User manual QUANTUM SCSI MAN CDXXXXXX STDXXXXXX

.................................. DDS-4 and DAT 72 Tape Drives and Autoloaders .................................. .................................. .................................. .................................. SCSI Interface Manual .................................. Copyright © 2003 Certance LLC. All rights reserved Part Number 50000697 June 2003, Rev.B Certance and the Certance logo are trademarks of Certance LLC. Seagate is a trademark of Seagate Technology LLC. Other product names are trademarks or registered trademarks of their respective owners. Certance reserves the right to change, without notice, product offerings or specifications. No part of this publication may be reproduced in any form without written permission from Certance LLC. Important Information About This Manual All information contained in or disclosed by this document is considered proprietary by Certance. By accepting this material, the recipient agrees that this material and the information contained therein are held in confidence and in trust and will not be used, reproduced in whole or in part, nor its contents revealed to others, except to meet the purpose for which it was delivered. It is understood that no right is conveyed to reproduce or translate any item herein disclosed without express written permission from Certance. Certance provides this manual "as is," without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. Certance reserves the right to change, without notification, the specifications contained in this manual. Certance assumes no responsibility for the accuracy, completeness, sufficiency, or usefulness of this manual, nor for any problem that might arise from the use of the information in this manual. Contents Page iii Contents Introduction 1 Overview...................................................................................................................... 1 Drive Models ............................................................................................................... 1 About This Manual ...................................................................................................... 1 SCSI Interface Description 2 Overview...................................................................................................................... 2 ANSI SCSI Bus Standards ................................................................................... 2 Cabling and Connectors ....................................................................................... 2 Signal Descriptions...................................................................................................... 3 Command Set Description .......................................................................................... 4 ANSI X3.131, 199x Conformance Statement (SCSI-2) .............................................. 5 General features ................................................................................................... 5 Interface standards conformance ......................................................................... 5 Commands............................................................................................................ 6 * Autoloader only commands................................................................................ 6 Messages.............................................................................................................. 6 Vendor-unique commands.................................................................................... 6 SCSI Bus Protocol....................................................................................................... 6 Waiting and Control Phases........................................................................................ 7 Arbitrating Systems............................................................................................... 7 Selection and Reselection Phases.............................................................................. 9 Select With Attention .......................................................................................... 10 Identify Message................................................................................................. 10 Information Transfer Phases..................................................................................... 11 Asynchronous Data Transfer .............................................................................. 11 Synchronous Data Transfer................................................................................ 13 Command Phase....................................................................................................... 14 Data Phase................................................................................................................ 15 Data-In Phase ..................................................................................................... 15 Data-Out Phase .................................................................................................. 15 Status Phase ............................................................................................................. 15 Message Phase......................................................................................................... 15 Message-In Phase.............................................................................................. 