Information Technology Reference
In-Depth Information
7
SCSI
7.1 Introduction
SCSI has many advantages over IDE, these include:
A single bus system for up to seven connected devices.
It supports many different peripherals, such as hard disks, tape drives, CD-ROMs, and so
on.
It supports device priority where a higher SCSI-ID has priority over a lower SCSI-ID.
It supports both high-quality connectors and cables, and low-quality connection and rib-
bon cable.
It supports differential signals, which gives longer cable lengths.
Extended support for commands and messaging.
Devices do not need individual IRQ lines (as they do in IDE) as the controller communi-
cates with the devices.
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.
SCSI-II. Support for SCSI-1 and with one or more of the following:
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.
15 devices per master device.
Tagged command queuing (TCQ) which greatly improves performance and is sup-
ported by Windows NT, NetWare and OS/2.
Multiple commands sent to each device.
Commands executed in whatever sequence will maximize device performance.
Ultra SCSI (SCSI-III). Operates either as 8-bit or 16-bit with either 20 MB/s or 40 MB/s
transfer rate.
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