ISO/IEC 14776-251:2014

Information technology — Small computer system interface (SCSI) — Part 251: USB attached SCSI (UAS)

ISO/IEC 14776-251:2014(E) describes a SCSI transport protocol for USB-2 and USB-3 with the following properties: a) mechanism to send commands associated with any SCSI standard to a USB device; b) complies with SCSI Architecture Model - 4 (e.g., autosense and command queuing) and c) other capabilities.

Technologies de l'information — Interface de petit système d'ordinateur (SCSI) — Partie 251: Titre manque

General Information

• Status: Published
• Publication Date: 16-Apr-2014
• ICS: 35.200 - Interface and interconnection equipment
• Technical Committee: ISO/IEC JTC 1/SC 25 - Interconnection of information technology equipment
• Drafting Committee: ISO/IEC JTC 1/SC 25 - Interconnection of information technology equipment
• Current Stage: 6060 - International Standard published
• Due Date: 09-May-2015
• Completion Date: 17-Apr-2014

Relations

• Consolidated By: ISO 21487:2012/Amd 2:2015 - Small craft — Permanently installed petrol and diesel fuel tanks — Amendment 2
• Effective Date: 06-Jun-2022

Standards Content (Sample)

ISO/IEC 14776-251
Edition 1.0 2014-04
INTERNATIONAL
STANDARD

colour
inside
Information technology – Small computer system interface (SCSI) –
Part 251: USB Attached SCSI (UAS)

ISO/IEC 14776-251:2014-04(en)

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ISO/IEC 14776-251


Edition 1.0 2014-04




INTERNATIONAL



STANDARD








colour

inside










Information technology – Small computer system interface (SCSI) –

Part 251: USB Attached SCSI (UAS)



























INTERNATIONAL

ELECTROTECHNICAL

COMMISSION

PRICE CODE
B



ICS 35.200 ISBN 978-2-8322-1507-4



  Warning! Make sure that you obtained this publication from an authorized distributor.

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– 2 – ISO/IEC 14776-251:2014 © ISO/IEC 2014
CONTENTS
FOREWORD. 6
INTRODUCTION . 8
SCSI standards family . 8
1 Scope .10
2 Normative references .10
3 Terms, definitions, symbols, abbreviations and conventions .10
3.1 Terms and definitions.10
3.2 Symbols and abbreviations .13
3.3 Keywords.13
3.4 Editorial conventions.14
3.5 Numeric and character conventions .15
3.5.1 Numeric conventions.15
3.5.2 Byte encoded character strings conventions.15
3.6 Sequence figure notation .16
3.7 Notation for procedures and functions.16
4 Model .17
4.1 Overview.17
4.2 Tag handling.18
4.3 Data transfers .18
4.4 UAS domain.19
4.5 Addressing.20
4.6 World wide name .20
4.7 Resets .20
4.8 I_T Nexus loss.21
4.9 Target power loss expected .21
4.10 USB error handling .21
5 USB.22
5.1 Overview.22
5.2 USB resource requirements .22
5.2.1 Overview.22
5.2.2 USB class specific requests .22
5.2.3 USB descriptors .22
6 Transport.27
6.1 Overview.27
6.2 IUs.27
6.2.1 Overview.27
6.2.2 COMMAND IU.28
6.2.3 READ READY IU .29
6.2.4 WRITE READY IU.29
6.2.5 SENSE IU .30
6.2.6 RESPONSE IU.30
6.2.7 TASK MANAGEMENT IU .31
6.3 Information unit sequences .33
6.3.1 Overview.33

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ISO/IEC 14776-251:2014 © ISO/IEC 2014 – 3 –
6.3.2 Non-data command/sense sequence.34
6.3.3 Non-data command/response sequence .35
6.3.4 Data-out command sequence.36
6.3.5 Data-in command sequence.37
6.3.6 Task management function sequence .37
6.3.7 Bi-directional command sequence.38
6.3.8 Multiple command example .38
6.4 Transport requirements.41
7 SCSI Application Layer transport protocol services.42
7.1 SCSI transport protocol services overview .42
7.2 Send SCSI Command transport protocol service .43
7.3 SCSI Command Received transport protocol service .44
7.4 Send Command Complete transport protocol service.44
7.5 Command Complete Received transport protocol service .45
7.6 Send Data-In transport protocol service .46
7.7 Data-In Delivered transport protocol service .46
7.8 Receive Data-Out transport protocol service.46
7.9 Data-Out Received transport protocol service.47
7.10 Terminate Data Transfer transport protocol service.47
7.11 Data Transfer Terminated transport protocol service.48
7.12 Send Task Management Request transport protocol service.48
7.13 Task Management Request Received transport protocol service .49
7.14 Task Management Function Executed transport protocol service .49
7.15 Received Task Management Function Executed transport protocol service .51
7.16 USB Acknowledgement.52
8 Device server error handling.53