15 Page iv DDS-4 SCSI Interface Manual, Rev. B Message-Out Phase ...........................................................................................15 Command Descriptor Block.......................................................................................16 Logical Unit Support ..................................................................................................17 SCSI Message Descriptions and Definitions.............................................................18 SCSI Status Code Descriptions and Definitions .......................................................23 Attention Condition ....................................................................................................24 Reset Condition .........................................................................................................24 Unit Attention Condition.............................................................................................24 Buffered Mode ...........................................................................................................25 Immediate Function ...................................................................................................25 Residual Length Function..........................................................................................25 Disconnect/Reconnect Function................................................................................26 SCSI Memory Address Pointers................................................................................27 Current Data Pointers .........................................................................................27 Saved Data Pointers ...........................................................................................27 Early Warning Function .............................................................................................27 Error Reporting ..........................................................................................................28 Soft Errors ...........................................................................................................28 Hard Errors..........................................................................................................28 Variable and Fixed Mode Recording .........................................................................28 Autoloader Operation ................................................................................................29 General SCSI Information...................................................................................29 SCSI Medium Changer Commands ...................................................................29 General Operation ..............................................................................................31 Loading and Initialization ....................................................................................31 Front Panel Operation.........................................................................................32 Cassette Selection ..............................................................................................32 Automatic Loading Mode ....................................................................................32 Magazine Ejection...............................................................................................33 Stray Tape Recovery ..........................................................................................33 Commands 34 Introduction ................................................................................................................34 Command Reference List ...................................................................................34 Conventions ........................................................................................................35 Command Descriptor Blocks (CDBs) ........................................................................35 Command Descriptor Block Formats..................................................................35 Command Descriptor Block Field Descriptions ..................................................36 Flag and Link Bit Descriptions ............................................................................36 ERASE (19h) .............................................................................................................37 Contents Page v ERASE Command Descriptor Block................................................................... 37 Command Descriptor Block Field Description.................................................... 37 Completion Status .............................................................................................. 37 EXCHANGE MEDIUM (AutoLoader Only) (A6h) ...................................................... 39 EXCHANGE MEDIUM Command Descriptor Block........................................... 39 Command Descriptor Block Field Descriptions .................................................. 39 Completion Status .............................................................................................. 40 INITIALIZE ELEMENT STATUS (AutoLoader Only) (07h) ....................................... 41 INITIALIZE ELEMENT STATUS Command Descriptor Block............................ 41 Completion Status .............................................................................................. 41 INQUIRY (12h) .......................................................................................................... 42 INQUIRY Command Descriptor Block................................................................ 42 Command Descriptor Block Field Description.................................................... 42 Standard INQUIRY Data Format ........................................................................ 43 Standard INQUIRY Data Format Field Descriptions .......................................... 43 INQUIRY Drive Serial Number Data Format Page............................................. 45 Drive Serial Number Data Format Field Descriptions ........................................ 