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BSR X3.131-198_ NOTE: Revision 17B consists of changes made by the X3T9/84-40 REV 1B X3T9.2 task group at their December 10, 1985 meeting. X3T9.2/82-2 These changes were made in order to make the X3T9.2 REV 17B draft proposed standard consistent with the ISO/TC97/SC13 draft proposal. See page 1.1 for a changed page list. draft proposed American National Standard for information systems - SMALL COMPUTER SYSTEM INTERFACE (SCSI) December 16, 1985 Secretariat Computer and Business Equipment Manufacturers Association Abstract: This standard defines mechanical, electrical, and functional requirements for attaching small computers with each other and with low- to medium-performance intelligent peripherals such as rigid disks, flexible disks, magnetic tape devices, printers, and optical disks. The resulting interface facilitates the interconnection of small computers and intelligent peripherals and thus provides a common interface specification for both systems integrators and suppliers of intelligent peripherals. POINTS OF CONTACT: William E. Burr (X3T9.2 Chairman) John B. Lohmeyer (X3T9.2 Vice Chairman) U.S. Department of Commerce NCR Corporation National Bureau of Standards 3718 N. Rock Road Technology A-216 Wichita, KS 67226 Gaithersburg, MD 20899 (316) 688-8703 (301) 921-3723 .pa .he Small Computer System Interface X3T9.2/82-2 - Rev. 17B 12/16/85 .fo 1.1 CHANGED PAGE LIST At their December 10, 1985 meeting, the X3T9.2 task group made a number of changes to Revision 17 of this document. (Revision 17A was distributed at the meeting proposing some of the changes; the remainder of the changes were made at the meeting.) The primary reason for making these changes is to keep this document consistent with the ISO/TC97/SC13 SCSI draft proposal. The pages that have changed since Revision 17 are listed below: Page Change ------- ------------------------------------------------------------------- 1 Changed revision number and date. 1.1 Replaced changed page list. 2 Editorial revisions to the foreword. 7-7.1 Revised the Table of Contents to reflect the document changes. 9 Added description of the contents of appendixes D - G. 12 Moved section 4.3.2, Shielded Connectors into Appendix D. Merged section 4.3.1 into section 4.3. 12.1-12.2 \ 17-19 \ Deleted pages 19.1-19.4 / (Information moved to Appendix D). 21.1-21.2 / 20-21 Deleted second line of Table titles. 24 Redrew figures 4-5 through 4-7. 26 Redrew figure 4-9. 37 Clarified that multiple messages may be sent during a message phase. 38 Clarification to the MESSAGE OUT phase error handling. 41 Redrew figures 5-1 and 5-2. 43 Editorial clarification. 49 Deleted "immediately". 50-50.1 Clarified that unit attention condition is on a per logical unit basis. Also clarified "other" command is other than REQUEST SENSE. 65-66 Added ISO version field to the INQUIRY data. 93.1 Revised Table 8-14.1 to reference the latest X3B5 document number and to refer to Appendix F for additional standards. 94-99 Editorial clarifications to the RESERVE and RELEASE commands. 113 Clarified which status codes are to be returned on SEARCH DATA commands. 119 Clarified that READ BLOCK LIMITS returns the target's capability as opposed to its current setting. 132 Added code values 0BH through 0DH to Table 9-14.1. 132.2-133 Editorial clarifications to the RESERVE UNIT and RELEASE UNIT commands. 147-149 Editorial clarifications to the RESERVE UNIT and RELEASE UNIT commands. 174-176.2 Revised Appendix A to make the figure legible. 182-183 Redrew figures C1 and C2. 184-192 Added Appendix D, Recommended Shielded Connectors. Most of this information was previously in section 4.3.2. 193-194 Added Appendix E, Conformance. 195-196 Added Appendix F, Additional Medium Type and Density Code Standards. 197 Appendix G was previously Appendix D. Editorial clarification. .pa .pn 2 .fo # FOREWORD (This Foreword is not part of American National Standard X3.131-198_.) The development of comparatively inexpensive VLSI device controllers have recently changed the economics of interfaces for small system storage devices. Where expensive controller logic was once shared among as many devices as possible, in many cases it now makes economic sense to build a controller in each device. This is particularly true for high-performance storage devices, where the intimate interactions of the recording medium, the recording mechanism, and the recording code cause intersymbol interference and error recovery problems that are highly specific to the chosen technologies, and are best resolved within the device itself. Moreover, the number of types of storage devices for small computers, and the industry that builds them, have grown dramatically in the past few years. In particular, the emergence of physically small, but comparatively high-capacity and high-performance fixed-medium magnetic disk devices (virtually non- existent in the late 1970s, but a multibillion dollar business in the mid- 1980s) has driven the development of small computer systems and caused a need for other classes of devices, such as streaming cartridge tape drives, for backup and data interchange. Because device interfaces are very specific to device types, many device level interface standards would be required to service all small computer device types. Because backplane buses reside at the center of computers, and have dramatic performance effects, many different ones are needed for different system requirements. To connect every backplane bus to every device interface through a controller would require an almost unbounded number of specific controller products. In addition, in many systems today, it is not the computer which is "central", it is the storage facility. That is, one or two large capacity storage subsystems serve several computers. An interface adapted to this reality was needed. By 1982, all the needs given above were widely recognized in the industry and by the members of X3T9 and its Task Groups. A commercial small system parallel bus, the Shugart Associates System Interface (SASI), generally met the small system requirements for a device-independent peripheral or system bus and had enjoyed