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Figure 1 – SCSI document structure.8
Figure 2 – Example Sequence figure .16
Figure 3 – USB Model .17
Figure 4 – Example Simple UAS domain .19
Figure 5 – Example Complex UAS Domain.20
Figure 6 – UAS sequence figure notation .34
Figure 7 – Non-data transfer with Sense.34
Figure 8 – Non-data Transfer with Response.35
Figure 9 – Write Data Transfer .36
Figure 10 – Read Data Transfer .37
Figure 11 – Task management .37
Figure 12 – Bi-directional Data Transfer .38
Figure 13 – Multiple Command example.40

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ISO/IEC 14776-251:2014 © ISO/IEC 2014 – 5 –
Table 1 – Numbering conventions . 15
Table 2 – Device descriptor . 22
Table 3 – Configuration descriptor. 23
Table 4 – Interface Descriptor. 24
Table 5 – Bulk-in endpoint descriptor. 24
Table 6 – Bulk-out endpoint descriptor . 25
Table 7 – Pipe Usage Descriptor . 25
Table 8 – Pipe ID. 26
Table 9 – IU ID field summary. 27
Table 10 – IU Header . 27
Table 11 – COMMAND IU. 28
Table 12 – TASK ATTRIBUTE field . 28
Table 13 – READ READY IU . 29
Table 14 – WRITE READY IU. 29
Table 15 – SENSE IU . 30
Table 16 – RESPONSE IU. 30
Table 17 – RESPONSE CODE field . 31
Table 18 – TASK MANAGEMENT IU . 31
Table 19 – task management function field. 32
Table 20 – Execute Command procedure call transport protocol services . 42
Table 21 – Execute Command procedure call transport protocol services . 43
Table 22 – Send SCSI Command transport protocol service arguments . 43
Table 23 – SCSI Command Received transport protocol service arguments. 44
Table 24 – Send Command Complete transport protocol service arguments . 45
Table 25 – Command Complete Received transport protocol service arguments . 45
Table 26 – Send Data-In transport protocol service arguments. 46
Table 27 – Data-In Delivered transport protocol service arguments . 46
Table 28 – Receive Data-Out transport protocol service arguments. 47
Table 29 – Data-Out Received transport protocol service arguments. 47
Table 30 – Terminate Data Transfer transport protocol service arguments . 48
Table 31 – Data Transfer Terminated transport protocol service arguments . 48
Table 32 – Send Task Management Request transport protocol service arguments . 48
Table 33 – Task Management Request Received transport protocol service arguments . 49
Table 34 – Task Management Function Executed transport protocol service arguments. 50
Table 35 – Received Task Management Function Executed transport
protocol service arguments . 51
Table 36 – USB Acknowledgement. 52
Table 37 – Delivery Result to additional sense code mapping . 53

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– 6 – ISO/IEC 14776-251:2014 © ISO/IEC 2014
INFORMATION TECHNOLOGY –
SMALL COMPUTER SYSTEM INTERFACE (SCSI) –
Part 251: USB Attached SCSI (UAS)
FOREWORD
1) ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission)
form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC
participate in the development of International Standards through technical committees established by the
respective organization to deal with particular fields of technical activity. ISO and IEC technical committees
collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental,
in liaison with ISO and IEC, also take part in the work. In the field of information technology, ISO and IEC have
established a joint technical committee, ISO/IEC JTC 1.
2) The formal decisions or agreements of IEC and ISO on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested IEC National Committees and ISO member bodies.
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9) Attention is drawn to the possibility that some of the elements of this ISO/IEC Publication may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
International Standard ISO/IEC 14776-251 was prepared by subcommittee 25: Interconnection of
information technology equipment, of ISO/IEC joint technical committee 1: Information technology.
The list of all currently available parts of the ISO/IEC 14776 series, under the general title Information
technology – Small computer system interface (SCSI), can be found on the IEC web site.
This International Standard has been approved by vote of the member bodies, and the voting results
may be obtained from the address given on the second title page.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

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ISO/IEC 14776-251:2014 © ISO/IEC 2014 – 7 –
IMPORTANT – The “colour inside” logo on the cover page of this publication indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this publication using a colour printer.