45 Completion Status .............................................................................................. 45 LOAD/UNLOAD (1Bh)............................................................................................... 46 LOAD/UNLOAD Command Descriptor Block..................................................... 46 Command Descriptor Block Field Description.................................................... 46 Completion Status .............................................................................................. 47 LOCATE (2Bh) .......................................................................................................... 48 LOCATE Command Descriptor Block ................................................................ 48 Command Descriptor Block Field Description.................................................... 48 LOCATE and the BT Bit...................................................................................... 49 Completion Status .............................................................................................. 50 LOG SELECT (4Ch).................................................................................................. 51 LOG SELECT Command Descriptor Block ........................................................ 51 Command Descriptor Block Field Description.................................................... 52 Errors Detected in the Command Descriptor Block............................................ 52 Use of the PC Bits............................................................................................... 52 LOG SELECT Parameter Data........................................................................... 53 LOG SENSE (4Dh).................................................................................................... 58 LOG SENSE Command Descriptor Block .......................................................... 58 Command Descriptor Block Field Description.................................................... 59 Using Page Control Bits...................................................................................... 60 Using the Parameter Pointer Field ..................................................................... 61 Log Sense Pages ............................................................................................... 61 Completion Status .............................................................................................. 73 Page vi DDS-4 SCSI Interface Manual, Rev. B MODE SELECT (15h) ...............................................................................................75 MODE SELECT Command Descriptor Block .....................................................75 Command Descriptor Block Field Description ....................................................76 MODE SELECT Parameters...............................................................................76 Parameter List Header Field Descriptions ..........................................................77 Parameter List--Block Descriptor.......................................................................77 Parameter List--Block Descriptor Field Descriptions.........................................78 Mode Page Format .............................................................................................78 Completion Status...............................................................................................90 MODE SENSE (1Ah).................................................................................................91 MODE SENSE Command Descriptor Block .......................................................91 Command Descriptor Block Field Description ....................................................92 MODE SENSE Parameters ................................................................................93 Parameter List Header Field Descriptions ..........................................................93 Parameter List--Block Descriptor.......................................................................94 Parameter List Block Descriptor Field Descriptions ...........................................94 Mode Page Format .............................................................................................95 Completion Status.............................................................................................103 MOVE MEDIUM (Autoloader only) (A5h) ................................................................104 MOVE MEDIUM Command Descriptor Block...................................................104 Completion Status.............................................................................................105 PREVENT/ALLOW MEDIA REMOVAL (1Eh) .........................................................106 PREVENT/ALLOW MEDIA REMOVAL Command Descriptor Block ...............106 Command Descriptor Block Field Description ..................................................106 Completion Status.............................................................................................106 READ (08h) .............................................................................................................107 READ Command Descriptor Block ...................................................................107 Command Descriptor Block Field Description ..................................................107 Description of the READ Command .................................................................108 Completion Status.............................................................................................111 READ BLOCK LIMITS (05h) ...................................................................................112 READ BLOCK LIMITS Command Descriptor Block .........................................112 Command Descriptor Block Field Description ..................................................112 Block Size Definition .........................................................................................112 Completion Status.............................................................................................113 READ DATA BUFFER (3Ch) ..................................................................................114 READ DATA BUFFER Command Descriptor Block.........................................114 Command Descriptor Block Field Description ..................................................114 READ DATA BUFFER Header .........................................................................115 READ DATA BUFFER Header Field Descriptions ...........................................115 Contents Page vii Completion Status ............................................................................................ 