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SCSI Interface

Posted on 1998-09-16
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What is SCSI Interface? Describe.
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Question by:ritua
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brosenb0 earned 30 total points
ID: 1125211
Abbreviation of Small Computer System Interface.
Pronounced scuzzy, SCSI is a parallel interface standard used by Apple Macintosh computers, PCs, and many UNIX systems for attaching peripheral devices to computers. All Apple Macintosh computers starting with the Macintosh Plus come with a SCSI port for attaching devices such as disk drives and printers.

SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

Athough SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.

While SCSI is the only standard interface for Macintoshes, PCs support a variety of interfaces in addition to SCSI. These include IDE, enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs
come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.

The following varieties of SCSI are currently implemented:

SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps
SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.
Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers.
Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MBps.
Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI.
Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.
Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.
 

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by:simonlam
ID: 1125212
SCSI - Small Computer Systems Interface.  An intelligent peripheral I/O interace with a standard, device independent protocol that allows many different peripheral devices to be attached to the host's SCSI port.

There are few different versions around at the moment.

SCSI-1: Originally included sync. and async. data transfers at 5Mb/sec for 8-bit, and allows up to 8 devices on the same SCSI chain.  Each device is assigned with 1 ID starts from 0.  Usual market standard is to have bootable device to be assigned as ID0 and ID1 and the host adaptor itself is ID7.  The SCSI chain must be properly terminated on both ends to avoid signal bouncing.

SCSI-2: Improved version and allows transfer rate at 10Mb/sec for 8-bit, 20Mb/sec for 16-bit and 40Mb/sec for 32-bit.  This version also adds the smaller 50-pin high-density micro-D connector and recommends to have active terminators to be installed on the chain.  SCSI-2 is backward compatible with SCSI-1.

SCSI-3: Another improved version and allows transfer rate at 20Mb/sec for 8-bit, 40Mb/sec for 16-bit and 80Mb/sec for 32-bit.  This version introduces an enhancement over Parallel SCSI called UltraSCSI and doubles the throughput.  A single channel can have 15 devices joined together and some adaptor can even have multiple channel capability.

Hope it helps!
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by:danpaul091498
ID: 1125213
There is a lot of disagreement about SCSI. It is theoretically faster if you are using multiple drives and a system that can take advantage of it. For the most part it is the interface of choice for high-end users.
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by:ritua
ID: 1125214
Thanks brosenb0. I need an answer like that only.
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