SCSI'S 29th Anniversary Review

This year SCSI, the industry-recognized naming acronym for "Small Computer Systems Interface" is 20 years young.
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(click thumbnail)This year SCSI, the industry-recognized naming acronym for "Small Computer Systems Interface" is 20 years young. SCSI standards began their evolution under a different name – SASI – the Shugart Associates Standard Interface. Working with NCR (National Cash Register), Shugart Associates in 1981 developed an 8-bit parallel interface that connected hard disk drives to host computers. Later in that year ANSI (American National Standards Institute) formed the X3T9 committee that began its work on what would become the SCSI-1 standard, which was formally approved in 1986.

The SCSI interface is found in nearly every peripheral interface for not just small computers, but larger servers and even in the onerous mainframes of old. SCSI protocol commands are fluent in systems from Fibre Channel to FireWire. The entire foundation of disk storage technologies has its roots in SCSI.

SCSI-1, an almost obsolete and rare standard, allows for asynchronous or synchronous transfers at up to 5 MBps. There were no common command set parameters understood by SCSI-1, which resulted in numerous driver incompatibilities between hosts and devices.


SCSI-1 incorporated bipolar technologies for its line drivers and receivers, and used a simple passive termination at the end of the SCSI daisy-chain bus. Only eight devices could be attached to the bus.

SCSI-2, the next generation, begat a series of both 8-bit and then 16-bit interfaces. The 8-bit SCSI-2 system employs asynchronous commands with synchronous data transfer rates up to 5 MBps. SCSI-2 moved beyond interfacing just hard disk drives by incorporating a set of additional commands to control devices such as CD-ROMs, printers, scanners and communications equipment.

The SCSI parallel interface (SPI) in 8-bit form uses CMOS technology with open drain or active negation single-ended (unbalanced wire per signal with common ground) bus transceivers. Either passive or active terminations are permitted. EIA 485-based HVD (high-voltage differential) bus transceivers are allowed.

SPI-2 was essentially the same as SPI, but allowed both LVD (low-voltage differential) and HVD bus transceivers. By adjusting the timing and synchronous data transfers, transfer bandwidth moved from 5 MBps to 10 MBps. Called Fast SCSI, the 8-bit SCSI-2 extension remained similar to SCSI-2, but increased transfer rates to 10 MBps with only the HVD bus transceivers allowed.


The next generation of SCSI came out of a SCSI-2 extension to 16-bit bus technology. Fast Wide SCSI is a mixture of "fast," which altered some timing parameters for gains in higher throughput, and "wide," which offered a greater bus width for increased performance. SCSI-2 now included a 16-bit SCSI flavor with asynchronous commands and synchronous data transfer rates, moving the transfer rate up to 20 MBps. With SCSI-2 16-bit came SCSI Parallel Interfaces, SPI-2, the 16-bit version with the same parameters as the 8-bit SPI-2, but now providing for twice the bandwidth.

Wide Ultra SCSI emerged with data transfers of 40 MBps and were first defined as SCSI Fast-20, a 16-bit SCSI asynchronous command set with synchronous data transfer rates up to 40 MBps. Active negation CMOS-based single-ended bus transceivers and EIA 485 based-HVD bus transceivers emerged; now active terminations were required for Fast-20. The SPI-2 version was similar but included LVD as well as HVD bus transceiver capability.

Ultra2 SCSI and Wide Ultra2 SCSI appeared with SPI-2 interfaces at 40 MB and 80 MB, respectively. This SPI-2 implementation ended the use of HVD bus transceivers as the next generation of SCSI (SCSI-3) began to change the face of SCSI-interfaces forever.

SCSI-3 moved beyond the SCSI-2 parallel-only interfaces into support for serial SCSI based on the then P1394 (now IEEE 1394) and known in the Macintosh domain as FireWire, the 100 Mbps (and faster) serial interface standard, fiber-optic connections, and several others. The foundation of Fibre Channel evolved out of the SCSI-3 mechanism, and in parallel wire-mode SCSI-3 allows from 16 to 32 devices to be connected to a single bus channel.


Cable-based SCSI-3 systems include a high-speed mode called Ultra SCSI, with 20 MBps transfers in 8-bit and 40 MBps in 16-bit. Beyond SCSI-2, Ultra3 SCSI with SPI-3 interface pushed the well-established 16-bit SCSI from 40 MBps to 160 MBps transfers (Ultra160). Many changes were necessary to achieve this higher data rate including the deletion of the HVD option, abandonment of single-ended interfaces and the banishment of the Q-cable, a secondary cable that married with the P-cable – or primary cable – scheme for extension of a second bus.

SPI-3 launched SCSI into the 32-bit domain and defined double transition (DT) clocking for LVD on both the rising and the falling edges of the of the REQ/ACK clock. Cyclic Redundancy Check (CRC) and domain validation is also defined in SPI-3 and SPI-x.

The next, and most current application, is also the seventh generation of SCSI. Ultra320 provides for data transfers of up to 320 MBps, with clock speeds twice that of Ultra160. Ultra320 SCSI is best suited for servers, network storage, high-end workstations and RAID storage applications.

First defined in the predecessor Fast-160, Ultra320 utilizes SPI-4 with 16-bit SCSI synchronous data transfers and a new set of clock and driver parameters. SPI-4 employs an 80 MHz free-running clock to eliminate Intersymbol Interference (ISI) problems with the clock signal. It offers support for packetized protocol on all devices, with optional support for Quick Arbitration and Selection (QAS).


To reduce protocol overhead, bundled commands, messages and status bytes are packetized and transferred at full data transfer speeds. Packetization further allows single-connection transfer of multiple commands or data from multiple I/O processes. QAS manages the arbitration and selection processes by eliminating bus free time, the interval at which a device can hand off the bus to another bus without entering a new arbitration phase.

Paced data transfers and a training sequence at the start of each transfer series is coupled with pre-compensation drivers and Adaptive Active Filter receivers (AAF) for high-frequency attenuation control. Skew compensation further minimizes attenuation of the signal at the higher data rates and allows the same 25-meter cable length allowance in Ultra160 to be maintained.

Ultra320 products are in production, and an Ultra640 is already under development.