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The Small Computer System Interface (SCSI), (pronounced scuzzy), interface is a an old storage technology but many computer systems may still employ some type form of this hard disk protocol. SCSI is most popular in server computers and high end workstation systems where system performance is a critical to the proper functioning of a system.

There is a fixed set of of standards that physically connects and transfers data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.

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Internal PC Cables

External PC Cables

Internal PC Connectors

External PC Connectors
Ultra ATA (44 Pin)
Ultra ATA (44 Pin)

Ultra ATA (50 Pin)
Ultra ATA (50 Pin)
Small Computer Systems Interface

Serial Advanced Technology Attachment
Serial ATA


Apple Power
Book Ata

Apple Power

Novell Procomp External SCSI

Paravision SX-1 External IDE

This illustration shows what a typical Small Computer Systems Interface cable looks like.

There are numerous variations to the SCSI interface, with each providing a slightly faster and more efficient method for transferring data between SCSI drives and other components inside a computer. The SCSI interface works well with removable storage devices including tape and zip drives. A SCSI computer can employ both internal and external connectors and basic SCSI controllers can support up to fifteen devices.

SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. SCSI uses hand shake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a CRC32 checksum. The SCSI protocol defines communication from host to host, host to a peripheral device, peripheral device to a peripheral device. However most peripheral devices are exclusively SCSI targets, incapable of acting as SCSI initiators—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.

Creation Year: 1986
Superseded By: IDE and EIDE
Cable Capacities: 5, 10, 20, 40, 80, 160, 320, and 640 mbps.
Data Width: 8 and 16 bits Bandwidth
Maximum Devices Supported: 15 (chain of command mode) utilizing SCSI identification.
Protocols Used: SCSI-I, Fast SCSI, Fast-Wide SCSI, Ultra SCSI, Ultra Wide SCSI, Ultra2 SCSI, Ultra2 Wide SCSI, Ultra3 Scsi, Ultra-320 SCSI, Ultra-640 SCSI
Cable Style: SCSI Parellel, Serial attached SCSI, ISCSI, USB Attached SCSI.
Hotplugging Support? No
External Interfaces: Low Density 50 pin, High Density 50 pin, High density 68 pin, 80 pin Alt-4, DB-25
Supported Devices:

Hard Disk Drives
Tape and Zip Drives.
CD-ROM Drives.
DVD-ROM Drives.

Explanations Of Connectors:
# Purpose
1 50 pin SCSI male connector
50 pin male connector for connection to SCSI device.
50 pin female connector for connection to controller card.
Low-density, 50-pin SCSI connector
Older narrow (8-bit) SCSI adapters and external devices use a full-sizes Centronics-type connector that normally has wire latches on each side to secure the cable connector. The SCSI-2 revision added a high-density, 50-position, D-shell connector option for the A-cable connectors. This connector now is called Alternative 1. Figure 8.5 shows the 50-pin high-density SCSI connector.
High-density, 50-pin SCSI connector
The Alternative 2 Centronics latch-style connector remains unchanged from SCSI-1. A 68-conduc- tor B cable specification was added to the SCSI-2 standard to provide for 16- and 32-bit data transfers; the connector, however, had to be used in parallel with an A cable.
High-density, 68-pin SCSI connector.
80-pin Alt-4 SCSI connector

DB-25 SCSI connector

Typical Lengths:
Typical lengths of SCSI cables range from 18 to 36 inches.
Advantages and Disadvantages:
High performance. Old technology.
High realiability. Expensive.
Easily scalable. Difficult to implement
Can support up to 15 devices on a single channel. Requires a seperate host controller card to control functioning of storage devices.
Cable Features:
  • Devices Supported: 3
  • Cable Type: SCSI-3
  • Supports transfer speeds up to 40 Bytes/sec
  • Connector Spacing: 0-20-28-36in
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  • CableFeatures04
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