Overview
ISO/IEC 14776-251:2014 - commonly called USB Attached SCSI (UAS) - defines a SCSI transport protocol that operates over USB-2 and USB-3 links. The standard specifies how to deliver any SCSI command set to a USB device while conforming to the SCSI Architecture Model – 4 (SAM-4) behaviors such as autosense and command queuing. ISO/IEC 14776-251 describes the transport model, USB resource and descriptor requirements, information units (IUs), task management, and error handling for SCSI over USB.
Key Topics and Requirements
- Transport model and IUs: Specifies Information Units (e.g., COMMAND IU, RESPONSE IU, READ READY, WRITE READY, SENSE IU) and sequences for non‑data, data-in, data-out, bi‑directional and multi-command transfers.
- SCSI semantics: Ensures compliance with SCSI Architecture Model‑4 features such as autosense, tagged command queuing and expected SCSI command semantics.
- USB integration: Defines required USB class-specific requests, device/interface/configuration descriptors, and bulk endpoint behavior for UAS operation on USB‑2 and USB‑3.
- Task management: Describes task management IUs, task attributes, and procedures for executing and reporting task management functions.
- Error handling and recovery: Covers USB error reporting, mapping delivery results to SCSI sense data, I_T Nexus loss, resets, and target power‑loss considerations.
- Protocol services: Lists SCSI Application Layer transport protocol services (e.g., Send SCSI Command, Send Data‑In/Out, Command Complete) and their expected interactions.
Applications and Practical Value
- Enables high-performance storage devices (SSDs, HDD enclosures, NVMe-to-USB bridges) to expose SCSI command sets over USB with lower latency and improved parallelism compared to legacy USB mass-storage transports.
- Useful for device firmware developers, storage device vendors, USB controller and host-stack implementers, and OS kernel engineers implementing UAS drivers.
- Helps integrators and test engineers validate interoperability, command queuing behavior and error recovery between USB hosts and SCSI-capable USB devices.
- Supports deployment scenarios where standard SCSI features (autosense, tagged queuing) are required over commodity USB physical layers.
Who Would Use This Standard
- Firmware and hardware engineers designing USB storage devices or bridges
- Operating system and driver developers implementing UAS support
- Test labs and QA teams performing conformance and interoperability tests
- Product managers and architects evaluating storage transport options for USB-based products
Related Standards
- Other parts of the ISO/IEC 14776 SCSI family (SCSI architecture and command specifications)
- Relevant USB specifications (USB‑2.0, USB‑3.x) and USB class documentation (for device descriptors and endpoint usage)
Keywords: ISO/IEC 14776-251, USB Attached SCSI, UAS, SCSI over USB, SCSI Architecture Model‑4, USB‑2, USB‑3, command queuing, autosense, SCSI transport protocol.
Open preview (PDF)Information technology — Small computer system interface (SCSI) — Part 251: USB attached SCSI (UAS)