115 READ ELEMENT STATUS (Autoloader Only) (B8h).............................................. 116 READ ELEMENT STATUS Command Descriptor Block ................................. 116 Element Status Data ......................................................................................... 116 Element Status Data Header ............................................................................ 117 Completion Status ............................................................................................ 120 READ POSITION (34h)........................................................................................... 121 READ POSITION Command Descriptor Block ................................................ 121 READ POSITION Command Descriptor Block Field Descriptions................... 121 READ POSITION Data Format......................................................................... 122 READ POSITION Data Format Field Descriptions........................................... 122 READ POSITION Data Format......................................................................... 123 READ POSITION Data Format Field Descriptions........................................... 123 Completion Status ............................................................................................ 125 RECEIVE DIAGNOSTIC RESULTS (1Ch) ............................................................. 126 RECEIVE DIAGNOSTIC RESULTS Command Descriptor Block.................... 126 Command Descriptor Block Field Description.................................................. 126 Completion Status ............................................................................................ 127 RELEASE UNIT (17h / 57h) .................................................................................... 129 RELEASE UNIT Command Descriptor Block................................................... 129 Command Descriptor Block Field Description.................................................. 130 Completion Status ............................................................................................ 130 REPORT DENSITY SUPPORT (44h)..................................................................... 131 REPORT DENSITY SUPPORT Command Descriptor Block........................... 131 Command Descriptor Block Field Description.................................................. 131 REPORT DENSITY Response Data ................................................................ 131 REPORT DENSITY SUPPORT Header........................................................... 132 REPORT DENSITY SUPPORT Data block descriptor..................................... 132 DENSITY SUPPORT Data Block Descriptor Field Descriptions...................... 132 Completion Status ............................................................................................ 133 REQUEST BLOCK ADDRESS (02h)...................................................................... 134 REQUEST BLOCK ADDRESS Command Descriptor Block............................ 134 Command Descriptor Block Field Description.................................................. 134 REQUEST BLOCK ADDRESS Address Data Format ..................................... 134 REQUEST BLOCK ADDRESS Address Data Field Description...................... 135 Completion Status ............................................................................................ 135 REQUEST SENSE (03h) ........................................................................................ 136 REQUEST SENSE Command Descriptor Block .............................................. 136 Sense Data Format........................................................................................... 136 Sense Data Field Descriptions ......................................................................... 137 Page viii DDS-4 SCSI Interface Manual, Rev. B Definition of Sense Keys...................................................................................138 Additional Sense Code and Code Qualifier ......................................................138 Vendor-Specific Code Qualifiers.......................................................................143 Completion Status.............................................................................................148 RESERVE UNIT (16h / 56h)....................................................................................149 RESERVE UNIT Command Descriptor Block ..................................................149 Command Descriptor Block Field Description ..................................................150 Completion Status.............................................................................................151 REWIND (01h).........................................................................................................152 REWIND Command Descriptor Block ..............................................................152 Command