1. SCSI Reference
  2. Overview
  3. SCSI Bus Interface

SCSI Bus Interface

The libraries' SCSI interface conforms to SCSI specifications and is accepted by:

  • American National Standards Institute (ANSI X3.131)

  • European Computer Manufacturing Association (ECMA-111)

  • Federal Information Processing Standard (FIPS-131)

  • International Standards Organization (ISO-9316)

Overview

The small computer system interface operates locally as an input and output (I/O) bus that uses a common command set to transfer controls and data to all devices. The main purpose of this interface, called the SCSI bus, is to provide host computer systems with connections to a variety of peripheral devices, including disk subsystems, tape subsystems, printers, scanners, optical devices, communication devices, and libraries.

The SCSI bus design for the library provides a peer-to-peer, I/O interface that supports up to 16 devices and accommodates multiple hosts.

Peer-to-peer interface communication can be from:

  • Host to host

  • Host to peripheral device

  • Peripheral device to peripheral device

SCSI terms defining communication between devices on the SCSI bus include:

  • Initiator is the device that requests an operation.

  • Target is the device that performs the operation requested.

Some targets are control units that can access one or more physical or virtual peripheral devices addressable through the control unit. These peripheral devices are called logical units and are assigned specific addresses or logical unit numbers (LUNs).

The library supports SCSI-3 commands.

The library and the tape drives have separate connections for attachment to the SCSI bus. Daisy-chain cables are available to interconnect devices on the SCSI bus but keep the total cable length to a minimum. The following figure shows an example of a library and four tape drives that are daisy-chained to two initiators (or hosts). It is recommended that the drives be connected to a separate SCSI bus from the library.

Figure 1-1 Example of a Library Configuration on the SCSI Bus

SCSI Bus Example, two initiators connected to tape drives

Benefits

A small computer system interface also provides these benefits:

  • Low overhead

  • High transfer rates

  • A high-performance buffered interface

  • Conformance to industry standards

  • Plug compatibility for easy integration

  • Error recovery, parity, and sequence checking provides high reliability

  • Provisions in the command set for vendor-unique fields

  • Standard or common command sets with an intelligent interface that provides device independence

Implementation

Implementation of the SCSI bus for the library supports:

  • 8-bit wide transfers, asynchronous; 16-bit wide selection

  • Disconnect and reselect

  • Multiple initiator

  • Hard resets

  • Single-ended LVD

  • SCSI-3, 68-pin P-cable

Implementation for the library does not support:

  • Soft resets

  • Command queuing

  • Command linking

  • Asynchronous event notification

  • Extended contingent allegiance