Information Technology Reference
In-Depth Information
7
SCSI
7.1 Introduction
SCSI has many advantages over IDE, these include:
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A single bus system for up to seven connected devices.
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It supports many different peripherals, such as hard disks, tape drives, CD-ROMs, and so
on.
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It supports device priority where a higher SCSI-ID has priority over a lower SCSI-ID.
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It supports both high-quality connectors and cables, and low-quality connection and rib-
bon cable.
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It supports differential signals, which gives longer cable lengths.
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Extended support for commands and messaging.
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Devices do not need individual IRQ lines (as they do in IDE) as the controller communi-
cates with the devices.
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It has great potential for faster transfer and enhanced peripheral support.
7.2 SCSI types
SCSI has an intelligent bus subsystem and can support multiple devices cooperating cur-
rently. Each device is assigned a priority. The main types of SCSI are:
•
SCSI-I. Transfer rate of 5 MB/s with an 8-bit data bus and seven devices per controller.
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SCSI-II. Support for SCSI-1 and with one or more of the following:
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Fast SCSI which uses a synchronous transfer to give 10 MB/s transfer rate. The initia-
tor and target initially negotiate to see if they can both support synchronous transfer.
If they can they then go into a synchronous transfer mode.
•
Fast/wide SCSI-2 which doubles the data bus width to 16 bits to give 20 MB/s trans-
fer rate.
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15 devices per master device.
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Tagged command queuing (TCQ) which greatly improves performance and is sup-
ported by Windows NT, NetWare and OS/2.
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Multiple commands sent to each device.
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Commands executed in whatever sequence will maximize device performance.
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Ultra SCSI (SCSI-III). Operates either as 8-bit or 16-bit with either 20 MB/s or 40 MB/s
transfer rate.