Descriptor Block Field Description ..................................................152 Completion Status.............................................................................................152 SEEK BLOCK (0Ch)................................................................................................154 SEEK BLOCK Command Descriptor Block ......................................................154 Command Descriptor Block Field Description ..................................................154 Completion Status.............................................................................................155 SEND DIAGNOSTIC (1Dh) .....................................................................................156 SEND DIAGNOSTIC Command Descriptor Block ...........................................156 Completion Status.............................................................................................157 SPACE (11h) ...........................................................................................................158 SPACE Command Descriptor Block.................................................................158 Command Descriptor Block Field Descriptions ................................................159 Space-by-Count Functions ...............................................................................159 Space by Position Functions.............................................................................160 Space and the RSMK Bit ..................................................................................160 Completion Status.............................................................................................161 TEST UNIT READY (00h) .......................................................................................162 TEST UNIT READY Command Descriptor Block .............................................162 TEST UNIT READY Detailed Operation...........................................................162 Completion Status.............................................................................................163 VERIFY (13h) ..........................................................................................................164 VERIFY Command Descriptor Block ................................................................164 Command Descriptor Block Field Description ..................................................164 Data Blocks Verified..........................................................................................164 Completion Status.............................................................................................165 WRITE (0Ah) ...........................................................................................................166 WRITE Command Descriptor Block .................................................................166 Command Descriptor Block field description ....................................................166 Completion Status.............................................................................................166 WRITE DATA BUFFER (3Bh) .................................................................................168 Contents Page ix WRITE DATA BUFFER Command Descriptor Block ....................................... 168 Command Descriptor Block Field Descriptions ................................................ 169 WRITE DATA BUFFER Data Header............................................................... 169 Completion Status ............................................................................................ 170 WRITE FILEMARKS (10h) ...................................................................................... 171 WRITE FILEMARKS Command Descriptor Block............................................ 171 Command Descriptor Block Field Description.................................................. 171 Completion Status ............................................................................................ 172 Introduction Page 1 Introduction 1 The Seagate DDS-4 and DAT 72 DAT drives are designed for computer environments requiring high performance, high capacity data storage. These drives are available in 3.5-inch or 5.25-inch internal configurations or as an external subsystem. The Autoloaders contain a DDS-4 or DAT 72 drive with an enclosure and tape manipulation mechanism. Seagate DDS-4 and DAT 72 drives contain an embedded Small Computer Systems Interface (SCSI) controller. These drives provide synchronous or asynchronous SCSI and a high speed burst synchronous data transfer rate of 80 Mbytes/second. The internal drive form factors are tailored for easy installation in today's computers and the full-featured embedded SCSI controller facilitates easy integration into a variety of systems. Overview Drive Models This manual provides detailed information about the SCSI interface and SCSI commands that apply to Seagate DDS-4 and DAT 72 tape drives and autoloaders. These drives offer electronically erasable, programmable, read-only memory (flash EEPROM) for SCSI firmware, which enables qualified Seagate OEMs to download revised firmware to the drive via two methods: using the SCSI bus or using a specialized Seagate firmware tape cartridge. About This Manual The information in this manual applies to DDS-4 and DAT 72 drives and autoloaders. For specific installation and operational information--including SCSI connection information--refer to the Installation Guide or Product Description Manual for the specific model of your drive or Autoloader. The following table outlines the chapters in this manual. Chapter 1 2 3 Title Introduction SCSI Interface Description Commands Description Introduces the SCSI interface and explains the structure of this manual. Provides general information about the