Where can I find an overview of the SCSI Standards and Cables as defined by the SCSI Trade Assocation

Version 1

    The following information is also available at: http://www.scsita.org/aboutscsi/terms160/Overview_SCSI_Standards.html


    SCSI-1 is a complete document with all the physical and protocol Layers. It was developed in the mid 80s and is obsolete. 8 bit SCSI bus used single-ended open collector drivers with Asynchronous transfer for all commands and data transfers. Bipolar technology was used for the drivers and receivers specifications. Bus DC loads a major consideration with the Bipolar receivers and the passive termination. The connectors are the low-density 50 pin internal and external connectors; the external low-density connector is also known as the Centronics connector, in Cable terms, the SCSI-1 Connector.


    SCSI-2 is a complete document with all the physical and protocol layers. SCSI-2 used asynchronous commands with Synchronous data transfer rates up to Fast 10 megatransfers per second or 20 megabytes per second. SCSI-2 is defined using bipolar technology with Open collector or active negation single-ended bus transceivers, passive or active termination, and Differential (now called HVD -- High Voltage differential) bus transceivers based on EIA 485 allowed in SCSI-2. The A cable connectors are the 50 pin low-density internal connector, low-density Centronics external connector and the high-density external connector. In cable terms the SCSI-2 connector is the high-density 50 pin. The B connectors for wide bus was not practical because it required a second cable 68 pin high density.

    SCSI-2 was considered an unmanageable document with all the SCSI command structure and electrical interfaces. It took years to finalize, becoming a standard 1994.

    SCSI-3 changed the document structure. SCSI-3 is not one document with all the different layers and electrical interfaces, but a collection of documents that cover the physical layer, the basic protocol specific to that electrical interface, the primary command set layer (SPC) and the specific protocol layer. The specific protocol layer contains the Hard Disk interface Commands in the Block Commands (SBC), Stream Commands for tape drives (SSC), Controller Commands for RAID arrays (SCC), Multimedia Commands (MMC), Media Changer Commands (MCC) and enclosure services commands (SES) for example. There is an overall architectural model (SAM). Each document has its own revision level, these are normally referred to as SCSI, the -3 has been dropped.
    Should match terminator impedance = 80ohms

    The SCSI Parallel interface (SPI) defines the Parallel bus electrical connections and Signals. SCSI Interlock Protocol (SIP) defines the parallel command set. SIP is include in the SPI-2 and SPI-3 revisions. SPI defines the P connector 68 pin high-density primary cable for a wide bus in a single cable. This 68 pin high-density cable is referred to as the SCSI-3 connector. SPI includes speeds up to Fast SCSI (Wide Fast SCSI is 20 MegaBytes per second), electrical interfaces are defined in CMOS terms, with the termination specified at 0.2 volts instead of 0.5 volts. The A cable is referenced to SCSI-2 but not included in SPI. (Became a standard approximately 1995, obsolete July 1999; use SPI-2.)

    Fast-20 commonly known as Ultra SCSI is a speed doubling from the SPI document allowing Fast-20, 20 megatransfers per second or on a wide bus 40 megabytes per second. This is not complete standard, it only has the sections required for the Fast-20, all other sections use the SPI standard. (Became a standard approximately 1996, obsolete July 1999; use SPI-2.)

    SPI-2 doubles the speed again to Fast-40 commonly known as Ultra2 SCSI, 40 megatransfers per second or on a wide bus 80 megabytes per second with a new electrical interface, low-voltage differential, LVD SCSI. Single-ended cannot be used for speeds above Fast-20. The SPI-2 standard included SIP, the Single Connector Attachment (SCA-2) 80 pin Host swap connector and the 68 pin Very High Density Connector (VHDCI). The SCSI-2 A and SPI P connectors are include in SPI-2.

    SPI-2 is a complete document and does not refer to older documents, one reference of all parallel interfaces up to Fast-40 speeds. (Became a standard in 1999 and replaced SPI, SIP and Fast-20.)

    SPI-3 doubles the speed again to Fast-80DT commonly known as Ultra3 SCSI and Ultra160, 80 megatransfers per second on only a wide bus 160 megabytes per second. There are a lot of changes with SPI-3; high-voltage differential (HVD SCSI) and the 32 bit data bus with the Q cable are obsolete, clocking is defined for LVD SCSI on both the rising and falling edges of the REQ/ACK clock -- Double Transition (DT) defined for wide bus only, Cyclic Redundancy Check (CRC) defined, Domain Validation defined part of the domain validation will be in SPC-x, Packetized Commands and Messaging defined and Quick Arbitration Defined. **

    SPI-3 is a complete document and does not refer to older documents, one reference of all parallel interfaces up to Fast-80DT speeds, but it does obsolete HVD, SCAM and 32 bit data buses.

    SPI-4 doubles the speed again to Fast-160DT commonly known as Ultra320 SCSI, 160 megatransfers per second on only a wide bus 320 megabytes per second. Example: LTO-3 is a Ultra320 device and would require the use of a SPI-4 cable to obtain maximum performance.

    EPI (Enhanced Parallel Interface) Technical Report describes how to design SCSI systems. The cable lengths and loads are defined by the electrical parameters, allowing system designs to take advantage of faster cables and reduced loads. The technical report describes how to work with wide (68 Pin) and narrow (50 pin) devices on the same system. EPI includes documentation of Expanders, Bridging expanders, Switches and common connectors not documented in the standards.

    SCSI-1 External A cable is the 50 pin Centronics connector cable. (C50M) 85 mil (2.16 mm) spacing
    SCSI-2 External A cable is the 50 pin High Density connector cable. (MiniD50M) 50 mil (1.27mm) Spacing
    SCSI-3 External P cable is the 68 pin High Density connector cable. (MiniD68M) 50 mil (1.27mm) Spacing
    SCSI SPI-2 very high density connector (VHDCI) external P cable is some times incorrectly called the SCSI-4 or 5 cable uses the 68 pin VHDCI connector cable. (.8 mm 68M) *
    SCSI-1, SCSI-2 Internal A cable is the 50 pin connector Ribbon cable. 100 mil (2.54mm) spacing
    SCSI-3 Internal P cable is the 68 pin High Density ribbon Cable. 50 mil (1.27mm) Spacing

    * SCSI-4 and SCSI-5 are not standards; SCSI-3 divided the standard into separate standards, each a stand alone document.

    ** Ultra160/m is a specific implementation of Ultra3 that includes Fast-80DT, CRC, and Domain Validation. Ultra160/m was developed by a group of SCSI suppliers and customers.