significant market success. It was offered to X3T9.2 as the basis for a standard. X3T9.2 chose the name Small Computer System Interface (SCSI) for that standard and began work at its April 1982 meeting. The present SCSI dpANS is a formalization and extension of the SASI. Many existing SASI devices are SCSI compatible. Since April 1982, X3T9.2 has held plenary sessions, at two month intervals, plus numerous informal working meetings. The original SASI has been extended in a number of ways, including: (1) A differential electrical option has been added to allow use of longer cables (up to 25 meters) in environments where common mode noise is a concern. (2) A synchronous transfer option has been developed allowing maximum transfer rates in the 3 to 4 megabyte per second range. (3) An optional "extended" command set has been added, allowing for very large capacity storage devices (a block address space of 232 blocks versus 221 for SASI), and Inquiry commands that allow self-configuring driver software. (4) Command sets for magnetic tape (both start/stop and streaming), printers, processors, optical disks, and read-only optical disks have been added to the proposed SCSI standard, in addition to those for magnetic disks. Although it might have been premature in April 1982 to claim that SASI was then a de facto standard, this surely is the case for SASI/SCSI today. SCSI compatible host adapters, controllers, and peripheral devices are now widely manufactured around the world. Host adapters are available for most small computers with accessible backplane buses. SCSI controllers are widely available for all the de jure and de facto standard magnetic disk and magnetic tape device interfaces. Small high-capacity fixed-medium magnetic disks, rigid removable-medium magnetic disks, high-capacity Bernoulli-effect flexible disks, and other closed-loop high-capacity flexible disk products, as well as optical disk products, are all available with integral SCSI controllers. SCSI subsystems that integrate both a rigid-disk and a streaming-tape drive into a single package are also available. SCSI interface chips are available, and some disk controller chip sets also provide SCSI support. Since a large number of companies have implemented and tested the SCSI during the development of the proposed standard, no separate test program has been deemed necessary. At the February 1984 meeting of X3T9.2, representatives of the following companies stated, for the record, that their companies had implemented and tested SCSI: Adaptec, Adaptive Data, Data Technology Corp., Fujitsu America, Inc., NCR Corp., and Shugart Corp. This is not a complete list of companies implementing products using SCSI nor is it a promise by these companies to offer SCSI products. The SCSI fills an urgent need, provides for the future, and is consonant with actual commercial practice. Most important, the timing is right. SCSI catches the floodtide of new, high-performance storage devices for small systems, and promises to bring a measure of needed order to what would otherwise be a chaotic and fragmented market. This standard specifies the mechanical, electrical, and functional requirements for a small computer input/output bus interface, and command sets for peripheral device types, particularly storage devices, commonly used with small computers. Suggestions for improvement of this standard will be welcome. They should be sent to the Computer and Business Equipment Manufacturers Association, 311 First Street, NW, Suite 500, Washington, DC 20001. This standard was processed and approved for submittal to ANSI by American National Standards Committee on Computers and Information Processing, X3. Committee approval of the standard does not imply that all committee members voted for its approval. .fo 2.1 .pa .pn 3 .fo # At the time it approved this standard, the X3 Committee had the following members: TO BE DETERMINED .pa Subcommittee X3T9 on I/O interfaces, which reviewed this standard, had the following members: Delbert L. Shoemaker (Chairman) Ron Tranquilli (Vice Chairman) Bob Bender G. Atterbury (Alt) John Blagaila Charles Brill (Alt) Fred Ciechowski William E. Burr (Alt) George Clark Roger Cormier (Alt) Stephen W. Cooper Hank Dorris (Alt) Louis C. Domshy Thomas A. Fiers (Alt) Robert Dugan Henry Ginter (Alt) Ross H. Jaibaji William J. McClain (Alt) Patrick Lannan William Mosenthal, Jr. (Alt) Gene Milligan Kirk Moulton (Alt) Tom Morrow Mike Newton Gary S. Robinson Arnold John Roccati Floyd E. Ross Holly S. White (Note: The name lists on this page and the following page are incomplete and they will be updated upon final approval of this standard.) .pa Task Group X3T9.2 on Lower-Level Interfaces, which developed this standard, had the following members: William E. Burr (Chairman) John B. Lohmeyer (Vice Chairman) Ezra R. Alcudia Keith Amundsen (Alt) J. L. Amstutz Karen Anneberg (Alt) Bob Bender Charles Brill (Alt) John Blagaila Larry Boucher (Alt) Tom Briggs Paul Clement (Alt) David T. Cornaby George E. Clark (Alt) David F. Craft, Jr. Steve Cooper (Alt) Jay Cunningham Gary Crowell (Alt) Willard S. Davidson Jon Ericson (Alt) Terry Dawson Tom Fiers (Alt) Phil Devin Stephen Fitzgerald (Alt) Louis C. Domshy Marty Francis (Alt) Norm Dornseif William Homans (Alt) Alan Ebright J. V. Howell (Alt) Anita Freeman Skip Kilsdonk (Alt) Abe Gindi Jim Korpi (Alt) William A. Horton Lawrence J. Lamers (Alt) Frank Krulc Keith Mueller (Alt) Patrick E. Lannan Don Nanneman (Alt) Daniel Loski Doug Nolff (Alt) William C. Mavity Richard Reiser (Alt) Gene Milligan William H. Roberts (Alt) Bob Mortensen Floyd E. Ross (Alt) Gary S. Robinson D. Michael Robinson (Alt) Don Rodgers Jay Seashore (Alt) Arnold J. Roccati Chuck Spatafore (Alt) Jack Schiffhauer Jeff Stai (Alt) Ralph H. Schultz Paul Stavish (Alt) Moshe Segal Delbert L. Shoemaker Tim Slaton Robert N. Snively Adrienne Turenne Norm Zimmerman .pa T A B L E O F C O N T E N T S 1. Scope...................................................................8 2. Referenced Standard.....................................................9 3. Glossary and Conventions................................................9 3.1 