ISO IEC 14776-251-2014 PDF

Full title and description

Information technology — Small computer system interface (SCSI) — Part 251: USB Attached SCSI (UAS). This international standard specifies a SCSI transport protocol for USB (USB‑2.0 and USB‑3.x) that provides a mechanism to send SCSI commands to USB devices and implements features of the SCSI Architecture Model‑4 (for example autosense and command queuing).

Abstract

ISO/IEC 14776‑251:2014 describes the USB Attached SCSI (UAS) transport layer: how SCSI commands and data are encapsulated, queued and returned over USB transports (both USB‑2 and USB‑3), and how the protocol interoperates with SCSI Architecture Model‑4 services and features. The standard enables improved performance and native SCSI semantics for USB mass‑storage devices.

General information

• Status: Current / Published international standard.
• Publication date: 15 April 2014 (Edition 1.0).
• Publisher: ISO/IEC (joint international publication).
• ICS / categories: 35.200 (Interface and interconnection equipment).
• Edition / version: Edition 1.0 (2014).
• Number of pages: 46 pages (official published text).

Scope

Defines a SCSI transport protocol layered over USB‑2.0 and USB‑3.x that: a) provides a mechanism to transmit commands associated with any SCSI standard to a USB device; b) is consistent with SCSI Architecture Model‑4 features such as autosense and command queuing; and c) specifies related protocol behaviour and capabilities required for UAS operation. The scope covers protocol semantics, command/data flow and interoperability considerations for device and host implementations.

Key topics and requirements

• UAS transport framing and command encapsulation for USB‑2 and USB‑3.
• Support for SCSI command queuing and out‑of‑order completion consistent with SAM‑4.
• Error reporting, autosense and status return mechanisms aligned with SCSI architecture.
• Host and device behavior requirements to achieve higher throughput and lower latency than legacy USB mass‑storage transport.
• Interoperability considerations and references to related SCSI/USB specifications (e.g., SPC, SAM, USB specifications).

Typical use and users

Device and firmware engineers building USB mass‑storage devices (HDDs, SSDs, flash enclosures), host controller and operating system developers implementing UAS drivers, test and certification laboratories, and integrators seeking improved performance and native SCSI semantics over USB. The standard is used where SCSI command semantics and high performance over USB are required.

Related standards

Part of the ISO/IEC 14776 SCSI series. Closely related documents include the SCSI Architecture Model (SAM‑4 / ISO/IEC 14776‑414), SCSI Primary Commands (SPC family), USB specifications (USB 2.0 / USB 3.x) and other 14776 series parts (e.g., SBC, SSC, SES). Implementers should consult SAM and SPC as normative references for command semantics.

Keywords

SCSI, UAS, USB Attached SCSI, USB 2.0, USB 3.x, SCSI Architecture Model‑4, SAM‑4, storage device protocol, command queuing, autosense.

FAQ

Q: What is this standard?

A: ISO/IEC 14776‑251:2014 is the international standard that specifies the USB Attached SCSI (UAS) transport protocol — a method for carrying SCSI commands and data over USB transports.

Q: What does it cover?

A: It covers the transport-level framing and behaviour for sending SCSI commands over USB‑2.0 and USB‑3.x, including command queuing, status and autosense semantics consistent with SAM‑4, and interoperability requirements for host and device implementations.

Q: Who typically uses it?

A: Firmware and hardware engineers for USB storage devices, OS and driver developers implementing UAS support, test labs, and system integrators seeking higher performance and native SCSI features over USB.

Q: Is it current or superseded?

A: As published on 15 April 2014 (Edition 1.0) it is published and listed as the active edition; users should confirm whether a newer revision exists before assuming long‑term currency (the IEC/ISO stores list the 2014 edition as the published edition).

Q: Is it part of a series?

A: Yes — it is one part of the ISO/IEC 14776 SCSI series (which includes parts for SAM, SBC, SSC, SES, etc.). Implementers commonly use the series together (e.g., SAM‑4 and SPC documents) for complete SCSI behavior.

Q: What are the key keywords?

A: UAS, USB Attached SCSI, SCSI, USB 2.0, USB 3.x, SAM‑4, command queuing, autosense, storage protocol.