SCSI-3 is a term used to describe a set of standards currently being developed. Simply put, it is the next generation of documents a product conforms to. Unlike SCSI-1 and SCSI-2, SCSI-3 is not one document that covers all the layers and interfaces of SCSI, but is instead a collection of documents that covers the primary commands, specific command sets, and electrical interfaces and protocols.

The command sets include hard disk interface commands, commands for tape drives, controller commands for redundant array of inexpensive drives (RAID), and other commands. Additionally, an overall SCSI Architectural Model (SAM) exists for the physical and electrical interfaces, as does a SCSI Parallel Interface (SPI) standard that controls the form of SCSI most commonly used.

Each document within the standard is now a separate publication with its own revision level—for example, within SCSI-3 three versions of the SCSI Parallel Interface have been published. Usually, we don't refer to SCSI-3 anymore as a specific interface and instead refer to the specific subsets of SCSI-3, such as SPI-3 (Ultra3 SCSI).

The main additions to SCSI-3 include:

  • Ultra2 (Fast-40) SCSI

  • Ultra3 (Fast-80DT) SCSI

  • Ultra4 (Fast-160DT) SCSI

  • Ultra5 (Fast-320DT) SCSI

  • New Low Voltage Differential signaling

  • Elimination of High Voltage Differential signaling

Breaking SCSI-3 into many smaller individual standards has enabled the standard as a whole to develop more quickly. The individual substandards can now be published rather than waiting for the entire standard to be approved.

The most recent changes or additions to SCSI are the Fast-40 (Ultra2), Fast-80DT (Ultra3), and Fast-160DT (Ultra4) high-speed drives and adapters. These have taken the performance of SCSI up to 320MBps. Also new is the LVD electrical interface standard, which enables greater cable lengths.

The older High Voltage Differential signaling has also been removed from the standard. A number of people are confused over the speed variations in SCSI. Part of the problem is that speeds are quoted as either clock speeds (MHz) or transfer speeds.

With 8-bit transfers, you get 1 byte per transfer, so if the clock is 40MHz (Fast-40 or Ultra2 SCSI), the transfer speed is 40MBps. On the other hand, if you are using a Wide (16-bit) interface, the transfer speed doubles to 80MBps, even though the clock speed remains at 40MHz.

With Fast-80DT, the bus speed technically remains at 40MHz; however, two transfers are made per cycle, resulting in a throughput speed of 160MBps. The same is true for Ultra4 SCSI, which runs at 80MHz, transfers 2 bytes at a time, and has two transfers per cycle. Ultra4 is also called Ultra320 and is the fastest form of parallel SCSI available today—the Ultra5 (Ultra640) standard is still under development.

Finally, confusion exists because SCSI speeds or modes are often discussed using either the official terms—such as Fast-10, Fast-20, Fast-40, and Fast-80DT—or the equivalent marketing terms, such as Fast, Ultra, Ultra2, and Ultra3 (also called Ultra160). The further evolution of the most commonly used form of SCSI is defined under the SPI standards within SCSI-3.

SPI or Ultra SCSI

The SCSI Parallel Interface standard was the first SCSI standard that fell under the SCSI-3 designation and is officially known as ANSI X3.253-1995. SPI is also called Ultra SCSI by most marketing departments and defines the parallel bus electrical connections and signals.

A separate document called the SCSI Interlock Protocol (SIP) defines the parallel command set. SIP was included in the later SPI-2 and SPI-3 revisions and is no longer carried as a separate document. The main features added in SPI or Ultra SCSI are:

  • Fast-20 (Ultra) speeds (20MBps or 40MBps)

  • 68-pin P-cable and connectors defined for Wide SCSI

SPI initially included speeds up to Fast SCSI (10MHz), which enabled transfer speeds up to 20MBps using a 16-bit wide bus. Later, Fast-20 (20MHz), commonly known as Ultra SCSI, was added through an addendum document (ANSI X3.277-1996), allowing a throughput of 40MBps on a 16-bit wide bus (commonly called Ultra/Wide).

SPI-4 or Ultra4 SCSI (Ultra320)

SPI-4, also known as Ultra4 or Ultra320 SCSI, has all the same features as the previous Ultra3 (Ultra160) and adds several new features to ensure reliable data transmission at twice the speed.

Ultra320 SCSI integrates both the Packetization and Quick Arbitrate and Select features from Ultra160+ SCSI as mandatory features. Ultra320 SCSI then adds the following new features:

  • Transfer speed. Ultra320 transfers data 2 bytes (16 bits) at a time at 80MHz using double-transition (DT) cycling, meaning it transfers twice per cycle (hertz). This results in a burst transfer rate of 320MBps.

  • Read/Write data streaming. This minimizes the overhead for queued data transfers by enabling a device to send one data stream queue-tag packet followed by multiple data packets. Previously, only one data packet could be sent with each queue-tag packet. Write performance is also increased because there are fewer bus turnarounds from data in to data out.

  • Flow control. This allows a target device to indicate when the last packet of a data stream will be transferred, which enables the initiator to terminate the data prefetch or begin flushing data buffers sooner than previously possible.

SPI-5 or Ultra5 SCSI (Ultra640)

Work has begun on the SPI-5 SCSI standard, also called Ultra5 or Ultra640 SCSI. All that is known at this time is that it is based on the Ultra320 standard but will transfer at twice the speed—an amazing 640MBps.

RAID Arrays

Most servers, especially at levels above the workgroup, use SCSI drives rather than ATA drives because of their superior performance. You can enhance performance and data reliability further by creating a drive array. RAID (redundant array of individual drives) technologies are used by both SCSI and ATA drives.

Current SCSI-based RAID products primarily support Ultra320 and Ultra160 drives and are used in traditional servers and rackmounted computers. For a complete description of RAID levels and terminologies.

Fibre Channel SCSI

Fibre Channel SCSI is a specification for a serial interface using a fibre channel physical and protocol characteristic, with a SCSI command set. It can achieve 200MBps or 400MBps over either fiber or coaxial cable of several kilometers in length.

Fibre Channel is designed for long-distance connectivity (such as several kilometers) and connecting multiple systems, and it has become a popular choice for storage area networks (SANs) and server clusters. Fibre Channel SCSI complements, rather than replaces, Ultra160 and Ultra320 SCSI, which are designed for direct connection to servers.

200MBps versions of Fibre Channel SCSI use the gigabit interface connector (GBIC), whereas 400MBps versions use either the small form factor pluggable (SFP) connector for optical connections or the high-speed serial data connector (HSSDC) for copper cable.


The latest variation on SCSI, iSCSI, combines the performance of SCSI drives with Ethernet networking up to gigabit speeds. Because iSCSI uses Ethernet to transport data between systems, iSCSI storage can be located anywhere an Ethernet network can reach, including Internet access.

In addition, iSCSI storage enables secure remote storage for computers that could be hundreds of kilometers away. Because iSCSI data can be routed the same way any other type of Ethernet data can be routed, it enables data to be transported even when some connections between the server and the storage devices are unavailable.

Eventually, iSCSI is expected to replace Fibre Channel in uses such as network attached storage (NAS), SANs, and storage clusters. Similar to Fibre Channel, iSCSI cards can be purchased in both copper-wire and fiber-optic versions to match the Ethernet network already in use. The first iSCSI devices were produced by Adaptec and Cisco Systems in mid-2002.