Glossary...........................................................9 3.2 Editorial Conventions.............................................11 4. Physical Characteristics...............................................11 4.1 Physical Description..............................................11 4.2 Cable Requirements................................................11 4.3 Connector Requirements............................................12 4.4 Electrical Description............................................22 4.5 SCSI Bus..........................................................25 4.6 SCSI Bus Signals..................................................27 4.7 SCSI Bus Timing...................................................30 5. Logical Characteristics................................................31 5.1 SCSI Bus Phases...................................................31 5.2 SCSI Bus Conditions...............................................38 5.3 SCSI Bus Phase Sequences..........................................40 5.4 SCSI Pointers.....................................................42 5.5 Message System Specification......................................42 6. SCSI Commands .........................................................49 6.1 Command Implementation Requirements...............................50 6.2 Command Descriptor Block........................................50.1 6.3 Command Examples..................................................55 7. Command Descriptions for All Device Types..............................57 7.1 Group 0 Commands for All Device Types.............................57 7.2 Group 1 Commands for All Device Types.............................76 7.3 Group 2 Commands for All Device Types.............................79 7.4 Group 3 Commands for All Device Types.............................79 7.5 Group 4 Commands for All Device Types.............................79 7.6 Group 5 Commands for All Device Types.............................79 7.7 Group 6 Commands for All Device Types.............................79 7.8 Group 7 Commands for All Device Types.............................79 8. Command Descriptions for Direct-Access Devices.........................80 8.1 Group 0 Commands for Direct-Access Devices........................80 8.2 Group 1 Commands for Direct-Access Devices.......................104 9. Group 0 Command Descriptions for Sequential-Access Devices............117 10. Group 0 Command Descriptions for Printer Devices......................140 11. Group 0 Command Descriptions for Processor Devices....................152 12. Command Descriptions for Write-Once Read-Multiple Devices.............155 12.1 Group 0 Commands for Write-Once Read-Multiple Devices...........155 12.2 Group 1 Commands for Write-Once Read-Multiple Devices...........164 .pa 13. Command Descriptions for Read-Only Direct-Access Devices..............170 13.1 Group 0 Commands for Read-Only Direct-Access Devices............170 13.2 Group 1 Commands for Read-Only Direct-Access Devices............171 14. Status ...............................................................172 L I S T O F F I G U R E S 4-1 Nonshielded SCSI Device Connector.....................................13 4-2 Nonshielded Cable Connector...........................................15 4-5 Termination for Single-Ended Devices..................................24 4-6 Termination for Differential Devices..................................24 4-7 Differential Driver Protection Circuit (Optional).....................24 4-8 SCSI ID Bits..........................................................25 4-9 Sample SCSI Configurations............................................26 5-1 Phase Sequences without Arbitration...................................41 5-2 Phase Sequences with Arbitration......................................41 5-3 Simplified SCSI System................................................42 Appendixes Appendix A................................................................174 SCSI Signal Sequence Example Appendix A Figures A1: SCSI Timing Chart ...............................................175 Appendix B................................................................177 Typical Bus Phase Sequence Appendix C................................................................179 SCSI System Operation C1. Host Memory / Host Adapter / SCSI Controller Relationship........179 C2. SCSI READ Command Example........................................180 C3. I/O Channel Concept..............................................181 Appendix C Figures C1: Snapshot Prior to Initial Selection..............................182 C2: Snapshot Prior to Data Transfer..................................183 Appendix D................................................................184 Recommended Shielded Connectors D1. Shielded Connector, Alternative 1................................184 D2. Shielded Connector, Alternative 2................................184 D3. EUROCARD Boxes...................................................184 .pa .fo 7.1 Appendix D Figures D1: Female Shielded SCSI Cable Connector, Alternative 1..............185 D2: Male Shielded SCSI Device Connector, Alternative 1...............187 D3: Shielded SCSI Device Connector, Alternative 2....................189 D4: Shielded SCSI Cable Connector, Alternative 2.....................190 Appendix E................................................................193 Conformance E1. Alternatives.....................................................193 E2. Levels of Conformance............................................193 E3. Options..........................................................194 E4. Statement of Conformance.........................................194 Appendix F................................................................195 Additional Medium Type and Density Code Standards Appendix G................................................................197 Future Standardization .pa (This page is intentionally blank.) .fo 7.2 .pa .fo Section 1 # 1. Scope This American National Standard provides the mechanical, electrical, and functional requirements for a small computer input/output bus and command sets for peripheral device types commonly