AS 3748-1990

Provides the mechanical and functional requirements for a small computer input/output bus and command sets for peripheral device types commonly used with small computers. The interface is intended for use with data rates up to four megabytes per second. This Standard is identical with and has been reproduced from ISO 9316:1989, Information processing systems - Small Computer System Interface (SCSI).

The International Standard provides the mechanical, electrical, and functional requirements for a small computer input/output bus and command sets for peripheral device types commonly used with small computers.The small computer system interface, described in this International Standard, is a local I/O bus that can be operated at data rates up to 4 Mbytes/s depending upon circuit implementation choices. 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, and even communication devices can be added to the host computer(s) without requiring modifications to generic system hardware or software. Provision is made for the addition of nongeneric features and functions through vendor unique fields and codes.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.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.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). Optional 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 ms.The physical characteristics are described in clause 4. There are two electrical alternatives: single-ended and differential. Single-ended and differential devices are electrically different and shall not be mixed on the same bus. In addition, there are several options: shielded or unshielded connectors may be used and parity may or may not be implemented.Clause 5 describes the logical characteristics of the interface. An arbitration option is defined to permit multiple initiators and to permit concurrent I/O operations. AU SCSI devices are required to be capable of operating with the defined asynchronous transfer protocol. In addition, an optional synchronous transfer protocol is defined. Clause 5 also specifies a message protocol for control of the interface. In most cases, messages are not directly apparent to the host computer software. Only one message, COMMAND COMPLETE, is mandatory, all others are optional and are not necessarily implemented. Note that some options (e.g., synchronous transfer) require the implementation of certain messages.The SCSI command structure is specified in clause 6. Commands are classified as mandatory (M), extended (E), optional (O), or vendor unique (V). SCSI devices shall implement all mandatory commands defined for the appropriate device type and may implement other commands as well. Extended SCSI devices shall implement all extended plus all mandatory commands and may implement other commands as well. Extended 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). Extended commands for direct access devices also implement a very large logical block address space (232blocks), although mandatory commands for direct access devices implement a somewhat smaller logical block address space (221blocks).Clause 7 specifies those commands that have a consistent meaning for all device types.Clauses 8 through 13 contain commands for direct-access (e.g., magnetic disk), sequential-access (e.g., magnetic tape), printer, processor, write-once-read-multiple (e.g., optical disk), and read-only direct-access devices, respectively. 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.Clause 14 describes the status byte for all device types. Status is returned by targets at the end of each command.