SCSI interface for the DDS-4 and DAT 72 drives. Lists and describes the SCSI commands. Page 2 DDS-4 SCSI Interface Manual, Rev. B SCSI Interface Description 2 Overview The Seagate DDS-4 and DAT 72 drives are designed to operate with the Small Computer System Interface (SCSI) bus. This chapter discusses SCSI bus operation as it pertains to drive functions. SCSI is a standard interface established to support peripheral equipment such as printers, tape drives, magnetic disks, optical disks for microcomputers and other computer systems. The SCSI bus can support up to eight devices consisting of any multiple of host adapters and peripheral devices. The Seagate DDS-4 and DAT 72 drives comply with SCSI-2 and SCSI-3 specifications. In a few cases, vendor unique features are available. These features are compatible with the SCSI standards. The interface is a sixteen-port daisy-chained bus using twenty-seven signal lines: eighteen data-bit signal lines and nine control lines. The eighteen data-bit lines are made up of two pairs of eight data signal lines and one parity bit line. The remaining nine lines provide control and status signals to coordinate data transfer operations between the host controller and the selected drive. The drives have an internal SCSI controller integrated into the drive electronics. Each device ID on the SCSI bus may drive up to 8 logical units (LUN). These drives support only LUN 0, except for the Autoloader, which also uses LUN 1. ANSI SCSI Bus Standards In addition to the information presented in this manual, we recommend that for SCSI-2, read the ANSI Working Draft Revision 10 standard before writing host software drivers. Also, see the conformance statements, which are given in the Product Description Manual for the each model of drive or Autoloader. Cabling and Connectors The cabling requirements and pin outs for the SCSI connector for the internal drive models are given in the respective installation guide and Product Description Manual for each model of DAT drive or Autoloader. SCSI Interface Description Page 3 Signal Descriptions The drive SCSI interface consists of eighteen signals. Nine are control lines and eighteen are data lines. Data lines include the parity signal option. These signals are described in the following table. Signal -BSY -SEL Name Busy Select Description OR-tied signal used to show that the data bus is in use Signal used by an Initiator to select a Target or by a Target to reselect an Initiator. SEL is driven by the Initiator during the Selection Phase and driven by the Target during a Reselection Phase. -C/D Control/Data Target-driven signal used to indicate whether Control or Data information is on the data bus. True (low) indicates Control, and false (high) indicates Data. -I/O Input/Output Target-driven signal used to control data movement direction on the data bus with respect to an Initiator. This signal is also used to distinguish between the Selection and Reselection Phases. True (low) indicates input to the Initiator, and false (high) indicates output from the Initiator. -MSG Message Target-driven signal used to indicate the presence of a Message Phase on the bus. True (low) indicates Message Phase, and false (high) indicates Data, Command, or Status Phase. -REQ -ACK -ATN -RST DB(7-0) Request Acknowledge Attention Reset Data Bus Target-driven signal used to indicate a request for a REQ/ACK data transfer handshake. Initiator-driven signal used to indicate an acknowledgment for a REQ/ACK data transfer handshake. Initiator-driven signal used to indicate the Initiator has a message to communicate to the Target. OR-tied signal used to indicate a Reset condition. Eight data-bit signals plus a parity bit signal that form the odd bytes of the data bus. DB (7) is the MSB and has the highest priority (ID 7) during the Arbitration Phase. DB(8-15) DB(P0) Data Bus Data Bus Eight data-bit signals that form the even byte of the data bus. DB(15) is the MSB The data parity bit for the odd byte of the data bus. Parity is odd and is a jumper-selectable option. Parity is not valid during the Arbitration Phase. DB (P) is not to be driven False (high) during the Arbitration Phase. DB(P1) NOTE: Data Bus Data parity bit for the even byte of the data bus. The BSY and RST signals are the only OR-tied signals. In ordinary bus operation, these signals may be simultaneously driven by two or more drivers. There is no operational problem in mixing OR-tied and three-state drivers on signals other than BSY and RST. Page 4 DDS-4 SCSI Interface Manual, Rev. B Command Set Description The following table shows the SCSI commands for sequential access devices implemented by the drive and autoloader. Code 00h 01h 02h 03h 05h 07h 08h 0Ah 0Ch 10h 11h 12h 13h 15h 16h 17h 19h 1Ah 1Bh 1CH 1Dh 1Eh 2Bh 34h 3Bh 3Ch 44h 4Ch 4Dh 56H 57H A5H A6H B8H Type M M V M M A M M V M M M O M M M M M O O M O O O O O M O O O O A A A Command TEST UNIT READY REWIND REQUEST BLOCK ADDRESS REQUEST SENSE READ BLOCK LIMITS INITIALIZE ELEMENT STATUS READ WRITE SEEK BLOCK WRITE FILEMARKS SPACE INQUIRY VERIFY MODE SELECT RESERVE UNIT RELEASE UNIT ERASE MODE SENSE LOAD/UNLOAD RECEIVE DIAGNOSTIC RESULTS SEND DIAGNOSTIC PREVENT/ALLOW MEDIUM REMOVAL LOCATE READ POSITION WRITE DATA BUFFER READ DATA BUFFER REPORT DENSITY SUPPORT (SCSI-3) LOG SELECT LOG SENSE RESERVE UNIT (SCSI-3) RELEASE UNIT (SCSI-3) MOVE MEDIUM EXCHANGE MEDIUM READ ELEMENT STATUS Group 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 2 2 2 2 2 5 5 5 M = Mandatory Command O = Optional Command V = Vendor Unique Command A = Autoloader Command Only SCSI Interface Description Page 5 ANSI X3.131, 199x Conformance Statement (SCSI-2) General features · · · · · · · · · · · Disconnect/reconnect, arbitration (required in SCSI-2). Supports LVD and single-ended drivers. Fixed and variable block