used with small computers. The small computer system interface, described in this standard, is a local I/O bus that can be operated at data rates up to 4 megabytes per second depending upon circuit implementation choices. The primary objective of the interface is to provide host computers with device independence within a class of devices. Thus, different disk drives, tape drives, printers, and even communication devices can be added to the host computer(s) without requiring modifications to generic system hardware or software. Provision is made for the addition of nongeneric features and functions through vendor unique fields and codes. The interface uses logical rather than physical addressing for all data blocks. For direct access devices, each logical unit may be interrogated to determine how many blocks it contains. A logical unit may coincide with all or part of a peripheral device. Provision is made for cable lengths up to 25 meters using differential drivers and receivers. A single-ended driver and receiver configuration is defined for cable lengths of up to 6 meters and is primarily intended for applications within a cabinet. The interface protocol includes provision for the connection of multiple initiators (SCSI devices capable of initiating an operation) and multiple targets (SCSI devices capable of responding to a request to perform an operation). Optional distributed arbitration (i.e., bus-contention logic) is built into the architecture of SCSI. A priority system awards interface control to the highest priority SCSI device that is contending for use of the bus. The time to complete arbitration is independent of the number of devices that are contending and can be completed in less than 10 microseconds. The physical characteristics are described in Section 4. There are two electrical alternatives: single-ended and differential. Single-ended and differential devices are electrically different and shall not be mixed on the same bus. In addition, there are several options: shielded or unshielded connectors may be used and parity may or may not be implemented. Section 5 describes the logical characteristics of the interface. An arbitration option is defined to permit multiple initiators and to permit concurrent I/O operations. All SCSI devices are required to be capable of operating with the defined asynchronous transfer protocol. In addition, an optional synchronous transfer protocol is defined. Section 5 also specifies a message protocol for control of the interface. In most cases, messages are not directly apparent to the host computer software. Only one message, COMMAND COMPLETE, is mandatory; all others are optional and are not necessarily implemented. Note that some options (e.g., synchronous transfer) require the implementation of certain messages. The SCSI command structure is specified in Section 6. Commands are classified as mandatory (M), extended (E), optional (O), or vendor unique (V). SCSI devices shall implement all mandatory commands defined for the appropriate device type and may implement other commands as well. Extended SCSI devices shall implement all extended plus all mandatory commands and may implement other commands as well. Extended SCSI devices contain commands that facilitate the writing of self-configuring software drivers that can "discover" all necessary attributes without prior knowledge of specific peripheral characteristics (such as storage capacity). Extended commands for direct access devices also implement a very large logical block address space (232 blocks), although mandatory commands for direct access devices implement a somewhat smaller logical block address space (221 blocks). Section 7 specifies those commands that have a consistent meaning for all device types. Sections 8 through 13 contain commands for direct-access (e.g., magnetic disk), sequential-access (e.g., magnetic tape), printer, processor, write- once-read-multiple (e.g., optical disk), and read-only direct-access devices, respectively. The commands in each of these sections are unique to the device type, or they have interpretations, fields, or features that are specific for the device type. Thus, for example, although the WRITE command is used for several device types, it has a somewhat different form for each type, with different parameters and meanings. Therefore, it is specified separately for each device type. Section 14 describes the status byte for all device types. Status is returned by targets at the end of each command. Appendixes A through C provide examples of SCSI signal sequences, timing, and phase sequences. Appendix D contains information on recommended shielded connectors. Appendix E contains information on conformance statements. Appendix F contains information on other standards related to medium types and density codes for flexible disks and magnetic tapes. Appendix G contains information on future extensions to SCSI that are being considered by X3T9.2 However, the appendixes are not part of this standard. .fo Section 3 # 3. Glossary and Conventions 3.1 Glossary byte. In this standard, this term indicates an 8-bit (octet) byte. command descriptor block (CDB). The structure used to communicate requests >From an initiator to a target. connect. The function that occurs when an initiator selects a target to start an operation. ____________ 1 Available from the Electronic Industries Association, 2001 Eye Street NW, Washington, D.C. 20006. .cp 56 disconnect. The function that occurs when a target releases control of the SCSI bus, allowing it to go to the BUS FREE phase. initiator. An SCSI device (usually a host system) that requests an operation to be performed by another SCSI device. INTERMEDIATE status. A status code sent from a target to an initiator upon completion of each command in a set of linked commands except the last command in the set. logical unit. A physical or virtual device addressable through a target. logical unit number. An encoded three-bit identifier for the logical unit. LSB. Least significant byte. LUN. Logical unit number. mm. Millimeter. ms. Millisecond. MSB. Most significant byte. ns. Nanosecond. one. A true signal value. peripheral device. A