Overview
ISO/IEC 14776-351:2007 - "Information technology - Small Computer System Interface-3 (SCSI-3) - Part 351: Medium Changer Commands (SMC)" defines the SCSI command set and behavioural model for medium changer devices. The standard covers both independent medium changers (stand‑alone robotic media handlers) and attached medium changer functions integrated into other SCSI devices. It specifies the commands, element models and external behaviour of device servers that present themselves as medium changers in SCSI INQUIRY responses.
Keywords: ISO/IEC 14776-351, SCSI-3, Medium Changer Commands, SMC, medium changer, tape library, robotic library.
Key topics and technical requirements
- Device and element models: Definitions for element types used in medium changers - transport elements (robot arms), storage elements (slots), import/export elements, and data transfer elements (drive slots).
- Command set: Standardizes commands such as EXCHANGE MEDIUM, MOVE MEDIUM, POSITION TO ELEMENT, READ ELEMENT STATUS, INITIALIZE ELEMENT STATUS, REQUEST VOLUME ELEMENT ADDRESS, RESERVE/RELEASE ELEMENT, and SEND VOLUME TAG. Tables in the standard list command formats, parameters and expected behaviour.
- Element status and descriptors: Formats for element status data, identification descriptors and element-type codes that enable hosts to discover element properties and media status.
- Volume tag handling: Volume tag information formats and primary/alternate tag handling for identifying media.
- Reservation and concurrency rules: Logical unit and element reservation/release semantics (including third‑party reservation behaviours).
- Mode and parameter pages: Device capabilities, element address assignment, and transport geometry parameter pages to describe capabilities and mapping of elements.
- Architecture compliance: SMC is transport‑agnostic and conforms to the SCSI-3 Architecture Model (SAM), allowing use over different transports (Fibre Channel, parallel SCSI, Serial Bus protocols).
Applications and who uses it
- Automated tape libraries, optical jukeboxes and archival systems that require standardized control of robotic media handling.
- Storage device manufacturers and firmware engineers who implement medium changer device servers and SCSI command handlers.
- System integrators and storage architects designing backup, archive and media management solutions that rely on predictable SCSI behaviour.
- Software developers of backup and library management software that issues SMC commands to control media movement and inventory.
Practical benefits include interoperable control across vendors, standardized inventory reporting, consistent volume tagging and predictable multi‑initiator reservation behaviour.
Related standards
- SCSI-3 Architecture Model (ISO/IEC 14776-411)
- SCSI-3 Primary Commands-2 (ISO/IEC 14776-452)
- Related transport and protocol parts of the SCSI-3 family (Fibre Channel, parallel and serial protocols)
For implementers, ISO/IEC 14776-351 is a definitive reference when building or integrating SCSI medium changers and robotic media libraries.
Open preview (PDF)Information technology — Small Computer System Interface-3 (SCSI-3) — Part 351: Medium Changer Commands (SCSI-3 SMC)