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– 8 – ISO/IEC 14776-251:2014 © ISO/IEC 2014
INTRODUCTION
This International Standard standard encompasses the following:
Clause 1 describes the scope.
Clause 2 provides normative references for the entire standard.
Clause 3 provides definitions, abbreviations, and conventions used within the entire standard.
Clause 4 describes the model.
Clause 5 describes USB requirements.
Clause 6 describes transport requirements (e.g., IUs).
Clause 7 describes the SCSI Application Layer Transport Protocol Services.
Clause 8 describes device server error handling.
SCSI standards family
Figure 1 shows the relationship of this standard to the other standards and related projects in the SCSI
family of standards as of the publication of this standard.
Primary command set
Device-type specific command sets
(shared for all device types)
(e.g., SBC-2, SSC-2)
(SPC-3)
SCSI transport protocols
(e.g., SAS-2, FCP-4, this standard)
Interconnects
(e.g., SAS-2, Fibre Channel)
Figure 1 — SCSI document structure
The SCSI document structure in figure 1 is intended to show the general applicability of the documents
to one another. Figure 1 is not intended to imply a relationship such as a hierarchy, protocol stack, or
system architecture.
SCSI Architecture Model: Defines the SCSI systems model, the functional partitioning of the SCSI
standard set and requirements applicable to all SCSI implementations and implementation standards.
Device-Type Specific Command Sets: Implementation standards that define specific device types
including a device model for each device type. These standards specify the required commands and
behaviors that are specific to a given device type and prescribe the requirements to be followed by a
SCSI initiator device when sending commands to a SCSI target device having the specific device type.
The commands and behaviors for a specific device type may include by reference commands and
behaviors that are shared by all SCSI devices.
Shared Command Set: An implementation standard that defines a model for all SCSI device types.
This standard specifies the required commands and behavior that is common to all SCSI devices,
regardless of device type, and prescribes the requirements to be followed by a SCSI initiator device
when sending commands to any SCSI target device.
SCSI Transport Protocols: Implementation standards that define the requirements for exchanging
information so that different SCSI devices are capable of communicating.
SCSI Architecture Model
(SAM-4)

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ISO/IEC 14776-251:2014 © ISO/IEC 2014 – 9 –
Interconnects: Implementation standards that define the communications mechanism employed by
the SCSI transport protocols. These standards may describe the electrical and signaling requirements
essential for SCSI devices to interoperate over a given interconnect. Interconnect standards may allow
the interconnection of devices other than SCSI devices in ways that are outside the scope of this
standard.
The term SCSI is used to refer to the family of standards described in this introduction.

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– 10 – ISO/IEC 14776-251:2014 © ISO/IEC 2014
INFORMATION TECHNOLOGY –
SMALL COMPUTER SYSTEM INTERFACE (SCSI) –
Part 251: USB Attached SCSI (UAS)
1 Scope
This part of ISO/IEC 14776 describes a SCSI transport protocol (see ISO/IEC 14776-414) for USB-2
and USB-3 with the following properties:
a) mechanism to send commands associated with any SCSI standard to a USB device;
b) complies with SCSI Architecture Model - 4 (e.g., autosense and command queuing); and
c) other capabilities.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 14776-414, Information technology, Small Computer System Interface (SCSI) – Part 414:
1
SCSI Architecture Model-4 (herein referred to as SAM-4)
IEC 62680-1, Universal serial bus interfaces for data and power – Part 1: Universal serial bus
specification, revision 2.0 (herein referred to as USB-2)
2
ANSI INCITS 513-2004, SCSI Primary Commands-4 (herein referred to as SPC-4) [T10/1731-D]
Universal Serial Bus 3.0 Specification Revision 1.0 (herein referred to as USB-3). November 12, 2008
Universal Serial Bus Mass Storage Class Specification Overview Rev 1.3 (herein referred to as MSC).
3
September 5, 2008
3 Terms, definitions, symbols, abbreviations and conventions
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1
application client
object that is the source of SCSI commands
Note 1 to entry: See ISO/IEC 14776-414.
3.1.2
Bulk-in Endpoint Descriptor
USB Endpoint Descriptor with the BM ATTRIBUTES field set to 02h and bit 7 of the bEndPoint address
field set to 1
1. ANSI INCITS 447-2008
2. planned as ISO/IEC 14776-454
   For more information on the current status of these documents, contact the INCITS Secretariat at
202-737-8888 (phone), 202-638-4922 (fax) or via E-mail at [e-mail]. To obtain copies of these
documents, contact Global Engineering at 15 Inverness Way, East Englewood, CO 80112-5704 at
303-792-2181 (phone), 800-854-7179 (phone), or 303-792-2192 (fax) or see http://www.incits.org.
3. For information on the current status of USB documents, see the USB Implementers Forum at
http://www.usb.org.

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ISO/IEC 14776-251:2014 © ISO/IEC 2014 – 11 –
3.1.3
Bulk-in pipe
pipe (see 3.1.9) used to transfer data and status from the UAS target port to the UAS initiator port
3.1.4
Bulk-out Endpoint Descriptor
USB Endpoint Descriptor with the BM ATTRIBUTES field set to 02h and bit 7 of the bEndPoint address
field set to 0
3.1.5
Bulk-out pipe
pipe (see 3.1.9) used to transfer data and commands from the UAS initiator port to the UAS target port
3.1.6
default pipe
message pipe (see 3.1.9) created by the USB System Software to pass control and status information
between the host and a USB device’s endpoint zero (see USB-2)
3.1.7
Information Unit
IU
formatted collection of data that carries command, task management function, sense, response, read
ready, or write ready information (see 6.2)
3.1.8
Logical Unit Number
LUN
64-bit identifier for a logical unit
Note 1 to entry: See ISO/IEC 14776-414.
3.1.9
pipe
logical abstraction using USB endpoints representing an association between a function (see USB-2
and USB-3) of a USB device and an application client
3.1.10
read d
...