peripheral that can be attached to an SCSI device (e.g., magnetic-disk, printer, optical-disk, or magnetic-tape). reconnect. The function that occurs when a target selects an initiator to continue an operation after a disconnect. reserved. The term used for bits, bytes, fields, and code values that are set aside for future standardization. SCSI address. The octal representation of the unique address (0-7) assigned to an SCSI device. This address would normally be assigned and set in the SCSI device during system installation. SCSI ID. The bit-significant representation of the SCSI address referring to one of the signal lines DB(7-0). SCSI device. A host computer adapter or a peripheral controller or an intelligent peripheral that can be attached to the SCSI bus. signal assertion. The act of driving a signal to the true state. signal negation. The act of driving a signal to the false state or allowing the cable terminators to bias the signal to the false state (by placing the driver in the high impedance condition). signal release. The act of allowing the cable terminators to bias the signal to the false state (by placing the driver in the high impedance condition). status. One byte of information sent from a target to an initiator upon completion of each command. target. An SCSI device that performs an operation requested by an initiator. us. Microsecond. vendor unique. In this standard, this term indicates bits, fields, or code values that are vendor specific. xxH. Numbers followed by capital H subscript are hexadecimal values. All other numbers are decimal values. zero. A false signal value. 3.2 Editorial Conventions. Certain words and terms used in this standard have a specific meaning beyond the normal English meaning. These words and terms are defined either in the glossary (see 3.1) or in the text where they first appear (e.g., initiator). Names of signals, phases, conditions, messages, commands, statuses, and sense keys are in all uppercase (e.g., REQUEST SENSE). Lowercase is used for words having the normal English meaning. .fo Section 4 # 4. Physical Characteristics This section contains the physical definition of the SCSI. The connectors, cables, signals, terminators, and bus timing needed to implement SCSI are covered. 4.1 Physical Description. SCSI devices are daisy-chained together using a common cable. Both ends of the cable are terminated. All signals are common between all SCSI devices. Two driver/receiver alternatives are available: (1) Single-ended drivers and receivers, which allow a maximum cable length of six meters (primarily for connection within a cabinet) (2) Differential drivers and receivers, which allow a maximum cable length of 25 meters (primarily for connection outside of a cabinet) 4.2 Cable Requirements. An ideal impedance match with cable terminators implies a cable characteristic impedance of 132 ohms (singled-ended option) or 122 ohms (differential option). In general, cables with this high of a characteristic impedance are not available; however, impedances that are somewhat lower are satisfactory. A characteristic impedance of 100 ohms + 10% is recommended for unshielded flat or twisted pair ribbon cable. A characteristic impedance greater than 90 ohms is preferred for shielded cables; however, most available cables have a somewhat lower characteristic impedance. To minimize discontinuities and signal reflections, cables of different impedances should not be used in the same bus. Implementations may require trade-offs in shielding effectiveness, cable length, the number of loads, transfer rates, and cost to achieve satisfactory system operation. A minimum conductor size of 28 AWG shall be employed to minimize noise effects and ensure proper distribution of optional terminator power. .cp 5 4.2.1 Single-Ended Cable. A 50-conductor flat cable or 25-signal twisted- pair cable shall be used. The maximum cable length shall be 6.0 meters. A stub length of no more than 0.1 meters is allowed off the mainline interconnection within any connected equipment. SCSI bus termination may be internal to the SCSI devices that are at the ends of the cable. 4.2.2 Differential Cable. A 50-conductor cable or 25-signal twisted-pair cable shall be used. The maximum cable length shall be 25 meters. A stub length of no more than 0.2 meters is allowed off the mainline interconnection within any connected equipment. SCSI bus termination may be internal to the SCSI devices that are at the ends of the cable. 4.3 Connector Requirements. Nonshielded connectors are specified. The nonshielded connectors are typically used for in-cabinet applications. Appendix D defines recommended shielded connectors and their pin assignments. These connectors are typically used for external applications where electromagnetic compatibility (EMC) and electrostatic discharge (ESD) protection may be required. Either type of connector may be used with the single-ended or differential drivers. The nonshielded SCSI device connector (Figure 4-1) shall be a 50-conductor connector consisting of two rows of 25 male pins with adjacent pins 2.54 mm (0.1 in) apart. A shroud and header body should be used. The nonmating portion of the connector is shown for reference only. The nonshielded cable connector (Figure 4-2) shall be a 50-conductor connector consisting of two rows of 25 female contacts with adjacent contacts 2.54 mm (0.1 in) apart. It is recommended that keyed connectors be used. The unshielded connector pin assignments shall be as shown in Table 4-1 for single-ended drivers and as shown in Table 4-2 for differential drivers. .pa Figure 4-1a. Nonshielded SCSI Device Connector .pa ============================================================================== Dimensions Millimeters Inches ------------------------------------------------------------------------------ D1 2.54 0.100 D2* 82.80 3.260 D3 2.54 0.100 D4 4.83 0.190 D5* 8.51 0.335 D6* 72.64 2.860 D7* 78.74 3.100 D8* 13.94 0.549 D9 4.19 0.165 D10 6.09 0.240 D11 6.60 0.260 ============================================================================== NOTES: (1) Fifty Contacts on 2.54-mm (0.100-inch) spacing = 60.96 mm (2.40 inch). (2) Tolerances + 0.127 mm (0.005 