SCSI: The next generation

There have been many iterations of SCSI to cope with the demands of modern computer users. Modern computers have faster...

There have been many iterations of SCSI to cope with the demands of modern computer users. Modern computers have faster processors and buses and SCSI performance must be improved if it isn’t to become the weak link in the data chain

What is Ultra2 SCSI?

Ultra2 SCSI, also known as low-voltage differential SCSI (LVDS), is the next-generation parallel SCSI interface. Beginning as an ANSI standard intended as an I/O device attachment interface for microcomputers, SCSI has evolved to support today's faster, more complex peripherals and their demands for ever-increasing flexibility, distance and bandwidth. Today SCSI encompasses not only magnetic disk drives but magnetic tape drives, optical disk drives, processors, communications devices, disk array subsystems and other devices. It is used in systems of all sizes, from small systems up to and beyond high-end UNIX systems.

The SCSI standard has also evolved to use more intelligent hardware and a more sophisticated command protocol. Since SCSI became a standard in 1986, the data path width has increased from 8 to 16 bits, and the data transfer speed has increased from 5Mbit/s to a maximum of 40Mbit/s. Ultra2 SCSI technology also increases cable distances to 12m (40ft). Initially Ultra2 SCSI technology will support transfer speeds of 40Mbit/s. Over time, speeds will increase to 80Mbit/s, although system-level performance improvements will be small, because SCSI bus data transfer rates typically do not limit a system's performance.

SCSI became popular for many reasons, some of which are listed below:

( New types of peripheral devices can be added to a system without hardware changes; only a new I/O device driver is needed

( SCSI satisfies the high-performance requirements of medium and large systems

( Intelligence can be moved from the host to a peripheral device, thereby off-loading the system or controller's processor

( SCSI's ability to logically disconnect and reconnect devices from the bus means that slow operations can be performed offline, thus allowing several operations in a system to run concurrently

( The parallel SCSI interface enables systems to perform at a higher potential because the I/O bus is not the system bottleneck

( SCSI and the enhanced versions that have followed it have all offered backward compatibility, thus protecting existing investment in both hardware and software

SCSI originally offered two electrical interfaces: "single-ended" for devices up to 6m (20ft) apart, and "differential" for devices up to 25m (82 ft) apart. Differential provides better protection against noise from an adjacent circuit, and thus much-improved signal quality, than does single-ended SCSI. However, the increased power required for differential drivers means that they cannot be built into a single CMOS SCSI controller chip. Therefore, differential is significantly more expensive and requires more circuit board space and power than does single-ended SCSI. As the speed of the SCSI bus has increased with the adoption of Fast and Ultra SCSI technology, the cable lengths of the bus have steadily decreased to 1.5m (5ft), which has created a significant problem when trying to connect large numbers of devices to a SCSI bus.

Ultra2 SCSI has been developed to solve these problems. Because it uses low-voltage differential signals, it increases the maximum cable length to 12m (40 ft), which is eight times the length allowed by single-ended Ultra SCSI. Low-voltage differential circuits consume much less power than conventional differential SCSI drivers do, which allows the circuits to be built into standard CMOS SCSI controller chips.

Ultra2 SCSI technology also offers greater flexibility in peripheral configuration and the ability of current SCSI users to upgrade their systems easily.

What are the advantages of Ultra2 SCSI?

Ultra2 SCSI is a low-voltage interface that takes advantage of the benefits of differential signalling. The high signal quality provided by Ultra2 SCSI will support the reliable, sustained data transfer rate required by storage subsystems attached to network servers. As a low-voltage interface, it will enable bus drivers to be built inside controller chips, thus making its cost closer to that for single-ended SCSI.

Integration of low-power drivers in the standard CMOS SCSI chip will reduce the power dissipated by the chip, lower the amplitude of noise reflections and provide higher transmission rates.

The backward compatibility with legacy SCSI buses will extend the life of existing SCSI designs and peripherals, thus protecting current SCSI hardware and software investments. Reduced integration and support costs will follow because it will not be necessary to adopt an entirely new I/O interface.

The enhancements of Ultra2 SCSI will support both intra and inter-rack connectivity between a CPU, an Ultra2 SCSI RAID adaptor, an external storage enclosure supporting Ultra2 SCSI, and Ultra2 SCSI hard-disk drives.

Customers running applications that manipulate large amounts of data, such as Lotus Notes or data-mining applications, may see modest system-level performance improvements due to the 80Mbit/s data transfer rate capability of Ultra2 SCSI technology. Because Ultra2 SCSI is based on the single-ended SCSI bus, it supports both single-ended traditional SCSI devices and newer, Ultra2 SCSI devices. When both are attached, a signal in the bus called Differential Sense (DiffSens) automatically detects the presence of a single-ended device and switches all devices in the system to single-ended. This means that all devices run at a maximum speed of 40Mbit/s with the same cable length limitations, as supported by non-Ultra2 SCSI products. It also means that systems continue to operate, as long as the cable length restrictions have been followed, instead of failing because of mismatched transmission speeds.

Compiled by Paul Phillips

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