus (host adapter, devices, cables, and terminators).
Configuring a SCSI bus correctly involves selecting and configuring the correct SCSI host
adapter, SCSI devices, cables, SCSI Ids, terminators, and ASPI Software.
What is SCSI?
SCSI, Small Computer System Interface, is a PC bus interface standard that defines
physical and electrical connections for devices. SCSI provides a standard interface that
enables many different kinds of devices, such as disk drives, magneto optical disks,
scanners, CD-ROM drives, and tape drives to interface with the host computer. Each
device, including the SCSI host adapter itself, must have a unique SCSI Id assigned to it
and the SCSI bus must be terminated at both physical ends.
The SCSI host adapter, which is the SCSI bus controller, contains an on-board processor
(CPU), a buffer area (RAM), I/O interfaces, and on some cards, a BIOS. These
components are all dedicated to one task: transfer data between the system and devices
at high speed. If the host adapter is an 8-bit standard SCSI 2 card then it can have Ids 0-
7. If the host adapter is a 16-bit ‘Fast Wide’ or ‘Ultra Wide’ card, then it can have SCSI Ids
0-15. SCSI host adapters are usually set to SCSI Id 7 by default. The Adaptec 152x
series are ISA cards. The Adaptec 29xx series are Bus-mastering PCI cards.
Figure 1.1: Adaptec SCSI Host Adapter
The SCSI devices, which are data input or output devices, contain a special-purpose
computer in the SCSI interface portion. This computer has only one task: to manage
communications on behalf of the device. If the device is a standard SCSI device, then it
can be set to SCSI Id 0 thru 7. If the device is a ‘Fast Wide’ or ‘Ultra Wide’ SCSI device,
then it can be set to SCSI Id 0 thru 15. The LUN (Logical Unit Number) essentially
allows one SCSI Id to be multiplexed/subdivided into more than one addressable device.
This is the case with the DVD-RAM drive that has a logical unit number for the DVD-RAM