inch) noncumulative. (3) Dimensions listed with asterisks (*) are shown for reference only. Figure 4-1b. Nonshielded SCSI Device Connector (Editors note: Figures 4-1a and 4-1b are to be combined into a single figure during the final editing.) .pa Figure 4-2a. Nonshielded Cable Connector .pa ============================================================================== Dimensions Millimeters Inches ------------------------------------------------------------------------------ C1 2.5400 0.100 C2 60.9600 2.400 C3 2.5400 0.100 C4 8.3570 0.329 C5 3.3025 0.130 C6 68.0720 2.680 C7 6.0960 0.240 C8* 8.1530 0.321 C9* 13.4870 0.531 C10* 3.8100 0.150 C11* 1.2700 0.050 C12* 6.0960 0.240 C13 32.3850 1.275 C14 3.3020 0.130 C15 7.4930 0.295 C16 2.6670 0.105 C17 1.6250 0.064 ============================================================================== NOTES: (1) Fifty contacts on 1.27-mm (0.05-inch)* staggered spacing = 62.23 mm (2.450 inch)*. (2) Tolerances + 0.127 mm (0.005 inch) noncumulative. (3) Dimensions listed with asterisks (*) are shown for reference only. Figure 4-2b. Nonshielded Cable Connector (Editors note: Figures 4-2a and 4-2b are to be combined into a single figure during the final editing.) (Pages 17-19 and 19.1-19.4 are deleted. The information previously contained on these pages has been moved to Appendix D.) .pa .pn 20 .fo Section 4 # Table 4-1 Single-Ended Pin Assignments ============================================================================== Signal Pin Number ------------------------------------------------------------------------------ -DB(0) 2 -DB(1) 4 -DB(2) 6 -DB(3) 8 -DB(4) 10 -DB(5) 12 -DB(6) 14 -DB(7) 16 -DB(P) 18 GROUND 20 GROUND 22 GROUND 24 TERMPWR 26 GROUND 28 GROUND 30 -ATN 32 GROUND 34 -BSY 36 -ACK 38 -RST 40 -MSG 42 -SEL 44 -C/D 46 -REQ 48 -I/O 50 ============================================================================== NOTES: (1) All odd pins except pin 25 shall be connected to ground. Pin 25 should be left open. Some products designed prior to the generation of this standard connected this pin to ground. (2) The minus sign next to the signals indicates active low. .pa Table 4-2 Differential Pin Assignments ============================================================================== Signal Name Pin Number Signal Name ------------------------------------------------------------------------------ SHIELD GROUND 1 2 GROUND +DB(0) 3 4 -DB(0) +DB(1) 5 6 -DB(1) +DB(2) 7 8 -DB(2) +DB(3) 9 10 -DB(3) +DB(4) 11 12 -DB(4) +DB(5) 13 14 -DB(5) +DB(6) 15 16 -DB(6) +DB(7) 17 18 -DB(7) +DB(P) 19 20 -DB(P) DIFFSENS 21 22 GROUND GROUND 23 24 GROUND TERMPWR 25 26 TERMPWR GROUND 27 28 GROUND +ATN 29 30 -ATN GROUND 31 32 GROUND +BSY 33 34 -BSY +ACK 35 36 -ACK +RST 37 38 -RST +MSG 39 40 -MSG +SEL 41 42 -SEL +C/D 43 44 -C/D +REQ 45 46 -REQ +I/O 47 48 -I/O GROUND 49 50 GROUND ============================================================================== NOTE: (1) SHIELD GROUND is optional on some cables. (Implementors note: Some shielded flat ribbon cables use pin 1 as a connection to the shield.) .pa .pn 22 .fo Section 4 # 4.4 Electrical Description NOTE: For these measurements, SCSI bus termination is assumed to be external to the SCSI device. An SCSI device may have the provision for allowing optional internal termination. 4.4.1 Single-Ended Alternative. All assigned signals shall be terminated with 220 ohms to +5 volts (nominal) and 330 ohms to ground at each end of the cable. (See Figure 4-5.) All signals shall use open-collector or three-state drivers. 4.4.1.1 Output Characteristics. Each signal driven by an SCSI device shall have the following output characteristics when measured at the SCSI device's connector: Signal assertion = 0.0 volts dc to 0.4 volts dc Minimum driver output capability = 48 milliamps (sinking) at 0.5 volts dc Signal negation = 2.5 volts dc to 5.25 volts dc 4.4.1.2 Input Characteristics. Each signal received by an SCSI device shall have the following input characteristics when measured at the SCSI device's connector: Signal true = 0.0 volts dc to 0.8 volts dc Maximum total input load = -0.4 milliamps at 0.4 volts dc Signal false = 2.0 volts dc to 5.25 volts dc Minimum input hysteresis = 0.2 volts dc 4.4.2 Differential Alternative. All signals consist of two lines denoted +SIGNAL and -SIGNAL. A signal is true when +SIGNAL is more positive than -SIGNAL, and a signal is false when -SIGNAL is more positive than +SIGNAL. All assigned signals shall be terminated at each end of the cable as shown in Figure 4-6. NOTE: As an option, the DIFFSENS signal of the connector is reserved for an active high enable for the differential drivers. If a single-ended device or terminator is inadvertently connected, this signal is grounded, disabling the drivers. (See Figure 4-7.) 4.4.2.1 Output Characteristics. Each signal driven by an SCSI device shall have the following output characteristics when measured at the SCSI device's connector: VOL (Low-level output voltage) = 2.0 V maximum at IOL (Low-level output current) = 55 milliamps. VOH (High-level output voltage) = 3.0 V minimum at IOH (High-level output current) = -55 milliamps. VOD (Differential voltage) = 1.0 V minimum with common-mode voltage ranges from -7 volts dc to +12 volts dc. VOL and VOH shall be as measured between the output terminal and the SCSI device's logic ground reference. The output characteristics shall additionally conform to EIA RS-485-1983. .pa 4.4.2.2 Input Characteristics. Each signal received by an SCSI device shall have the following input characteristics when measured at the SCSI device's connector: II (Input current on either input) = + 2.0 milliamps maximum. NOTE: These characteristics include both receivers and passive drivers. This requirement shall be met with the input voltage varying between -7 volts dc and +12 volts dc, with power on or off, and with the hysteresis equaling 35 millivolts, minimum. The input characteristics shall additionally conform to EIA RS-485-1983. 