SCSI is a local I/O bus that can be operated over a wide range of data rates. 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, optical media drives, and other devices can be added to the host computers without requiring modifications to generic system hardware or software. Provision is made for the addition of special features and functions through the use of vendor unique fields and codes. Reserved fields and codes are provided for future standardization.
A second key objective of SCSI-2 is to provide compatibility with those SCSI-1 devices that support bus parity and that meet conformance level 2 of SCSI-1. While some previously vendor unique commands and parameters have been defined by the SCSI-2 standard, devices meeting SCSI-1 and SCSI- 2 can co-exist on the same bus. It is intended that those operating systems providing support for both command sets be able to operate in environments mixing SCSI-1 and SCSI-2 devices. Properly conforming SCSI-1 devices, both initiators and targets, should respond in an acceptable manner to reject SCSI-2 protocol extensions. All SCSI-2 protocol extensions are designed to be permissive of such rejections and to allow the SCSI-1 device to continue operation without requiring the use of the extension.
A third key objective of SCSI-2 is to move device-dependent intelligence out to the SCSI-2 devices. The command set definitions allow a sophisticated operating system to obtain all required initialization information from the attached SCSI-2 devices. The formalized sequence of requests identify the type of attached SCSI-2 device, the characteristics of the device, and all the changeable parameters supported by the device. Further requests can determine the readiness of the device to operate, the types of media supported by the device, and all other pertinent system information. Those parameters not required by the operating system for operation, initialization, or system tuning are not exposed to the SCSI-2 interface, but are managed by the SCSI-2 device itself.
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.
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). 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 us.
There are two electrical alternatives: single-ended and differential. Single-ended and differential devices are electrically incompatible and can not be mixed on the same physical bus.
Provision is made for cable lengths up to 25 m using differential drivers and receivers. A single-ended driver and receiver configuration is defined for cable lengths of up to 6 m and is primarily intended for applications within a cabinet.
Arbitration 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. A message protocol for control of the interface is also specified. In most cases, messages are not directly apparent to the host computer software.
Commands are classified as mandatory, optional, or vendor-specific. SCSI devices are required to implement all mandatory commands defined for the appropriate device type and may implement other commands as well. 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). Many commands also implement a very large logical block address space (2^32 blocks), although some commands implement a somewhat smaller logical block address space (2^21 blocks).
Starting with clause 8 and for each clause on a specific device type, the clause is constructed of at least four subclauses. The first subclause is the model for the device type. The model establishes the framework for interpreting the commands for the device type. The attributes and capabilities of the device type are discussed and examples are given. The second subclause defines the commands applicable to the device type. The third subclause defines the parameters applicable to the device type. These are the diagnostic parameters, log parameters, mode parameters and vital product data parameters that are transmitted as part of the appropriate commands. Most of the parameters are formatted into pages. The fourth subclause gives the definition of terms that apply specifically to that device type.
Starting with clause 9 the commands in each of these clauses 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.
Certain words and terms used in this International Standard have a specific meaning beyond the normal English meaning. These words and terms are defined either in clause 3 or in the text where they first appear. Names of signals, phases, messages, commands, statuses, sense keys, additional sense codes, and additional sense code qualifiers are in all uppercase (e.g. REQUEST SENSE). Lower case is used for words having the normal English meaning.
Fields containing only one bit are usually referred to as the name bit instead of the name field.
Numbers that are not immediately followed by lower-case b or h are decimal values.
Numbers immediately followed by lower-case b (xxb) are binary values.
Numbers immediately followed by lower-case h (xxh) are hexadecimal values.
4.2 Conventions