SCSI Technologies

SCSI Technologies

SCSI is a system-level parallel channel or I/O bus designed for interconnecting peripheral devices that have intelligent controllers, thereby allowing control signals and data to flow to all peripherals.

Each SCSI drive consists of a disk drive, intelligent controller, and a SCSI port. The advantage that SCSI offers is that the bus supports a total of eight compatible peripherals or logical units. One of these logical units (LUNs) is an adapter card or host bus adapter (HBA) that is the interface between the seven other logical units and the system.

The HBA does not need to know how many cylinders, Heads, or sectors are available on each device. The local device intelligence is capable of managing these functions, including errors. The device may be Tape Drive,DAT Drive, CD-ROMs and other SCSI compliant devices.

This flexibility has caused compatibility problems in the past. Later versions of the standard, such as SCSI3 and Low Voltage Differential (LVD) SCSI, have more closely defined the standard and largely eliminated compatibility issues.

SCSI Basics

The SCSI standard, regardless of versions and implementation, features a number of unique characteristics. As part of the architecture,the bus offers distributed arbitration or bus-connection logic. The priority scheme awards interface control to the highest priority SCSI device that is contending for use of the bus.

SCSI IDs are usually set up on the drives by selecting a unique id number through an array of jumpers. SCSI devices are daisy chained together using a common 50-conductor A Cable or optional 68 conductor B Cable for Fast wide SCSI-2. All signals are common between all SCSI devices on the 50 or 68 pin cable.

Both ends of the SCSI chain are terminated and the termination may be internal to the devices that are on the ends of the cable. Communication on the SCSI bus is allowed between only two SCSI devices at any given time. When two SCSI devices communicate on the SCSI bus, one acts as an initiator and the other acts as a target. The initiator originates an operation and is responsible for target selection, and for providing commands and data. The target manages these commands and performs the operations. Information transfers on the data bus are asynchronous and conform to a REQ/ACK handshake.

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