4.4.3 Terminator Power (Optional). Single-ended SCSI devices providing terminator power (TERMPWR) shall have the following characteristics: VTerm = 4.0 volts dc to 5.25 volts dc 800 milliamps minimum source drive capability 1.0 milliamp maximum sink capability (except for the purposes of providing power to an internal terminator) with 1.0 amp recommended current limiting (e.g., a fuse). Differential SCSI devices providing terminator power (TERMPWR) shall have the following characteristics: VTerm = 4.0 volts dc to 5.25 volts dc 600 milliamps minimum source drive capability 1.0 milliamp maximum sink capability (except for the purposes of providing power to an internal terminator) with 1.0 amp recommended current limiting (e.g., a fuse). The use of keyed connectors is recommended in SCSI devices that provide terminator power to prevent accidental grounding or misconnection of terminator power. SCSI devices that supply terminator power shall do so through a diode or similar semiconductor that prevents the backflow of power to the SCSI device. .pa Figure 4-5. Termination for Single-Ended Devices Figure 4-6. Termination for Differential Devices Figure 4-7. Differential Driver Protection Circuit (Optional) .pa 4.5 SCSI Bus. Communication on the SCSI bus is allowed between only two SCSI devices at any given time. There is a maximum of eight SCSI devices. Each SCSI device has an SCSI ID bit assigned as shown in Figure 4-8. 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 the target performs the operation. An SCSI device usually has a fixed role as an initiator or target, but some devices may be able to assume either role. An initiator may address up to eight peripheral devices that are connected to a target. An option allows the addressing of up to 2,048 peripheral devices per target using extended messages. Three sample system configurations are shown in Figure 4-9. DB(7) DB(6) DB(5) DB(4) DB(3) DB(2) DB(1) DB(0) <-- DATA BUS | | | | | | | | | | | | | | | SCSI ID = 0 | | | | | | | | | | | | | SCSI ID = 1 | | | | | | | | | | | SCSI ID = 2 | | | | | | | | | SCSI ID = 3 | | | | | | | SCSI ID = 4 | | | | | SCSI ID = 5 | | | SCSI ID = 6 | SCSI ID = 7 Figure 4-8. SCSI ID Bits .pa Figure 4-9. Sample SCSI Configurations .pa Up to eight SCSI devices can be supported on the SCSI bus. They can be any combination of initiators and targets. Certain SCSI bus functions are assigned to the initiator and certain SCSI bus functions are assigned to the target. The initiator may arbitrate for the SCSI bus and select a particular target. The target may request the transfer of COMMAND, DATA, STATUS, or other information on the DATA BUS, and in some cases it may arbitrate for the SCSI bus and reselect an initiator for the purpose of continuing an operation. Information transfers on the DATA BUS are asynchronous and follow a defined REQ/ACK handshake protocol. One byte of information may be transferred with each handshake. An option is defined for synchronous data transfer. 4.6 SCSI Bus Signals. There are a total of eighteen signals. Nine are used for control and nine are used for data. (Data signals include the parity signal option). These signals are described as follows: BSY (BUSY). An "OR-tied" signal that indicates that the bus is being used. SEL (SELECT). A signal used by an initiator to select a target or by a target to reselect an initiator. C/D (CONTROL/DATA). A signal driven by a target that indicates whether CONTROL or DATA information is on the DATA BUS. True indicates CONTROL. I/O (INPUT/OUTPUT). A signal driven by a target that controls the direction of data movement on the DATA BUS with respect to an initiator. True indicates input to the initiator. This signal is also used to distinguish between SELECTION and RESELECTION phases. MSG (MESSAGE). A signal driven by a target during the MESSAGE phase. REQ (REQUEST). A signal driven by a target to indicate a request for a REQ/ACK data transfer handshake. ACK (ACKNOWLEDGE). A signal driven by an initiator to indicate an acknowledgment for a REQ/ACK data transfer handshake. ATN (ATTENTION). A signal driven by an initiator to indicate the ATTENTION condition. RST (RESET). An "OR-tied" signal that indicates the RESET condition. DB(7-0,P) (DATA BUS). Eight data-bit signals, plus a parity-bit signal that form a DATA BUS. DB(7) is the most significant bit and has the highest priority during the ARBITRATION phase. Bit number, significance, and priority decrease downward to DB(0). A data bit is defined as one when the signal value is true and is defined as zero when the signal value is false. Data parity DB(P) is odd. The use of parity is a system option (i.e., a system is configured so that all SCSI devices on a bus generate parity and have parity detection enabled, or all SCSI devices have parity detection disabled or not implemented). Parity is not valid during the ARBITRATION phase. 4.6.1 Signal Values. Signals may assume true or false values. There are two methods of driving these signals. In both cases, the signal shall be actively driven true, or asserted. In the case of OR-tied drivers, the driver does not drive the signal to the false state, rather the bias circuitry of the bus terminators pulls the signal false whenever it is released by the drivers at every SCSI device. If any driver is asserted, then the signal is true. In the case of non-OR-tied drivers, the signal may be actively driven false, or negated. In this standard, wherever the term negated is used, it means that the signal may be actively driven false, or may be simply released (in which case the bias circuitry pulls it false), at the option of the implementor. The advantage to actively drive signals false is that the transition from true to false occurs more quickly, and noise margins may be somewhat improved; this may permit somewhat faster data transfer. 