transfer lengths. Hard reset. Synchronous data transfers Parity implemented (switch option). Space blocks, filemarks, EOD, and setmarks. Supports third-party reservation Log Sense and Log Select for managing soft errors reporting. MODE SENSE/SELECT page to control and report operation of data compression in sequential access devices. Complies with SCSI-2, Section 16, "Medium Changer Devices*. Interface standards conformance The Small Computer System Interface is described in standards including several versions and a number of individual documents. The original Small Computer System Interface Standard, X3.131-1986, is referred to herein as SCSI-1. SCSI-1 was revised resulting in the Small Computer System Interface ­ 2 (X3.131-1994), referred to herein as SCSI-2. The set of SCSI-3 standards are collectively referred to as SCSI-3. The applicable SCSI-3 standards are as follows: SCSI-3 Architecture Model (SAM) X3.270:1996 SCSI Architecture Model - 2 (SAM-2) INCITS.366:2003 SCSI Parallel Interface - 2 (SPI-2) X3.302:1998 SCSI Parallel Interface - 3 (SPI-3) NCITS.336:2000 SCSI-3 Primary Commands (SPC) X3.301:1997 SCSI Primary Commands - 2 (SPC-2) NCITS.351:2001 SCSI-3 Medium Changer Commands (SMC) NCITS.314:1998 SCSI-3 Stream Commands (SSC) NCITS.335.2000 The term SCSI is used wherever it is not necessary to distinguish between the versions of SCSI. Page 6 DDS-4 SCSI Interface Manual, Rev. B Commands Erase Exchange Medium* Initialize Element Status* Inquiry Load/Unload Locate Log Select Log Sense Mode Select Mode Sense Move Medium* Prevent/Allow Media Removal Read Read Block Limits Read Data Buffer Read Element Status* Read Position Release Unit Receive Diagnostic Results Report Density Support Request Block Address Request Sense Reserve Unit Rewind Seek Block Send Diagnostic Space Test Unit Ready Verify Write Write Data Buffer Write Filemarks * Autoloader only commands Messages Abort Bus Device Reset Disconnect Identify Ignore Wide Residue Initiator Detected Error Linked Command Complete Linked Command Complete with flag Message Reject No Operation Parity Error Message Save Data Pointer Synchronous Data Transfer Request Wide Data Transfer Request Vendor-unique commands Seek Block Request Block Address SCSI Bus Protocol Communication on the SCSI bus occurs between a host computer's SCSI controller and a peripheral controller. The host controller is the Initiator, and the peripheral device is the Target. Some SCSI bus functions are assigned to the Initiator and others to the Target. The Initiator arbitrates (enters the Arbitration Phase of operation) for control of the SCSI bus and enters the Selection Phase to select a specific Target such as a DAT drive. The Target drive can request transfer of command, data, status, information, or disconnect from the bus. While the tape drive is disconnected, the bus is free to accomplish other tasks. While disconnected, the Target drive can process information obtained from the bus. If the Target wants to reconnect, it arbitrates for bus control. For example, a Target can reselect an Initiator or another Target to continue an operation. SCSI Interface Description Page 7 Data-transfer operations on the SCSI bus are either asynchronous or synchronous. Asynchronous data transfer operations follow a defined request/acknowledge (REQ/ACK) handshake protocol. Two eight-bit bytes of information can be transferred with each REQ/ACK handshake. The default data transfer mode is asynchronous. Synchronous data transfer operations are initiated through the SYNCHRONOUS DATA TRANSFER REQUEST message from the Initiator. The SCSI bus protocol is divided into three modes or phases of operation: Waiting Phases, Control Phases, and Information Transfer Phases. These phases are further subdivided into the eight operational phases as listed in the following table. Information on these operational phases is contained in the following paragraphs. Waiting Phases 1. Bus Free Control Phases 2. Arbitration 3. Selection 4. Reselection Information Phases 5. Command 6. Data (Data In/Data Out) 7. Status 8. Message (Message In/ Message Out) Waiting and Control Phases The status of the SCSI bus is a function of the control signals. These signals define the SCSI as in the Waiting Phase (Bus Free Phase), the Control Phases (Arbitration, Selection, or Reselection), or the Information Transfer Phases (Command, Data, Status, or Message). Seagate DDS-4 and DAT 72 drives support both a SCSI system with Arbitration Phase and a SCSI system without Arbitration Phase (non-arbitrating system). Arbitrating Systems In arbitration systems (required in SCSI-2), the sequence of SCSI bus phases follows the sequence shown in Figure 2-1 on the following page. Operation begins with the Bus Free Phase. Normal progression is from the Bus Free to the Arbitration Phase. During Arbitration, Initiators and Targets assert for control of the SCSI bus. The bus is awarded to the device with the highest priority SCSI bus address. Page 8 DDS-4 SCSI Interface Manual, Rev. B Figure 2-1 Phase Sequencing with Arbitration RESET CONDITION BUS FREE PHASE ARBITRATION PHASE SELECTION OR RESELECTION PHASE COMMAND, DATA, STATUS, OR MESSAGE PHASE Arbitration is won by the highest priority SCSI device when both BSY and SEL are asserted and a delay of at least 1200 nSec (1 bus clear delay + 1 bus settle delay) occurs before the Arbitration Phase ends and the Selection Phase begins. Signal timing is shown in Figure 2-3 and the complete Arbitration Phase protocol is summarized in the following table. Figure 2-2 Arbitration and Selection Phase Signal Timing Step 1. Procedure The SCSI device first waits for a Bus Free Phase to occur. The Bus Free Phase is detected when both the BSY and SEL signals are simultaneously and continuously false for a minimum of a bus settle delay of 400 nSec.