4.6.2 OR-Tied Signals. The BSY and RST signals shall be OR-tied only. In the ordinary operation of the bus, these signals are simultaneously driven true by several drivers. No signals other than BSY, RST, and DB(P) are simultaneously driven by two or more drivers, and any signal other than BSY and RST may employ OR-tied or non-OR-tied drivers. DB(P) shall not be driven false during the ARBITRATION phase. There is no operational problem in mixing OR-tied and non-OR-tied drivers on signals other than BSY and RST. 4.6.3 Signal Sources. Table 4-3 indicates which type of SCSI device is allowed to source each signal. No attempt is made to show if the source is driving asserted, driving negated, or is passive. All SCSI device drivers that are not active sources shall be in the passive state. Note that the RST signal may be sourced by any SCSI device at any time. .pa Table 4-3 Signal Sources ============================================================================== Signals ---------------------------------------------------------- C/D, I/O, Bus Phase BSY SEL MSG, REQ ACK/ATN DB(7-0,P) ------------------------------------------------------------------------------ BUS FREE None None None None None ARBITRATION All Winner None None SCSI ID SELECTION I&T Initiator None Initiator Initiator RESELECTION I&T Target Target Initiator Target COMMAND Target None Target Initiator Initiator DATA IN Target None Target Initiator Target DATA OUT Target None Target Initiator Initiator STATUS Target None Target Initiator Target MESSAGE IN Target None Target Initiator Target MESSAGE OUT Target None Target Initiator Initiator ============================================================================== All: The signal shall be driven by all SCSI devices that are actively arbitrating. SCSI ID: A unique data bit (the SCSI ID) shall be driven by each SCSI device that is actively arbitrating; the other seven data bits shall be released (i.e., not driven) by this SCSI device. The parity bit (DB(P)) may be undriven or driven to the true state, but shall never be driven to the false state during this phase. I&T: The signal shall be driven by the initiator, target, or both, as specified in the SELECTION phase and RESELECTION phase. Initiator: If this signal is driven, it shall be driven only by the active initiator. None: The signal shall be released; that is, not be driven by any SCSI device. The bias circuitry of the bus terminators pulls the signal to the false state. Winner: The signal shall be driven by the one SCSI device that wins arbitration. Target: If the signal is driven, it shall be driven only by the active target. .pa 4.7 SCSI Bus Timing. Unless otherwise indicated, the delay-time measurements for each SCSI device, shown in 4.7.1 through 4.7.14, shall be calculated from signal conditions existing at that SCSI device's own SCSI bus connection. Thus, these measurements (except cable skew delay) can be made without considering delays in the cable. 4.7.1 Arbitration Delay (2.2 microseconds). The minimum time an SCSI device shall wait from asserting BSY for arbitration until the DATA BUS can be examined to see if arbitration has been won. There is no maximum time. 4.7.2 Assertion Period (90 nanoseconds). The minimum time that a target shall assert REQ while using synchronous data transfers. Also, the minimum time that an initiator shall assert ACK while using synchronous data transfers. 4.7.3 Bus Clear Delay (800 nanoseconds). The maximum time for an SCSI device to stop driving all bus signals after: (1) The BUS FREE phase is detected (BSY and SEL both false for a bus settle delay) (2) SEL is received from another SCSI device during the ARBITRATION phase (3) The transition of RST to true. NOTE: For the first condition above, the maximum time for an SCSI device to clear the bus is 1200 nanoseconds from BSY and SEL first becoming both false. If an SCSI device requires more than a bus settle delay to detect BUS FREE phase, it shall clear the bus within a bus clear delay minus the excess time. 4.7.4 Bus Free Delay (800 nanoseconds). The minimum time that an SCSI device shall wait from its detection of the BUS FREE phase (BSY and SEL both false for a bus settle delay) until its assertion of BSY when going to the ARBITRATION phase. 4.7.5 Bus Set Delay (1.8 microseconds). The maximum time for an SCSI device to assert BSY and its SCSI ID bit on the DATA BUS after it detects BUS FREE phase (BSY and SEL both false for a bus settle delay) for the purpose of entering the ARBITRATION phase. 4.7.6 Bus Settle Delay (400 nanoseconds). The time to wait for the bus to settle after changing certain control signals as called out in the protocol definitions. 4.7.7 Cable Skew Delay (10 nanoseconds). The maximum difference in propagation time allowed between any two SCSI bus signals when measured between any two SCSI devices. 4.7.8 Data Release Delay (400 nanoseconds). The maximum time for an initiator to release the DATA BUS signals following the transition of the I/O signal from false to true. 4.7.9 Deskew Delay (45 nanoseconds). The minimum time required for deskew of certain signals. 4.7.10 Hold Time (45 nanoseconds). The minimum time added between the assertion of REQ or ACK and the changing of the data lines to provide hold time in the initiator or target, respectively, while using synchronous data transfers. 4.7.11 Negation Period (90 nanoseconds). The minimum time that a target shall negate REQ while using synchronous data transfers. Also, the minimum time that an initiator shall negate ACK while using synchronous data transfers. 4.7.12 Reset Hold Time (25 microseconds). The minimum time for which RST is asserted. There is no maximum time. 4.7.13 Selection Abort Time (200 microseconds). The maximum time that a target (or initiator) shall take from its most recent detection of being selected (or reselected) until asserting a BSY response. This timeout is required to ensure that a target (or initiator) does not assert BSY after a SELECTION (or RESELECTION) phase has been aborted. This is not the selection timeout period; see Sections 5.1.3.5 and 5.1.4.2 for a complete description. 4.7.14 Selection Timeout Delay (250 milliseconds, recommended). The minimum time that an initiator (or target) should wait for a BSY response during the SELECTION (or RESELECTION) phase before starting the timeout procedure. Note that this is only a recommended time period. The specifications for the peripheral devices shall be consulted for the actual timing requirements. 4.7.15 Transfer Period (set during a MESSAGE phase). The Transfer Period specifies the minimum time allowed between the leading edges of successive REQ pulses and of successive ACK pulses while using synchronous data transfers. (See Sections 5.1.5.2 and 5.5.5.)